In general, a LAN is internally owned in a business whereas a WAN is leased infrasctructure. (at least as far as the CCNA exam is concerned) (Lammle, p.478)
The key to understanding WAN technologies is to be familiar with the different WAN terms and connection types often used by service providers. (Lammle, p.478)
Wide Area Network Terms (Lammle, p.478)
Customer premises equipment (CPE) – equipment owned by the subscriber and located on the subscriber’s premises.
Demarcation point – the spot where the service provider’s responsibility ends and the CPE begins.
WAN Terms (cont.) (Lammle, p. 479)
Local loop – connects the demarc to the closet switching office, called a central office.
Central office (CO) – connects the customers to the provider’s switching network. Sometimes referred to as a point of presence (POP).
Toll network – a trunk line inside a WAN provider’s network. This network is a collection of switches and facilities owned by the ISP.
WAN Connection Types (Lammle, p.479)
Leased line – a pre-established WAN communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting data.
Circuit switching – uses dial-up modems or ISDN and is used for low-bandwidth data transfers. Think phone call.
WAN Connection Types (cont.) (Lammle, p. 479)
Packet switching – allows you to share bandwidth with other companies to save money. Can be thought of as a network that’s designed to look like a leased line, yet costs more like circuit switching.
The downside: if data transfer is needed constantly this option is not good. Speeds can range from 56Kbps to T3 (45 Mbps). Frame Relay and X.25 are packet-switching technologies.
Prominent WAN protocols used today: (Lammle, p.480)
ISDN – Integrated Services Digital Network
LAPB – Link Access Procedure, Balanced
HDLC – High-Level Data-Link Control
PPP – Point-to-Point Protocol
ATM – Asynchronous Transfer Mode
Frame Relay (Lammle, p.480)
A packet-switched technology that emerged in the early 1990s
A Data link and Physical layer specification that provides high performance
Can be more cost effective than point-to-point links
Can run at speeds of 64Kbps up to 45 Mbps (T3).
ISDN (Lammle, p. 480)
Integrated Services Digital Network
A set of digital services that transmit voice and data over existing phone lines.
A possible cost-effective solution for remote users who need a higher-speed connection than analog dial-up links offer.
Recommended as a backup link for other types of links such as Frame Relay or T-1 connections.
LAPB (Lammle, p.481)
Link Access Procedure, Balanced
Lammle says it’s not on the CCNA exam
Created to be a connection-oriented protocol at the Data Link layer for use with X.25
Causes high overhead because of its strict timeout and windowing techniques
HDLC (Lammle, p.481)
High-Level Data-Link Control
HDLC is a protocol at the Data Link layer, and has very little overhead compared to LAPB.
The HDLC header carries no identification of the type of protocol being carried inside the HDLC encapsulation. Because of this, each vendor’s HDLC is proprietary for their equipment.
PPP (Lammle, p.481)
An industry standard protocol
It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol. It allows authentication and multilink connections and can be run over asynchronous and synchronous links.
Asynchronous vs. Synchronous
Asynchronous transmission – digital signals sent without precise timing, usually with different frequencies and phase relationships. (Lammle, p. 554)
Synchronous transmission – signals transmitted with precision clocking.
ATM (Lammle, p.481)
Asynchronous Transfer Mode
Lammle says it’s not on the CCNA exam
Created for time-sensitive traffic (like video), providing simultaneous transmission of voice, video, and data
ATM uses cells instead of packets that are a fixed 53 bytes long.
Serial Transmission (Lammle, p.482)
Cisco serial connections support almost any type of WAN service.
Typical WAN connections are dedicated leased lines using HDLC, PPP, ISDN, and Frame Relay.
Typical speeds run anywhere from 2400 bps to 45 Mbps (T3).
HDLC, PPP, and Frame Relay can use the same Physical layer specifications, but ISDN has different pinouts and specifications at the Physical layer.
Serial Transmission (Lammle, p. 482)
WAN serial connectors use serial transmission, which takes place one bit at a time over a single channel.
Cisco routers use a proprietary 60-pin serial connector that you must get from Cisco or a provider of Cisco equipment.
Serial links are described in frequency or cycles-per-second (hertz).
The amount of data that can be carried within these frequencies is called bandwidth. Bandwidth is the amount of data in bits-per-second that the serial channel can carry.
DTE/DCE (Lammle, p.482)
Router interfaces are, by default, data terminal equipment (DTE), and they connect into data communication equipment (DCE) – for example a channel service unit/ data service unit (CSU/DSU).
The CSU/DSU then plugs into a demarcation location (demarc) and is the service provider’s last responsibility.
Most of the time, the demarc is a jack that has an RJ-45 female connector located in a telecommunications closet.
More on HDLC (Lammle, p.483)
The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented Data Link layer protocol.
HDLC is a point-to-point protocol used on leased lines.
No authentication can be used with HDLC.
HDLC is the default encapsulation used by Cisco routers over synchronous serial links.
Cisco’s HDLC is proprietary as are all HDLC implementations.
More on HDLC (Lammle, p.483)
In byte-oriented protocols, control information is encoded using entire bytes.
On the other hand, bit-oriented protocols may use single bits to represent control information.
Bit-oriented protocols include SDLC, LLC, HDLC, TCP, IP, and others.
Point-to-Point Protocol (PPP) (Lammle, p. 484)
Point-to-Point Protocol (PPP) is a Data Link layer protocol that can be used over either asynchronous serial (dial-up) or synchronous serial (ISDN) media.
It uses the LCP (Link Control Protocol) to build and maintain data-link connections.
The basic purpose of PPP is to transport layer 3 packets across a Data Link layer point-to-point link.
PPP Protocol stack compared to OSI Reference Model (Lammle, p. 485) Physical layer (such as EIA/TIA-232, V.24, V.35, ISDN) 1 Network Control Protocol (NCP) Link Control Protocol (LCP) High-Level Data Link Control Protocol (HDLC) 2 Upper-layer Protocols (such as IP, IPX, AppleTalk) 3 PPP Protocol Stack OSI Layer
PPP contains four main components: (Lammle, p.485)
1) EIA/TIA-232-C, V.24, V.35, and ISDN – a Physical layer international standard for serial communication.
2) HDLC – a method for encapsulating datagrams over serial links.
3) LCP – a method of encapsulating, configuring, maintaining, and terminating the point-to-point connection.
4) NCP – a method of establishing and configuring different Network layer protocols. NCP is designed to allow the simultaneous use of multiple Network layer protocols.
For more in depth PPP information see Lammle, Ch. 11, p. 485-487
Wendell Odom, INTRO, Ch. 4, p. 92 – 95
Also, ICND, Ch. 9, p. 311 - 313
Frame Relay (Lammle, p. 489)
Frame Relay has become one of the most popular WAN services deployed over the past decade due to cost.
Frame Relay is a packet-switched technology.
It doesn’t work like a dedicated point-to-point leased line.
Point-to-point vs. packet-switched example.
For more information refer to:
Todd Lammle, Ch. 11, p.489 – 503
Wendell Odom (Cisco Press)
ICND, Ch. 11, p.376 – 413
INTRO, Ch. 4, p.96 - 100
Integrated Services digital Network (ISDN) (Lammle, p. 503)
ISDN is a digital service designed to run over existing telephone networks.
Can support both data and voice.
Typical ISDN applications and implementations include high-speed image applications, high-speed file transfer, videoconferencing, and multiple links into homes of telecommuters.
ISDN (Lammle, p.503)
PPP is typically used with ISDN to provide data transfer, link integrity, and authentication.
ISDN is not a replacement for PPP, HDLC, or Frame Relay, because it’s really an underlying infrastructure that any of these could use.
PPP is the most common encapsulation across ISDN connections.
Benefits of ISDN (Lammle, p.503)
It can carry voice, video, and data simultaneously.
Call setup is faster than with an analog modem.
Data rates are faster than on an analog connection.
Full-time connectivity across the ISDN is spoofed by the Cisco IOS routers using dial-on-demand (DDR) routing.
Benefits of ISDN (cont.) (Lammle, p. 504)
Small office and home office sites can be economically supported with ISDN BRI services.
ISDN can be used as a backup service for a leased-line connection between the remote and central offices.
Modem racking and cabling can be eliminated by integration of digital modem cards on Cisco IOS Network Access Server (NAS).
ISDN Connections (Lammle, p. 504)
ISDN Basic Rate Interface (BRI) is two B (Bearer) channels of 64k each, and one D (Data) channel of 16k for signaling.
Primary Rate Interface (PRI) provides T-1 speeds (1.544 Mbps) – but according to Lammle, it’s not on the CCNA exam.
Basic Rate Interface (BRI) (Lammle, p.508)
ISDN BRI service, also known as 2B+D, provides two B channels and one D channel.
The BRI B-channel service operates at 64kps and carries data, while the BRI D-channel service operates at 16Kbps and usually carries control and signaling information.
Total bandwidth for ISDN BRI is then 144Kbps (64 + 64 + 16 = 144).
BRI (cont.) (Lammle, p.508)
The D-channel signaling protocols (Q.921 and Q.931) span the OSI reference model’s Physical, Data Link, and Network layers.
The D channel carries signaling information to set up and control calls.
Dial-on-Demand Routing (DDR) (Lammle, p. 510)
Used to allow two or more Cisco routers to dial an ISDN dial-up connection on an as-needed basis.
DDR is only used for low-volume, periodic network connections using either Plain Old Telephone Service (POTS) or ISDN connection.
Designed to reduce WAN costs if you’re billed on a per-minute or per-packet basis.