Voice over Internet Protocol (VoIP) Technologies

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Voice over Internet Protocol (VoIP) Technologies

  1. 1. Voice over Internet Protocol (VoIP) Technologies Nortel Press Nortel Networks Inc. 4001 E. Chapel Hill-Nelson Hwy. Research Triangle Park, NC 27709 www.nortel.com/nortelpress
  2. 2. i Published by: Nortel Press Nortel Networks Inc. 4001 E. Chapel Hill-Nelson Hwy. Research Triangle Park, NC 27709 Copyright © 2008 Nortel Networks. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the publisher, except for the inclusion of brief quotations in a review. Second Printing November 2008 Printed in Canada Library of Congress Control Number: 2007942551 ISBN 978-0-9801074-0-1 The information contained in this book is provided "AS IS" without any express, statutory, or implied warranties or conditions of any kind. Neither the authors nor Nortel Networks Inc. shall have any liability or responsibility for any damages arising either directly or indirectly from the use of this book or the information contained herein. The information and/or products described in this document are subject to change without notice. Nortel, Nortel (Logo), NORTEL NETWORKS, the GLOBEMARK, NT, CONTIVITY, DMS, MERIDIAN, MERIDIAN 1, PASSPORT, and SUCCESSION are trademarks of Nortel Networks. APPLETALK is a trademark of Apple, Inc. CERTICOM and MOVIAN are trademarks of Certicom Corporation ORACLE is a trademark of Oracle Corporation MICROSOFT is a trademark of Microsoft Corporation SYBASE is a trademark of Sybase, Inc. WI-FI is a trademark of Wi-Fi Alliance All other trademarks are the property of their respective owners. Warning Always refer to the procedures described in the most current documentation that are appropriate for the system and software release that you support. Failure to use the appropriate documentation can result in serious technical difficulties and damage to your system. To access Nortel technical documentation, visit www.nortel.com/support or contact your local system vendor for the most current documentation release.
  3. 3. xvi Table of Contents 1: Major Components of Voice over IP ..................................................... 1 Identify VoIP Networks..........................................................................................................3 Understand VoIP Network Components ............................................................................3 H.323 VoIP Networks ....................................................................................................4 SIP Call Signaling ...........................................................................................................5 Gauge the Viability of a Customer’s Existing Network ...........................................6 VoIP Network Infrastructure and Deployment................................................................12 Knowledge Check 1-1: Major Components of VoIP.........................................................15 2: VoIP Network Design Considerations................................................. 19 VoIP Network Considerations ............................................................................................21 Analog to Digital Conversion..............................................................................................23 CODECs ..................................................................................................................................26 G.711 ........................................................................................................................27 G.729 A/B ...............................................................................................................27 G.726 ........................................................................................................................27 G.723.1 .....................................................................................................................27 CODEC Summary ........................................................................................................28 Coding Delay ................................................................................................................29 Effects of Echo...............................................................................................................30 Echo and VoIP ........................................................................................................30 Controlling Echo ....................................................................................................31 Transcoding and Tandem Hops.................................................................................32 Recognize QoE Problems on the Users’ Side.....................................................33 Packet Behavior .....................................................................................................................34 How Packet Switching Works ....................................................................................34 Packet Overhead and Packet Size .......................................................................................36 Packet Obstructions...............................................................................................................37 Packet Delay ...........................................................................................................................38 End-to-End Delay.........................................................................................................38 Variable Delay ..............................................................................................................39 Ways to Minimize Delay and Jitter............................................................................40 Service and routing policies .................................................................................40 Delay and the WAN..............................................................................................................41 Router Performance and the WAN ...........................................................................41 Estimated Latency ........................................................................................................42 Link Management: Fast Pipe to Slow Pipe ...............................................................44
  4. 4. xvii VoIP Technologies Serialization Delay........................................................................................................46 Fragmentation and Packet Size ..................................................................................48 Delay and MTU ......................................................................................................49 Calculate Delay Based on the MTU ....................................................................................51 Demonstration 2-1: Calculate Serialization Delay ...................................................52 Demonstration 2-2: Calculate the Ideal MTU...........................................................53 Knowledge Check 2-1: Calculate the Maximum Transmission Unit .............................54 How Packet Loss Affects VoIP Traffic................................................................................55 Ways to Avoid or Compensate for Packet Loss.......................................................57 Measure Voice Quality..........................................................................................................58 Analyzing Voice Quality.......................................................................................59 Knowledge Check 2-2: VoIP Design Consideration .........................................................60 Practice 2-1: Considerations for VoIP ........................................................................61 Bandwidth Overview............................................................................................................63 Available Bandwidth ...................................................................................................63 Guaranteed Bandwidth ...............................................................................................63 Bandwidth Per Call: Layer 2 Overhead..............................................................64 Calculate Bandwidth.............................................................................................................66 Demonstration 2-3: Calculate Bandwidth for a Full-duplex Ethernet Link.........67 Practice 2-2: Calculate the Bandwidth for a Frame Relay Link .............................68 Practice 2-3: Calculate the Bandwidth for a PPP Link ............................................69 Calculate Effective Bandwidth with VAD .........................................................................70 Demonstration 2-4: Calculate Bandwidth for Ethernet with VAD Enabled........71 Practice 2-4: Calculate Bandwidth for PPP Link with VAD Enabled ...................73 Chapter Summary .................................................................................................................74 Resources ................................................................................................................................74 3: Transport and Session Layer Technologies ......................................... 75 The OSI Reference Model .....................................................................................................77 Communication Tasks and the OSI Model ........................................................................78 Protocol Stack................................................................................................................80 Layering Protocols........................................................................................................81 OSI Reference Model Layers.......................................................................................82 Layer 7: Application Layer ...................................................................................82 Layer 6: Presentation Layer ..................................................................................82 Layer 5: Session Layer ...........................................................................................82 Layer 4: Transport Layer.......................................................................................83 Layer 3: Network Layer ........................................................................................83 Layer 2: Data Link Layer.......................................................................................83 Layer 1: Physical Layer .........................................................................................83 IP Suite Model ........................................................................................................................84 Layer 4: Application Layer..........................................................................................86 Layer 3: Transport Layer .............................................................................................87
  5. 5. Table of Contents xviii Difference Between Transport Layer Protocols.................................................88 UDP and VoIP Applications.......................................................................................89 Transport Models ..................................................................................................................90 Ethernet..........................................................................................................................91 Carrier Class PBT/PBB .........................................................................................91 Voice over Frame Relay...............................................................................................92 Voice over Asynchronous Transfer Mode................................................................94 ATM Services .........................................................................................................94 Voice over Point-to-Point Protocol ............................................................................96 Voice over Wireless LAN ............................................................................................96 Voice over Multi Protocol Label Switching..............................................................97 Cable Modem................................................................................................................97 Digital Subscriber Line ................................................................................................98 VoIP Control Protocols .........................................................................................................98 IP Datagram ..................................................................................................................98 Major Components of Real-time Transport Protocol..............................................99 Real-time Transport Control Protocol.....................................................................100 Difference Between RTP and RTCP ..................................................................100 Real-time Transport Control Protocol Extended Reports ....................................100 Well Known Ports ......................................................................................................101 Socket Numbers..........................................................................................................102 Knowledge Check 3-1: Transport and Session Layer Technologies ............................103 4: Implementing Quality of Service ....................................................... 107 Methods to Achieve QoS ....................................................................................................109 QoS Methods at Layer 2 .....................................................................................................109 Ethernet 802 Prioritization ........................................................................................109 802.1Q VLAN ID..................................................................................................109 802.1p Priority Field ............................................................................................110 Prioritizing Based on 802.1p Priorities .............................................................111 Application Service Classes................................................................................113 Traffic Service Classes.........................................................................................118 Subscriber Traffic Categories .............................................................................119 Port-based Prioritization ...........................................................................................121 Virtual LANs...............................................................................................................122 Types of VLANs...................................................................................................123 Port-based VLANs...............................................................................................124 Policy-based VLANs ...........................................................................................124 VLANs and Broadcast Domains........................................................................127 Queuing Mechanisms ................................................................................................129 First In, First Out..................................................................................................129 Priority Queuing ..................................................................................................130 Custom Queuing..................................................................................................130
  6. 6. xix VoIP Technologies Fair Queuing.........................................................................................................130 Weighted Round Robin.......................................................................................130 Weighted Fair Queuing.......................................................................................130 QoS Methods at Layer 3......................................................................................................133 Resource ReSerVation Protocol ................................................................................133 Traffic Shaping............................................................................................................133 IP Address Prioritization...........................................................................................134 Differentiated Services...............................................................................................134 Per Hop Behavior.................................................................................................134 Differentiated Services Code Point ...................................................................135 Explicit Congestion Notification........................................................................135 Assured Forwarding PHB ..................................................................................136 Drop Precedence ..................................................................................................136 Expedited Forwarding PHB ...............................................................................136 Default PHB ..........................................................................................................137 Marking Packets...................................................................................................137 How DiffServ Works ...........................................................................................137 Trust Configuration .............................................................................................138 DiffServ Domains.................................................................................................138 QoS Methods at Layer 4 and Beyond ...............................................................................140 Best-effort Networks ..................................................................................................140 Layer 4 TCP/IP Classification ..................................................................................140 RTP Prioritization .......................................................................................................140 Packet Fragmentation ................................................................................................141 Policy Management....................................................................................................142 QoS Methods Summary......................................................................................................142 Traffic Flow Design Considerations .................................................................................144 Traffic Engineering.....................................................................................................144 Traffic Measurement ..................................................................................................144 Design Considerations...............................................................................................145 Network Availability and Redundancy ..................................................................145 LAN Traffic Convergence Issues .............................................................................146 WAN Traffic Convergence Issues ............................................................................146 Routing Considerations for the WAN ..............................................................147 Use Service Level Agreements...........................................................................147 Create Backup WAN Links ................................................................................148 Network Policies..................................................................................................................149 Why Implement Policy Services...............................................................................149 Typical Policy Manager Functions...........................................................................149 Common Open Policy Services - Provisioning ......................................................150 Policy-enabled Network ............................................................................................151 Understanding Network Management ...................................................................152 Identifying Problem Conditions...............................................................................153 Resolving the Problem ........................................................................................153 Knowledge Check 4-1: Implementing Quality of Service..............................................154
  7. 7. Table of Contents xx Practice 4-1: Implementing Quality of Service.......................................................156 Chapter Summary ...............................................................................................................158 Resources ..............................................................................................................................158 5: Wireless LAN ..................................................................................... 159 Radio Frequency Overview ...............................................................................................161 Avoiding Mid-air Collisions.....................................................................................161 Interframe Spacing .....................................................................................................163 The Contention Window...........................................................................................164 Purpose of the Contention Window .................................................................164 Request to Send/Clear to Send ................................................................................165 RTS/CTS in a Mixed-mode Environment........................................................165 Radio Signals...............................................................................................................166 Radio Frequency Behavior........................................................................................168 Major Components of a WLAN ........................................................................................170 Service Sets ..................................................................................................................170 Making the WLAN Work...................................................................................................172 Access Points...............................................................................................................172 Beacons and Probe Responses..................................................................................172 Scanning ................................................................................................................173 Authentication and Association...............................................................................173 Antenna Types............................................................................................................174 WLAN Power .............................................................................................................176 Demonstration 5-1: Calculate Loss and Gain in a WLAN....................................177 Narrowband Versus Spread Spectrum ...................................................................178 Frequency Hopping Spread Spectrum....................................................................179 How FHSS Works ................................................................................................179 Direct Sequence Spread Spectrum...........................................................................180 The Barker Code ..................................................................................................181 How DSSS Works ................................................................................................181 WLAN Standards .......................................................................................................182 The IEEE................................................................................................................182 Other Organizations............................................................................................182 IEEE Std 802.11.....................................................................................................183 IEEE Std 802.11b ..................................................................................................183 IEEE Std 802.11b Channels .................................................................................184 IEEE Std 802.11a...................................................................................................186 Modulation Techniques ......................................................................................187 IEEE Std 802.11g ..................................................................................................188 Orthogonal Frequency Division Multiplexing ................................................188 Radio Channel Limitations for Access Links .........................................................189 Data Rates and Distance............................................................................................190 802.11b Data Rates and Distance .......................................................................190
  8. 8. xxi VoIP Technologies 802.11b/g Versus 802.11a ...................................................................................192 Comparison of the 802.11 WLANS....................................................................192 IEEE 802.11b and 802.11g Mixed Environment .....................................................193 Hidden Nodes ......................................................................................................194 Delay Issues for VoIP over WLANs..................................................................................196 Understand WLAN Delay.........................................................................................196 Typical WLAN Delay.................................................................................................198 Knowledge Check 5-1: WLAN Delay Issues ...................................................................200 Chapter Summary ...............................................................................................................201 Resources ..............................................................................................................................201 6: VoIP Standardization and Signaling Protocols ................................. 203 VoIP Signaling Protocols: H.323........................................................................................205 Functions of H.323......................................................................................................205 H.323 Protocol Stack ..................................................................................................206 Major Components of H.323 ..............................................................................................208 H.323 Gateways ..........................................................................................................209 Gatekeepers .................................................................................................................210 Summary of Gatekeeper Functions ...................................................................211 Multipoint Control Units...........................................................................................212 Centralized Conferencing ...................................................................................213 Decentralized Conferencing ...............................................................................214 IP Terminals and Clients ...........................................................................................215 H.323 Call Setup...................................................................................................................216 VoIP Signaling Protocols: SIP ............................................................................................218 Functions of SIP ..........................................................................................................218 SIP Message Format.............................................................................................220 Requests and Responses ............................................................................................221 SIP Network Elements ...............................................................................................222 User Agent Client and User Agent Server .......................................................222 SIP Servers.............................................................................................................223 SIP INVITE............................................................................................................225 SIP INVITE Redirect ............................................................................................226 Conference Calling ..............................................................................................228 SIP and Hardware Implementations ................................................................................229 H.323 and SIP VoIP Support Comparison .......................................................................230 Interworking H.323 and SIP...............................................................................................231 Other Signaling Protocols...................................................................................................231 Media Gateway Control Protocol.............................................................................232 Major Components of MGCP.............................................................................232 Megaco/H.248 ............................................................................................................232 Megaco Architecture ...........................................................................................232 Interworking of SIP and Megaco/H.248..........................................................................233
  9. 9. Table of Contents xxii H.323 Design Considerations ............................................................................................234 Security ........................................................................................................................234 Version Mismatch ......................................................................................................234 Backward Compatibility ...........................................................................................234 Compare H.323 and SIP Standards .........................................................................235 Interworking Systems: IP Peer Networking Overview .................................................236 Gatekeeper Zones.......................................................................................................236 Meridian Customer Defined Network....................................................................237 Trunk Route Optimization .................................................................................237 Trunk Anti-tromboning ......................................................................................238 Knowledge Check 6-1: VoIP Standardization and Signaling Protocols ......................239 Practice 6-1: VoIP Standardization and Signaling Protocols ...............................240 Chapter Summary ...............................................................................................................241 Resources ..............................................................................................................................241 7: Security in VoIP Environments.......................................................... 243 VoIP Security Overview .....................................................................................................245 Firewalls................................................................................................................................245 Types of External Threats ...................................................................................245 Technological Solutions.............................................................................................246 Types of Firewalls ......................................................................................................247 Filtering Routers/Packet Filters...............................................................................247 Stateful Packet Filters/Stateful Inspection Engine................................................248 Circuit-level Gateways ..............................................................................................249 Application Layer Gateways ....................................................................................249 Bastion Hosts ........................................................................................................250 Firewall Topologies.............................................................................................................251 Dual-homed Host.......................................................................................................251 Screened Host .............................................................................................................252 Demilitarized Zones and Perimeter Networks......................................................253 Combined Router and Firewall................................................................................254 Isolated Perimeter Network......................................................................................255 Network Address Translation ...........................................................................................256 Static NAT ...................................................................................................................258 Dynamic NAT.............................................................................................................259 NAT Traversal Solutions: User Agent Involved....................................................261 Universal Plug and Play............................................................................................262 Difficulties with UPnP ........................................................................................263 Simple Traversal of UDP through NAT .................................................................264 STUN Difficulties.................................................................................................265 Traversal Using Relay Network Address Translation..........................................266 TURN Considerations.........................................................................................267 NAT and VoIP ............................................................................................................268
  10. 10. xxiii VoIP Technologies NAT SIP ALG .......................................................................................................268 NAT Hairpinning.................................................................................................268 Back-to-Back User Agent...........................................................................................269 Session Border Controller..........................................................................................270 Securing SIP Signaling ........................................................................................................270 .......................................................................................................................................271 TLS Authentication ....................................................................................................272 Secure SIP ....................................................................................................................273 Secure Real-time Transport Protocol ................................................................................274 How SRTP Works.......................................................................................................274 Session Description Protocol Security Descriptions .......................................................274 Virtual Private Networks ...................................................................................................275 VPN Security Requirements .....................................................................................277 VPN Encryption and Key Management..................................................................278 Tunneling.....................................................................................................................280 VPN Tunneling Protocols..........................................................................................282 Security Architecture for IP.......................................................................................283 Authentication for IPSec Tunnels ......................................................................284 VPN Session Authentication Protocols.............................................................285 Tunneling Protocols and Frames .......................................................................286 SSL/TLS VPNs and IPSec .........................................................................................287 TLS Handshake...........................................................................................................288 High-level Overview of TLS Handshake .........................................................288 Application-level Remote Access.............................................................................290 Authenticating the User ......................................................................................291 VPN-enabling Technologies Summary ............................................................................293 Wireless LAN Security........................................................................................................295 Hacking Tools .............................................................................................................295 Wireless LAN Attacks................................................................................................296 Basic Wireless LAN Protection ..........................................................................298 Authenticating Users .................................................................................................300 Wi-Fi Protected Access ..............................................................................................301 802.1x and Extensible Authentication Protocol......................................................301 EAP-MD5 ..............................................................................................................302 EAP-Transport Level Security ...........................................................................303 EAP-Tunneled Transport Layer Security .........................................................304 Protected EAP.......................................................................................................305 Temporal Key Integrity Protocol..............................................................................306 IPSec .............................................................................................................................306 Detecting Rogue Hardware ......................................................................................308 Knowledge Check 7-1: Security in VoIP Environments.................................................309 Practice 7-1: Security in VoIP Environments ..........................................................310 Chapter Summary ...............................................................................................................311 Resources ..............................................................................................................................311
  11. 11. Table of Contents xxiv 8: Emergency Services in VoIP Environments....................................... 313 Emergency Services in the United States .........................................................................315 Emergency Services in Europe ..........................................................................................315 How 9-1-1 Works in the PSTN ..........................................................................................315 Problems of 9-1-1 over VoIP ..............................................................................................317 Location and Location Discovery ............................................................................317 Layer 2 Switch Port Discovery .................................................................................318 ................................................................................................................................318 Layer 3 Subnet Discovery .........................................................................................319 ................................................................................................................................319 Support for 9-1-1 and Local Trunk Access Code + 9-1-1......................................320 On-Site Notification of 9-1-1 Events ........................................................................320 Local Termination of 9-1-1 Calls ..............................................................................320 Dedicated 9-1-1 Trunk Facilities ..............................................................................321 Next Generation 9-1-1 .........................................................................................................321 Nortel Development Efforts...............................................................................................321 Chapter Summary ...............................................................................................................322 Resources ..............................................................................................................................322 9: Perform a Network Assessment......................................................... 323 VoIP Network Health Assessments..................................................................................325 Needs Assessment Process for Existing Network .................................................325 Pre-Sales Planning Phase....................................................................................326 Step 1: Conduct a Readiness Audit ...................................................................328 Knowledge Check 9-1: Perform a Network Assessment...............................................334 Practice 9-1: Perform a Network Assessment ........................................................335 Chapter Summary ...............................................................................................................336 Knowledge Check 9-2: Final Review ................................................................................337 Appendix A: Answers to Knowledge Check Questions.......................... 349 Knowledge Check 1-1: Major Components of VoIP.......................................................350 Knowledge Check 2-1: Calculate the Maximum Transmission Unit...........................353 Knowledge Check 2-2: VoIP Design Consideration.......................................................354 Knowledge Check 3-1: Transport and Session Layer Technologies ............................356 Knowledge Check 4-1: Implement Quality of Service ...................................................359 Knowledge Check 5-1: WLAN Convergence Issues ......................................................361 Knowledge Check 6-1: VoIP Standardization and Signaling Protocols ......................362 Knowledge Check 7-1: Security in VoIP Environments ................................................364 Knowledge Check 9-1: Perform a Network Assessment...............................................366 Knowledge Check 9-2: Final Review ................................................................................367
  12. 12. xxv VoIP Technologies Appendix B: Answers to Practice Exercises .......................................... 381 Practice 2-1: Considerations for VoIP ......................................................................382 Practice 2-2: Calculate Bandwidth with Frame Relay ...........................................384 Practice 2-3: Calculate Bandwidth with PPP ..........................................................385 Practice 2-4: Calculate Bandwidth with VAD Enabled & PPP ............................386 Practice 4-1: Transport and Session Layer Technologies......................................387 Practice 6-1: VoIP Standardization and Signaling Protocols................................388 Practice 7-1: Security in VoIP Environments ..........................................................390 Practice 9-1: Manage Your VoIP Network ..............................................................392 Appendix C: 920-803 Preparation Guide.............................................. 393 Training Road Map .............................................................................................................394 Using this Preparation Guide ...................................................................................394 Tips on Solving Scenario-based Questions.............................................................394 Sample Questions ................................................................................................................396 Answers to Sample Questions ...........................................................................................402 Appendix D: 920-804 Preparation Guide.............................................. 429 Training Road Map .............................................................................................................430 Using this Preparation Guide ...................................................................................430 Tips on Solving Scenario-based Questions.............................................................431 Sample Questions ................................................................................................................432 Answers to Sample Questions ...........................................................................................444 Appendix E: Acronym List ..................................................................... 475 Glossary ................................................................................................. 485 Index....................................................................................................... 521
  13. 13. 1 1 1: Major Components of Major Components of Voice over IP Voice over IP What if it were possible to cost-effectively consolidate Chapter 1 Topics: and streamline disparate communication processes? What could this mean to consumers who might not be When deploying VoIP, whether on interested in technology solutions, but are very an existing data network or a new interested in enhanced communications and more customer site, you must know options for less money? Voice over Internet Protocol which components make up the (VoIP), a universal communications language within VoIP network. the Open Systems Interconnect (OSI) model, provides To better understand how VoIP just such an opportunity. networks function, you also VoIP is a scalable, flexible network technology, capable need to know the role each of working across any network infrastructure. VoIP component plays and how uses the IP network to transmit real-time voice traffic. components interact to provide By using IP, it is possible for computers to share voice transmission over the data resources across networks and share application network. protocols that support communication services, such as the World Wide Web (WWW). In this chapter, you learn: What does this mean to businesses and consumers? IP provides a foundation that allows internetworking of • The six major H.323 VoIP both dissimilar physical networks and equipment from network components a variety of vendors. Computers can understand IP • Five important VoIP without a translator and can function as coordinated units within an IP network. It means more flexibility considerations; what is with current network equipment and applications for recommended and what is business, and easier access to the Internet for required for VoIP networks consumers. • Which steps to take to analyze This chapter reviews the major components of VoIP and the specific considerations that you want to be either a customer’s current aware of when implementing VoIP in a network. data network for VoIP or an already VoIP-enabled network • How local area network (LAN) infrastructures are assessed and analyzed
  14. 14. 2 VoIP Technologies 1 Chapter 1 Goals Major Components of After studying this chapter, you will be able to: Voice over IP • Identify the major components of VoIP networks. • Define the key infrastructure considerations to support the addition of VoIP traffic in local area networks (LANs) or wide area networks (WANs) environments. Key Terms Auto-negotiation 6 Power management 9 Half-duplex and full-duplex 7 Protocols and applications 11 Internet Protocol (IP) 1 Voice over IP (VoIP) 1 Layer 2 and Layer 3 switches 10
  15. 15. Identify VoIP Networks 3 1 Identify VoIP Networks Major Components of Nearly everyone is familiar with the Public Switched Telephone Network (PSTN), even if they Voice over IP are not familiar with how it works. Pick up a telephone, dial, talk, and voice transmissions somehow “flow” across the wires to the recipient’s telephone. PSTN calls require processing through dedicated circuit-switched connections. VoIP calls, on the other hand, convert conversations into data packets, which then “flow” across the data network wires. However, because there are no dedicated circuits, VoIP traffic routes are most uncertain. As we will learn in later chapters, this uncertainty means that real-time VoIP communication requires a higher Quality of Service (QoS) than other applications. Furthermore, the packetization of voice conversations means that VoIP requires security measures for local area networks (LANs), wide area networks (WANs), and wireless networks if conversation confidentiality is to be maintained. In the following section, we discuss the major elements involved in a VoIP network. Understand VoIP Network Components It is helpful to have a basic understanding of the different VoIP network components and the tasks they perform as we cover more complex concepts and information about VoIP technologies in later chapters. In this chapter, we define each of the VoIP components and gain an understanding of how they work together. Signaling protocols and standards stipulate how networking equipment initiates and controls voice, video, and data communications. In this chapter, we focus on two of these standards: H.323 and Session Initiation Protocol (SIP). • H.323 is a suite of protocols that provides a foundation for audio, video, and data communications across IP-based networks, including the Internet. H.323 is network-, platform-, and application-independent. It is a packet-based signaling standard that allows interoperability between H.323-compliant devices. • SIP is a protocol that allows IP-capable end points to create media sessions, such as phone calls, with each other. SIP does exactly what its name implies: it allows two (or more) end points to initiate a media session, such as a phone call or video conference. In a SIP environment, the signaling protocol is a peer-to-peer signaling protocol that creates, modifies, and terminates sessions over an IP network. SIP uses a media-description language and has a HyperText Transfer Protocol (HTTP)-like structure, which makes it easy to read and comprehend. H.323 provides for the entire call: media translation, call access, and so on, while SIP Note only handles signaling (call setup and teardown). We discuss both H.323 and SIP in much more detail in Chapter 6: VoIP Standardization and Signaling Protocols.
  16. 16. 4 VoIP Technologies 1 H.323 VoIP Networks Major Components of The six major components of an H.323 VoIP network are the media gateway, H.323 gateway, Voice over IP call servers, the gatekeeper, IP terminals and clients, and the IP backbone network, as depicted in Figure 1-1. • The media gateway converts one media stream into another; for example, converting digital voice packets to analog signaling. The media gateway can interact with call controllers, proxies, and softswitches using proprietary or standard protocols, such as H.323. • The H.323 gateway transforms audio received from a telecommunications system into a format that the data network can use. The H.323 gateway acts as a bridge to the IP network from the voice network, and has built-in intelligence to select the voice compression CODECs (COder/DECoders) and adjust the protocols and timing between two dissimilar computer systems or voice over data networks. • Call servers receive call setup request messages, determine the status of the destination devices, and check the authorization of users to originate or receive calls. Call servers also create and send the necessary messages to process the call requests. • The gatekeeper has several functions within the VoIP network. It provides call control, media access, and bandwidth management between end points. It also performs address translation, admissions control, and zone control. The gatekeeper coordinates access to other servers and manages call routing. It also maps destination telephone numbers to their destination end point IP addresses. • IP terminals and clients are the end points on the network, such as hard or soft telephones (stationary or portable) and wireless devices. These devices bring voice and data communications to the end user (at which point, the call server completes the call processing). We discuss H.323 terminals and clients in more detail in Chapter 6: VoIP Standardization and Signaling Protocols. • The IP backbone network provides the universal communication language and foundation to allow dissimilar networks and equipment from a variety of vendors to interconnect. The IP backbone network can include both wired and wireless networking components. Wireless communications present special challenges to VoIP. We discuss the challenges unique to wireless communications in more detail in Chapter 5: Wireless LAN.
  17. 17. Understand VoIP Network Components 5 1 FIGURE 1-1: MAJOR COMPONENTS OF VOIP (H.323 ENVIRONMENT) Major Components of Voice over IP System Components Call Server Call Server Gateway Gatekeeper Applications Management Gateway Infrastructure LAN WAN WAN PSTN Gateway Gateway Wireless Access Point Workstation Wireless IP Terminals Handsets IP Terminals and Clients SIP Call Signaling VoIP uses SIP for call signaling. With SIP, users can locate and contact one another, regardless of media content or number of participants, using disparate computers, phones, televisions, and hand-held devices. A SIP gateway sits at the edge of the SIP domain and translates what goes on inside the SIP domain into forms the outside network can use. From the SIP point of view, the gateway is merely a special type of User Agent. From the outside network’s point of view, the gateway is the door into the SIP domain. SIP gateways have two basic functions: translating the signaling stream and, where appropriate, translating the media stream. For example, a SIP-to-H.323 gateway translates SIP signaling to H.323 signaling, but does not translate the media stream, since both SIP and H.323 use Real-time Transport Protocol (RTP) to convey the voice packets. However, the PSTN does not use RTP, so a SIP-to-PSTN gateway has to translate both the signaling path and the media path. We discuss SIP in greater detail in Chapter 6: VoIP Standardization and Signaling Protocols.
  18. 18. 6 VoIP Technologies 1 Gauge the Viability of a Customer’s Existing Network Major Components of To add VoIP to an existing data network, you need to evaluate the architecture of the network Voice over IP to determine if changes need to be made to be able to run voice traffic over it. When analyzing a customer’s network, you need to know the answers to the following questions: • Is the customer using half-duplex or full-duplex Ethernet connections? • Has the customer enabled or implemented auto-negotiation? • Does the customer’s network provide reliable, continuous power? • Does the customer’s network use Layer 2 or Layer 3 switches, or both, or is the customer using shared media? • Is the customer implementing the best protocols and applications for their particular situation? In the next section, we discuss each of these issues and why they are important when assessing a customer’s VoIP network viability.
  19. 19. Understand VoIP Network Components 7 1 Set the Transmission Mode Major Components of The original Ethernet transmitted signals over a coaxial cable. Stations could transmit or Voice over IP receive, but could not do both at the same time. The reason is simple: the stations used the same physical wire to talk and listen. If a station transmitted a frame while it was receiving a frame, the impulses it was sending interfered with the impulses it was receiving, resulting in a collision and garbled data. Engineers named the communication state in which a station can only communicate in one direction half-duplex. Half-duplex differs from full-duplex as explained in the text below and depicted in Figure 1-2: • Half-duplex data transmission is defined as alternating transmissions over a communications link. A network device has to stop transmitting to receive information or stop receiving to transmit information. Wireless transmission shares this limitation; a radio antenna cannot both transmit and receive information at the same time. Wireless transmissions operate in half-duplex mode. • Full-duplex data transmission is defined as the ability for communications to flow both ways simultaneously over a communications link. Full-duplex only works when a device (such as your workstation) has a dedicated pair of transmit wires connected to a dedicated pair of receive wires on the other device (typically a port on a switch). Similarly, your workstation must have a dedicated pair of receive wires connected to a dedicated pair of transmit wires on the other device (that same port). Half-duplex links are NOT recommended for VoIP; use full-duplex links. Most Gigabit Tip Ethernet products sold today are full-duplex. FIGURE 1-2: FULL-DUPLEX VERSUS HALF-DUPLEX TRANSMISSION MODES BothFirst, one direction,Time Directions at a Both directions at the sameOnce Only One Direction at time. Both then, the other direction. and Directions at a Time Full-duplex: Half-duplex: Two-way Communication One-way Communication over the Same Pipe using Alternating Transmission
  20. 20. 8 VoIP Technologies 1 Enable Auto-negotiation Major Components of Auto-negotiation enables two devices sharing a common link to advertise their speed and Voice over IP duplex mode capabilities, acknowledge receipt and understanding of shared modes of operation, and reject modes of operation that are not shared. To establish a link, the devices need to: • Match duplex behavior • Match transmission speeds • Match any required special protocols, such as remote fault indication Beware: Not all devices support auto-negotiation. Hubs, for example, do not support Note auto-negotiation—the most a hub can do is autosense, which is to determine the speed of a device at the other end of the link. As illustrated in Figure 1-3, devices that support auto-negotiation send a burst of closely spaced link integrity pulses that contain the highest link speed and duplex combination they can support, as well as any optional auto-negotiation capabilities. The receiving device sends a return burst containing the highest link speed and duplex combination that it can support, as well as optional capabilities. The devices continue exchanging these packets until they have reached the highest common denominator and the link is established. Auto-negotiation is recommended for VoIP, because it can leverage the maximum resources of each node. Auto-negotiation pulses on the link are exchanged when: • The link is initially connected • A device at either end of the link is powered up • A device is reset or initialized • A renegotiation request is made FIGURE 1-3: USE AUTO-NEGOTIATION TO LEVERAGE NETWORK RESOURCES
  21. 21. Understand VoIP Network Components 9 1 Major Components of To properly use auto-negotiation, both sides must support auto-negotiation. Failure to Note enable full-duplex auto-negotiation at both ends can cause call clipping. Voice over IP To ensure backward compatibility, auto-negotiation also provides a parallel detection function that allows the speed of the link to be established on legacy systems that do not have auto-negotiation capabilities. Device mismatch can occur when you have differing transmission speeds or duplex behavior. For example, you might have 10 Mbps running on one end of a link and 100 Mbps on the other end of a link, or you might have full-duplex on one end and half- Note duplex on the other end. In some cases, it might be necessary to disable auto-negotiation and set a fixed speed and duplex mode for the local port (10 Mbps or 100 Mbps, half- or full-duplex) so that the modes coincide. Provide Power Management Building power management into your IP network increases its reliability. For certain industries, such as health-care, even the occasional power outage or fluctuation is unacceptable. To provide continuous power, use Uninterruptible Power Supply (UPS) units as a battery backup. Use redundant power supplies and plug each device into a power supply, then plug each power supply into a separate UPS unit. And then, if possible, have each UPS on its own circuit—if one circuit fails, the other circuit should still have power. Also, make sure you provide sufficient cooling in the wiring closet to prevent equipment failure due to overheating. For IP terminals, you can implement Power over Ethernet (PoE), also referred to as Power over LAN (PoL). The Institute of Electrical and Electronics Engineers (IEEE) standard for PoE is 802.3af, which is an open standard. An open standard means that entities are permitted open access if they desire to utilize the results of that standard to further conformity across implementations. An open standard can contribute to greater interoperability between vendors and quicker integration of new technologies. Tip PoE eliminates the need to connect each telephone to an alternating current (AC) power outlet. This elimination means less wiring to the desktop, as well as the ability to perform centralized power backups. Many Ethernet switches are QoS-enabled and provide PoE capability. Keep in mind that older, first generation IP terminals might not support PoE.
  22. 22. 10 VoIP Technologies 1 Use Layer 2 and Layer 3 Switches Major Components of Layer 2 and Layer 3 switches are two types of network devices. The two Layer 2 switches in Voice over IP Figure 1-4 operate using Media Access Control (MAC) addresses. Layer 3 switches, also referred to as routing switches, combine the speed of a switch with the IP routing capability of a router. Traditional routers store their routing instructions in software. However, Layer 3 switches store their routing instructions in hardware, which means Layer 3 switches can reduce latency (fixed amount of delay) by routing millions or even billions of packets per second. We revisit Layer 2 and Layer 3 switches in Chapter 2: VoIP Network Design Considerations and Chapter 3: Transport and Session Layer Technologies. FIGURE 1-4: LAYER 2 AND 3 SWITCHES Gateway WAN Layer 2 Switch Router 3 Switch Switch In communications, the term layer refers to the layers in the OSI reference model. The OSI model defines the language and boundaries for establishing protocols so devices Note are open to one another, allowing communication between different networks. We discuss the OSI reference model in Chapter 3: Transport and Session Layer Technologies. At a minimum, Layer 2 switching is required for VoIP. Ethernet hubs are NOT supported Note due to the potential for congestion and collision.
  23. 23. Understand VoIP Network Components 11 1 Select Protocols and Applications Major Components of Also, consider the protocols and applications that run or will be run on your customer’s Voice over IP network; bandwidth requirements for different protocols and applications vary widely. For example, Simple Mail Transfer Protocol (SMTP) can be bandwidth intensive, while Telnet is not. Commonly used protocols and applications are listed in Table 1-1. TABLE 1-1: NINE COMMONLY USED PROTOCOLS AND APPLICATIONS Protocol/Application Description Domain Name Server Use DNS to map a domain name to its corresponding IP address. The (DNS) DNS server is extremely important, because without it, you must know the IP address of a computer before you can access that computer. Dynamic Host Computers use DHCP to gather their initial network configuration Configuration information, such as host name, gateway, and subnet mask. DHCP is Protocol (DHCP) not that talkative. File Transfer Use FTP to manage data file transfers between computers and Protocol (FTP) networks. Because FTP is a standard protocol, it permits the transfer of any type of data file between different types of computers or networks. HyperText Transfer When you access web pages from your computer, your computer is Protocol (HTTP) using HTTP. Depending on which web pages a user is accessing, this protocol can be bandwidth intensive. Network Time NTP is a complex protocol that uses multiple methods to synchronize Protocol (NTP) clocks on a computer to a more accurate time source. Use NTP to synchronize clocks on physically separated machines. NTP is defined in Request for Comment (RFC) 1129. Simple Mail Transfer When querying messages on a mail server, SMTP is used. SMTP can be Protocol (SMTP) very bandwidth intensive. Structured Query SQL queries information from a relational database. SQL is a very Language (SQL) talkative protocol; Nortel recommends you run SQL on a single virtual local area network (VLAN). Telnet Use Telnet to access other computers over Transmission Control Protocol/IP (TCP/IP). The TCP/IP remote login protocol is specified in RFC 854. Telnet is not very bandwidth intensive. Trivial File Transfer TFTP, a simplified version of FTP, is commonly used in devices to allow Protocol (TFTP) for the transfer of setup and configuration information. TFTP is defined in RFC 1350. For a searchable RFC index, go to http://www.ietf.org/rfc.html or Tip http://www.rfc-editor.org/rfc.html.
  24. 24. 12 VoIP Technologies 1 VoIP Network Infrastructure and Deployment Major Components of Before deploying VoIP at a customer site, you first need to understand the company’s Voice over IP network and overall corporate structure, departments, lines of business, partners, and remote offices. To do this, you must: 1. Obtain or create diagrams for the existing LAN/WAN infrastructure. These diagrams help identify traffic flows to better understand the path voice packets will eventually take. 2. Identify the physical and logical locations of important internetworking equipment. For example, note the locations of routers, switches, and firewalls, and document the names and IP addresses of existing network servers and interconnection devices. 3. Document the types and lengths of physical cables and circuits. 4. Using the information you collected, analyze the infrastructure, identify any problematic conditions that might arise in the future, and determine if your client’s business and technical goals are realistic.
  25. 25. VoIP Network Infrastructure and Deployment 13 1 Major Components of Real World Example 1-1 Voice over IP Consider the situation of a small (about 30 agents) outgoing telemarketing center. The owner provides telemarketing services on a contractual basis for a variety of firms that include: • A small traveling circus • Several charities • The local firefighter’s guild • The local policeman’s guild • Political candidates All of these clients use the center’s services on an intermittent basis: the circus for a week before it arrives in town, the charities during their fund drives, and so on. The center keeps small databases on each of their clients, which the agents use for reference and order entry as they make their calls. The demand on the servers at any given moment is fairly low, and the demand on the data network is light. An agent will typically access the server only at the start of a call, and only at the end of a call, either with order information or to record the called individual’s response. The owner has spoken with a Nortel sales representative, and believes she can realize substantial cost savings by moving to a VoIP contact center solution. Based upon what you have learned so far in this chapter, what questions would you ask her about her current data network? Real World Solution Based upon the material in this chapter, you decide to ask the customer questions along the lines of: • What kind of networking equipment are you using? Are you using hubs, switches, or routers? • At what speed is your Ethernet running and are you using half- or full-duplex? • What pieces of equipment are currently connected to your network? How much bandwidth do they currently demand? • On what kind of wiring is your network running? • What protocols or applications are you currently using on your network? How much bandwidth do they require? • Do you currently have UPS in place? If not, what provision have you made to protect against a power outage? Your purpose in asking these questions is to discover whether your customer’s current network can meet the demands that VoIP will place on it. Ideally, you want to hear that she is using Layer 2 or Layer 3 switches (although in this network, a Layer 3 switch is probably overkill), is running Category (Cat) 5 Ethernet cable or better, and that her current applications consume no more than 50 percent of the available bandwidth. The purpose in asking about the UPS is to help her realize that a VoIP system relies on AC power, and not on the -48v DC power supplied by the ground lines from the Telco central office. A power outage would shut down her business.
  26. 26. 14 VoIP Technologies 1 Major Components of Real World Example 1-2 Voice over IP While talking with the owner of the telemarketing center, you learn that her data network consists of the following items: • The network includes 30 desktop personal computers (PCs) running the agent version of a contact center software. This software provides each agent with access to the various databases. It also communicates with the supervisor version of the contact center software located on the supervisor’s PC. Each PC connects to the network through a 100 Mbs Ethernet NIC. • It also contains one desktop PC running the supervisor version of the contact center software. The supervisor version allows the supervisor to track activity on each of the agent PCs. The supervisor’s PC connects to the network through a 100 Mbs Ethernet NIC. • The network has three 100 Mbs hubs running at 100 Mbs. • She has two redundant, load-balancing database servers. • She also has two combination printer/copier/fax machines, connected to the network through a print server. • Cat-5 cable in the transmission medium. • Current bandwidth consumption is at 37 percent. Based upon what you have learned so far, what recommendations, if any, would you make to the owner about the readiness of her network to run a VoIP contact center implementation? Real World Solution You decide to recommend to your customer that she: • Swap out her hubs for one or more Layer 2 or Layer 3 switches. The number of switches you recommend will depend on whether you run separate cables to each phone and PC. If you implement a Nortel solution, in which you can run a single cable to an IP Phone, and then a cable from the phone to the PC, you would only need a single 48-port Layer 2 switch. • Implement a UPS. Your customer’s network is actually in pretty good shape for the demands VoIP will place on the network. Even if every agent is on the phone at once, the total additional bandwidth demand would be only slightly over 2 Mbps. Even considering the bandwidth taken up by the VoIP signaling protocols, her network has plenty of available bandwidth to run VoIP. You must replace the hubs, however, to provide 100 Mbps full-duplex to each agent. This situation would be much different if your client’s network was running at 10 Mbps over Cat-3 cable. In that case, you would need to replace the entire network.
  27. 27. Knowledge Check 1-1: Major Components of VoIP 15 1 Knowledge Check 1-1: Major Components of VoIP Major Components of Voice over IP Answer the following questions. Answers to these Knowledge Check questions are located in Appendix A: Answers to Knowledge Check Questions. 1. Half-duplex Ethernet links are recommended for VoIP. a. True b. False 2. Which statement describes one of the benefits of using auto-negotiation? a. Permits a few IP addresses to be shared by many users b. Leverages the maximum resources of each network node c. Reduces latency by routing millions of packets per second d. Lets a gateway reject traffic when a defined threshold is reached 3. At a minimum, Layer 2 switching is required for VoIP. a. True b. False 4. SIP is a suite of protocols that provides a foundation for audio, video, and data communications across IP-based networks. a. True b. False 5. Failure to enable half-duplex auto-negotiation on both the sending and receiving hubs on either side of a link can cause call clipping. a. True b. False 6. Which device is responsible for address translation and mapping destination telephone numbers to their destination end point IP addresses? a. Call server b. Gatekeeper c. H.323 gateway d. Media gateway e. IP backbone network f. IP terminals and clients
  28. 28. 16 VoIP Technologies 1 7. Which device is responsible for converting voice packets to analog and can interact with Major Components of call controllers and proxies? Voice over IP a. Call server b. Gatekeeper c. H.323 gateway d. Media gateway e. IP backbone network 8. Which device is responsible for receiving call setup requests and checking the authorization of users to originate or receive calls? a. Call server b. Gatekeeper c. H.323 gateway d. Media gateway e. IP backbone network 9. Which device acts as a bridge to the IP network and has the capacity to select the voice compression and adjust protocols and timing between two dissimilar voice over data networks? a. Call server b. Gatekeeper c. H.323 gateway d. Media gateway e. IP terminals and clients 10. Which device is considered an end point on a network? a. Call server b. Gatekeeper c. H.323 gateway d. Media gateway e. IP backbone network f. IP terminals and clients
  29. 29. Knowledge Check 1-1: Major Components of VoIP 17 1 Chapter Summary Major Components of With the information you learned in this chapter, you can now identify the six major Voice over IP components of an H.323 VoIP network and define the key infrastructure considerations needed to support the addition of VoIP traffic in a LAN or WAN environment. You know why examining and characterizing your customer’s existing network infrastructure can help you determine if your client’s business and technical goals are realistic, and if any problematic conditions might occur in the future. You know that half-duplex is not recommended for VoIP networks and that you must use a minimum of Layer 2 switching in VoIP networks. And, although auto-negotiation and power management are important considerations and recommended for VoIP networks, you realize they are not required. In Chapter 2: VoIP Network Design Considerations, we will learn more about: • How packet switching works • How CODECs work • How delay and packet loss occur • How to minimize the impact of delay and packet loss on voice quality • How to modify the Maximum Transmission Unit and packet size when necessary We also discuss network assessments in more detail again in Chapter 9: Performing a Network Assessment, where we put all of the information you learn in these chapters in practice to provide a knowledgeable recommendation for a customer’s existing network or for a new installation. Resources Refer to the documents listed in Table 1-2, which are found on the CD-ROM located at the back of this textbook, for additional resources. Use the documentation specified in the table, as needed, to gain additional insight into the topics discussed in this chapter. TABLE 1-2: RESOURCES FOR CHAPTER 1 Location on CD-ROM Applicable Section Introduction to Quality of Service • “QoS Performance Dimensions” (QoS) Performance Characteristics of • “Summary of Voice over IP Packetization Basics” Voice over IP Networks • “Recommendations” SIP and the New Network • “What is SIP?: Basic Introduction, History, Capabilities” Communications Model • “The SIP Value Proposition” Useful Websites • http://www.ietf.org/rfc.html • http://www.rfc-editor.org/rfc.html
  30. 30. 107 4: Implementing Quality of Service Most data networks are inherently structured to treat Chapter 4 Topics: all traffic the same. Data traffic experiences different amounts of delay and packet loss at any given time. Now that you know how traffic Because of this structure, data networks are referred to moves across the VoIP network as best-effort networks. and that VoIP traffic experiences In contrast, voice networks are structured so that voice different amounts of delay and traffic experiences a fixed amount of delay and packet loss, you need to know essentially no packet loss, which results in very high how to keep voice quality high. quality voice. You need to know about the Because a Voice over Internet Protocol (VoIP) network different methods used to can encompass many different technologies, no single implement QoS, and about traffic technology can ensure end-to-end Quality of Service flow design and convergence (QoS). As packets traverse the network, they traverse issues, along with performance different link layers. A layered approach to QoS is and fault management. sometimes required to create a QoS-enabled network. This chapter describes the methods used to implement In this chapter, you will learn: QoS on a customer’s data network to meet that • How to prioritize and classify customer’s voice quality targets. traffic • How to separate traffic using 4 virtual local area networks Implementing Quality (VLANs) of Service • How queuing mechanisms work • How traffic engineering and flow design work with VoIP • Performance and fault management in a VoIP environment
  31. 31. 108 VoIP Technologies Chapter 4 Goals After studying this chapter, you will be able to: • Discriminate between the various methods available for implementing QoS prioritization of VoIP traffic to achieve the best voice quality. • Choose the appropriate method to implement QoS. • Identify queuing mechanisms available to handle network congestion. • Identify QoS techniques that you can implement to ensure optimal traffic flows and meet customer requirements. • Analyze customer data protocols and application requirements to identify whether certain data traffic is to be given priority over voice traffic. • Make QoS recommendations to ensure optimal traffic flow. Key Terms 802.1p priority field 110 Queuing mechanisms 129 802.1Q VLAN ID field 109 Resource ReSerVation Protocol Differentiated Services (DiffServ) 113 (RSVP) 133 Packet fragmentation 141 Service classes 113 Policy management 142 Traffic flow design 144 Port-based prioritization 121 Traffic shaping 133 Virtual LAN (VLAN) 109 4 Quality of Service (QoS) 109 Implementing Quality of Service

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