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Basic Data Networks
 

Basic Data Networks

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  • Page Student Notes Global Wireless Education Consortium  © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws.     Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners .
  • Page Student Notes Global Wireless Education Consortium  Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039.    GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.        
  • Page Student Notes Global Wireless Education Consortium 
  • After completing this module, you will be able to: Compare and contrast the different hubs, routers, switches and gateways that exist in the basic Public Switched Telephone Network (PSTN). Read and analyze routing tables. Explain how errors are detected and how to correct them.
  • Data networks can be classified into three major categories: Local area networks (LANs) Access networks Backbone networks. Although part of the difference is in the speeds that data can be transferred, the main attribute that determines how a network is classified is its position in the interconnection between end users. For more information, refer to the GWEC module S-ATM.
  • We will first look at local area networks (LANs) as the closest link to the end user. To do this, we need to understand how users on a network are connected to each other. This is called the network topology.
  • Local area networks are constructed using one of three topologies: Bus Star Ring. Within any given LAN, there are physical, electrical, and logical topologies. Physical refers to the way cables are physically routed between devices on the LAN Electrical is the way the wires are connected Logical is the way data units are routed from one device to another. Most of the time, electrical and logical topologies are the same. However, the physical and electrical topologies of a LAN will frequently be different.
  • Bus Topologies are common in the data communications networks. Ethernet is an example of a bus topology that was originally developed as proprietary technology of Xerox Corporation. The IEEE standardized most of the original Ethernet technology in its 802.3 standard for Carrier Sense Multiple Access/Collision Detection (CSMA/CD). Cable modems use the physical part and some of the higher layers of the Ethernet protocol.
  • The Ring Topology is another common LAN topology. IBM introduced the Token Ring as its technology for LANs. The corresponding IEEE standard is 802.5.
  • This chart shows how local area network protocols correspond to layers of the OSI reference model. Notice that the layer above the Physical Layer is called the Medium Access Control (MAC) sub-layer. One of the most important functions of the MAC sub-layer is to regulate when a device may communicate on the network. This may be done either centrally in the network by a single controller, or on a decentralized basis, by stations on the network dynamically determining the order in which transmissions may occur. While centralized control is easier to implement, it creates a single point of failure, which can bring the entire network down, and may become a bottleneck during high network traffic periods. For these reasons, the majority of MAC protocols specify a decentralized operation. How a device is given permission to communicate is also a function of the MAC sub-layer. The technique used in LANs is called asynchronous access control. This type of control allows the network to allocate its physical capacity dynamically, in response to immediate demand by the attached devices. Most LAN’s use a shared communication medium, thus necessitating medium access control. A network made up of point-to-point links would not need a MAC layer.
  • Access networks are intermediate networks in an end-to-end connection. They provide a path from a LAN to a high-speed backbone network. Typically, these networks are entered at Layer 3 of the OSI Reference Model, via a router on a LAN. Two common technologies for access networks are X.25 and Frame Relay.
  • The X.25 protocol specifies data flow in the first three layers of the OSI Reference Model. As data moves from switch to switch through an X.25 network (a legacy system), each node processes all three lower layers of the OSI Reference Model. Level 3 error checking between each node slows data flow.
  • As networks became more reliable at the Physical Layer, protocol designers modified X.25, creating Frame Relay . Frame Relay processes Layers 1 to 3 only at endpoints of the network. All other data movement through the network is at Layers 1 and 2. The error correction that was done at Layer 3 in the intermediate points of the network for X.25 is now the responsibility of the endpoints and the lower layers of the protocol. Removing the need to go up one layer increases the speed at which data can move through the network.
  • Backbone networks have the highest bandwidth or speed of data transfer. They may be used to interconnect LANs into Wide Area Networks (WANs) or simply to provide a one time high-speed connection between lower speed networks. Backbone technology is constantly evolving. At one time, X.25 was considered a backbone technology, but it has been replaced by the higher speed Asynchronous Transfer Mode (ATM) transported on SONET, or a related cell technology, SMDS.
  • One of the reasons for the high speed of ATM is the small size of the ATM protocol data unit (PDU) , which is called a “cell.” In an ATM cell, information is contained in 48 bytes. A 5 byte header contains the address and cell priority information used in routing and delivery, thus yielding a 53 byte cell.
  • The Python snake skin is a good metaphor for the flexibility needed from data networks. ATM can support applications of different bandwidths. Everything going into the network is encoded onto the 53 byte cell structure, and then multiplexed onto the network. What this means is that ATM equipment can be gracefully added to a network. An ATM multiplexer can be used to bring a combination of high-speed data and lower speed voice or Ethernet LAN traffic onto a fiber operating at SONET OC-x rates. The information part of the cell can contain frame relay headers/trailers, T1, X.25, etc. So, several technologies can be carried over ATM in the network, and sorted out at convenient nodes by ATM multiplexers, hubs, and routers. For more information, refer to the GWEC module S-ATM.
  • Rather than allocating network times to equipment ports, even when no data is being sent, ATM uses network capacity only when a device has information to send. The result is “bandwidth on demand”, which means that each device on an ATM network can have the share of the ATM transport mechanism that it needs.
  • Page Student Notes Global Wireless Education Consortium  Data network concepts and equipment in telecommunications networks have been merged with the world of computer data networking. The design and operation of data networks can be understood in the context of the OSI Reference Model. Layering protocols provides structure to data communications systems. What may not be so apparent is that there can be several ways to layer the protocols. Without a standard, systems created from multiple vendor sources would still find it difficult to communicate. In 1977, the International Organization for Standardization (ISO) recognized this problem, and established a committee to develop such a standard. The result of that committee’s work is now known as the Seven Layer Open Systems Interconnection (OSI) Reference Model. This model is generally regarded as the framework for any layered protocol architecture. It is important to note that the OSI model is not a particular architecture. It is a reference model. This means that no one layered architecture will have all the elements of the model. However, most layered architectures can be directly related to the OSI model, as can individual standard specifications for types of networks.
  • Page Student Notes Global Wireless Education Consortium  It is useful to notice some general characteristics before looking at each of the layers. First, the lowest level deals with hardware. This is where system designers spent most of their time prior to the advent of computer systems. Second, the seven layers can be grouped into two broad categories.  Layers 1 through 3 specify the way that a computer will interact with a network. These layers also govern data communications between chains of computers or networks, and are implemented within those individual computers or networks. They are concerned with routing the individual protocol data units, rather than the total message.  Layers 4 through 7 look beyond the individual computers or networks in a chain, and specify the rules for end-to-end communications. These layers are implemented within host computers. These are the layers that ensure the complete message is transmitted. Wireless System,s because they are concerned only with communications and information transport, are limited to levels 1-3.
  • Page Student Notes Global Wireless Education Consortium  The seven questions above depict the type of function each layer of the OSI Seven Layer Model would perform.
  • Page Student Notes Global Wireless Education Consortium  Layer 1 is the hardware layer. It is where the physical and electrical characteristics of the transmission path are defined. The Protocol Data Unit is the bit. This layer allows basic data to pass over an electrical connection. Layer 2 groups bits into data transmission units (frames). Error checks are made on both the sending and receiving sides of a point-to-point link, but not end-to-end. Layer 2 looks at intermediate points in the network. Signals for error checks are defined. The protocol asks for retransmissions between two points in the network if errors occur, but does not monitor end-to-end. Layer 3 defines messages. The Protocol Data Unit is commonly known as the packet or datagram. This layer determines routing sequences and timing across a network, preventing misrouting. It also deals with flow control and with addressing, particularly when multiple networks are involved. Layer 4 manages end-to-end flows and error controls. It ensures end-to-end delivery of data across a network. Layer 5 specifies when it is time for either side to talk. It negotiates and reaches agreement on the nature and duration of communications. It also checks for security violations or terminal incompatibilities. Layer 6 translates codes (e.g. ASCII to EBCDIC) when necessary. It makes sure data is intelligible from one side to another. Layer 7 interfaces to end-user applications.
  • Page Student Notes Global Wireless Education Consortium  The Physical Layer deals with the mechanical, electrical, configuration, and functional characteristics. It defines how 1s and 0s are to be changed into signals (encoding) and sent either electrically or optically over the transmission medium.
  • Page Student Notes Global Wireless Education Consortium  Layer 2, the Data Link Layer, pertains to line protocols. These are the rules that govern the flow of data between two directly connected, and only two, entities. The objective of this layer is to provide a high throughput, fast response time, and minimal logic to account for data integrity. Functions performed at Layer 2 are: Flow control Synchronization, accomplished by software timers set by the protocol designer. Synchronization and flow control work together to regulate the rate of data transmission. Error detection of data sent from another immediately connected entity Identification of the entities involved in the communication Identification of whether data or control information is being sent. Note: When there is a Media Access Control (MAC) layer, it forms the lower half of Layer 2. This is a MAC 2 layer – bridging.
  • Page Student Notes Global Wireless Education Consortium  Layer 3 is the Network Layer. This is the layer that regulates how groups (networks) of individual entities communicate with each other. The following key issues are resolved by Layer 3 protocols: Different sizes and formats for Protocol Data Units in each network Different timers and timeouts Different quality of service Different addressing schemes Different levels of performance Different routing methods Different user interfaces (connectionless versus connection-oriented) Different levels of security Different troubleshooting, diagnostics, and network maintenance issues Cost and billing allocations Layer 3 is also concerned with routing and congestion control within a single network. This is a routable layer.
  • Page Student Notes Global Wireless Education Consortium  The Transport Layer defines the rules for information exchange and manages end-to-end delivery of information between end users who may be on the same or different networks including error recovery and flow control. The functionality that the OSI assigned to layers 5 and 6 are often contained within the applications at layer 7.
  • Page Student Notes Global Wireless Education Consortium  Messages in a telecommunications Common Channel Signaling System or an Integrated Services Digital Network (ISDN) are a type of PDU that begin and end with a flag, and contain information that spans multiple OSI layers. We will give an example in the applications section of this module as Common Channel Signaling. The key idea is that routing information is contained in a header field and customer information is in a separate field.
  • Page Student Notes Global Wireless Education Consortium  The general pattern of routing is common to all packet switched networks and is store and forward oriented, using a header field, user data field, and frequently a trailer field.
  • Page Student Notes Global Wireless Education Consortium  The above graphic shows both a packet as defined by Layer 3 of the OSI model and a message unit which may be sent the same as a packet in a packet switched network. Notice that the message unit contains information from multiple layers of the OSI model. This is because certain types of packet switched networks are built upon protocols that were defined before the OSI model. In many ways, the message illustrated conforms to a hierarchical, rather than layered, protocol architecture, because it includes information from lower, as well as higher, OSI-defined layers. However, it may be possible that some of the layers are not hierarchical, as they cannot directly access information from other layers without going through a lower level. In this sense, the protocol is itself a type of hybrid, containing elements of both layered and hierarchical architectures. In order to reconcile both views, the layered equivalent has been overlaid onto the hierarchical message. The key point is that the network is still processing the message as a unit of information, using a store and forward methodology. A store and forward methodology does not require strict layering. Layering is useful for a variety of reasons, but there are economies in being able to “cheat” occasionally. The Internet protocols do this when they allow TCP, at layer 4, to peek into the layer 3 datagram to look at the IP addresses. Otherwise the two 32-bit addresses would have to be repeated at each layer.
  • Page Student Notes Global Wireless Education Consortium  While discussing the rules that govern the communication of information, we need to be aware of how that information is structured as it moves through various systems and networks. This has been standardized as a basic block that is transferred between layers and across networks. This block is called the Protocol Data Unit (PDU) . A PDU has a data part, a header, and sometimes a trailer. A protocol data unit is transferred across networks. A service data unit is transferred between layers. The data part contains information from a higher layer that is being operated upon by the current layer of the protocol standard. The header is information that provides the layer with direction concerning what to do with the information in the data part? While the header is not a set of detailed instructions, quite often software will use header contents to structure those instructions. The trailer is supplemental information to that contained in the header. Trailers are typically only found in the second layer of the OSI Reference Model.
  • Page Student Notes Global Wireless Education Consortium  Routers simply read destination information and forward the packet to the next node. Examine the above graphic carefully and routing will no longer seem mysterious. A packet enters the network at Node A with customer data. Its destination is Node C. There is no direct path from Node A to Node C. The packet is routed by Node A to Node B based on an internal routing table containing Node B’s connections stored at Node A. Note the multiple addresses in the routing PDU, C for final destination and B for the next node in the path to Node C. Upon arrival at Node B the packet is stored, analyzed, and re-addressed to its final destination at Node C. Node B uses its internal routing table of direct connections to determine the path and the address. The IEEE 802.11 wireless LAN protocols use multiple addresses as shown above to route frames through an “infrastructure network” between two parts of a single wireless LAN. The Internet routing protocols do not use multiple addresses.
  • Page Student Notes Global Wireless Education Consortium  Packet networks store and forward blocks of information. The current view of packet switching includes all the layers of the OSI model, so PDUs, rather than packets, are being processed by nodes. The PDU may simply contain a header and an information part, or it may be a message that spans multiple layers of the OSI model. A store-and-forward network may be built along the lines of the OSI model or it may not. If it is, an OSI-type of network the protocols will be strictly layered, the PDUs will be packets, and the routing will take place at layer 3. If it is not, an OSI-type of network the protocols may not be layered, the PDUs may be messages, and the headers may contain information that spans multiple layers. The header (in the packet PDU) or some part of the message (in the message PDU) contains routing information. In the simplest case, this is the address of the intended receiver. Somewhere in the network, this routing information must be stored and processed. The table summarizes the ways this can occur.
  • Page Student Notes Global Wireless Education Consortium  Ethernet switching operates at Layer 2.
  • Page Student Notes Global Wireless Education Consortium  Each packet received by a bridge is stored, checked for errors, and then re-sent as follows: If the destination address is broadcast (FF:FF:FF:FF:FF:FF), the frame is sent out to all ports except the one it arrived on. If the source and destination are both reachable from the same port of the bridge, the frame is dropped If the source and destination are reachable from different ports of the bridge, then the frame is re-sent out to the destination port. Each bridge keeps an internal forwarding table that associates addresses with ports. Learning: For each arriving data packet, the switch examines the source address and adds/updates the entry in forwarding table containing: Source address (6-byte format) Port that this frame arrived on Current time.
  • Page Student Notes Global Wireless Education Consortium  Several common data network elements will be roughly mapped into their OSI reference level. Terms and equipment definitions can be moving targets in the data arena, but the reference model is fixed.
  • Page Student Notes Global Wireless Education Consortium  Repeaters are Layer 1 devices. A better term for a repeater is actually a regenerator. Regeneration is a digital signal processing technique. Repeaters use analog amplification.
  • Page Student Notes Global Wireless Education Consortium  Bridges are Layer 2 devices. A bridge can be used to connect two similar LANs. Bridges connect to hubs. Each bridge examines the destination address in a frame and either forwards this frame onto the next LAN or drops the frame.
  • Page Student Notes Global Wireless Education Consortium  A hub broadcasts every packet to every device port at the same data rate. Hubs are generally associated with bridges. Switches perform similar functions, but have significant processing power, speed, and features at a greater cost.
  • Page Student Notes Global Wireless Education Consortium  Switches have hybrid or multi-layer applications. They can be viewed as Layer 2 devices with expanded capabilities. Switches function like bridges connecting two or more network segments, and take the place of a hub. A switch can be used to connect two similar LANs. Switches can connect to PCs, hubs, or other switches. Each switch examines the destination address in a frame and either forwards this frame onto the next LAN or drops the frame. Switches can also function at layer 3, replacing routers .
  • Page Student Notes Global Wireless Education Consortium  Routers are Layer 3 devices. They operate at the third layer, or OSI Network Layer, of the packet. Routers modify Layer 2 frame headers and trailers so the packet can travel end-to-end over many links. These devices connect a LAN to a WAN or a WAN to a WAN. The router modifies an outgoing packet by removing any LAN headers and trailers, and encapsulating the necessary WAN headers and trailers. They they strip off the “Layer 2” header (such as Ethernet), and then create a new “Layer 2” header for the next hop to the next router or final destination. Routers often incorporate firewall functions to protect end-users.
  • Page Student Notes Global Wireless Education Consortium  Gateways are Layer 3+ devices. The term gateway was originally synonymous with router, but currently is used as a generic term for devices that connect two networks. Gateways connect dissimilar networks and manipulate or translate unlike protocols and timing.
  • We will look at several data network applications that are merging circuit switching and packet switching, and mixing data and telecommunications devices.
  • Page Student Notes Global Wireless Education Consortium  A typical data network application. LANs in Chicago and Detroit use routers to access a national carrier network. The national carrier network is based upon routers and not circuit switches.
  • Page Student Notes Global Wireless Education Consortium  The above network merges data network concepts and devices with PSTN concepts and devices.
  • Page Student Notes Global Wireless Education Consortium  Two types of networks coexist today; voice and data.. Voice switches are nodes in the voice network and gateways are the data nodes. The interworking of these networks is a critical development area for telecommunications.
  • Page Student Notes Global Wireless Education Consortium  IP telephony mixes traditional circuit switched elements with LAN-style packet switching elements. Voice packets are the data units that carry a voice conversation through the Internet. Like all packets, they are routed independently of each other. Because of this, individual packets may experience different delays as they move across a network, or may be dropped entirely. Dropped or delayed packets can adversely affect the quality of a voice call. The public Internet is more susceptible to delays or dropped packets than a managed private network.
  • Page Student Notes Global Wireless Education Consortium  IP telephony traffic is similar to the packet traffic that is generated by 3rd Generation mobiles. If you replace the land line telephone with a voice coded (VOCODER) mobile, you will see a future architecture for wireless. The Coder-Decoder (CODEC), like a VOCODER, is a two-way device. .
  • Error detection and correction are key to quality network performance. We will discuss these functions at a high level.
  • Page Student Notes Global Wireless Education Consortium  Protocol Errors Certain data errors are due to violation of various rules for data transmission. Two examples are: Exceeding a Committed Information Rate (CIR) , causing loss of bit content in excess of that rate. The CIR is a rate of data transmission that the carrier guarantees will be sent between points on the network. Although a network user may transmit at rates exceeding the CIR, there is no guarantee that data over that rate will be sent uncorrupted over the network, especially during periods of high traffic. Data collisions resulting when two terminals illegally transmit at the same time. Ethernet, for example, only allows one terminal at a time to transmit data. A certain level of collisions is normal, typically in the 1% range. When these occur, the data communications system stops all transmission of data and reinstates transmission, one terminal at a time. When the level of collisions exceed the “normal” threshold, detailed protocol tests are necessary to identify the offending terminals. These data errors are observable using test equipment called protocol analyzers , which allow the technician to directly observe data messages as they travel through all or part of a data network. Many protocol analyzers also feature statistical analysis of the condition of the network and its ports, providing a way to find error patterns.
  • Page Student Notes Global Wireless Education Consortium  Block Parity Check Block or parallel parity is one of the simpler forms of error detection. It operates on blocks of data words. During a data transmission of several words, a predefined number of these words are grouped together into a block, which will consist of rows and columns of data, one row of parity bits, and one column of parity bits. A parity bit is appended to the end of each row and also to the end of each column in the block. The parity bits are based on Boolean Algebra functions called exclusive OR (XOR) and exclusive NOR (XNOR) which have been performed on the data bits in each row and column. Note in the slide above, the result is that the sum of the 1s in each row and column is always an odd number. In other cases, it could be set to always be an even number. On the receiving end, the parity bits at the end of the rows and columns are verified as being correct for the data bits which are received. If they are not, an error has occurred somewhere in the transmission process, and the data must be either reconstructed with a more sophisticated algorithm, or resent.
  • Page Student Notes Global Wireless Education Consortium  The block parity character is sometimes called the Longitudinal Redundancy Check (LRC) . It is also known as the checksum , because it is formed by performing a binary addition without the carry of each successive character. The limitations of block error checking are that error correction is not possible by this method alone, and it cannot detect more than one error in the block of data. In many cases, the combination of additional cost for more sophisticated methods, the low probability of more than one error per block, and the degree that errors will be critical, justify the use of this simple scheme over more complex methods. XMODEM Checksum is another type of checksum used with the XMODEM protocol formerly used between personal computers. Under the XMODEM protocol, data is sent in 128 character blocks. The blocks are preceded by a Start of Header (SOH) character and block number information. At the end of the block, the XMODEM checksum is appended. This error detection device is calculated by adding all of the 128 bytes in the character block, dividing the result by 255, and taking the remainder as the checksum. In addition to being sent with the block of data, the checksum is computed at the receiving end. The result of the receiving end computation is compared with the value that was sent. If the two agree, the transmission is acknowledged by the receiver as error-free.
  • Page Student Notes Global Wireless Education Consortium  Cyclic Redundancy Check (CRC) Cyclic Redundancy Check is the method used to detect line errors in T-1 Extended Superframe transmission. Because it is very accurate, it has also become a standard for many other forms of block data transmission. The methodology of CRC is that the data communications system treats the bits in a data block as the coefficients of a polynomial (eg: x^ 15 + x^ 4 + 1). Internally, the system creates other polynomials which it uses in a mathematical function performed on the original polynomial. (An example is the division of the data block polynomial by the polynomial created by the system.) This function results in a remainder. The same function is performed on the data block at both ends of the data transmission, and remainders are compared. When the transmission is error free, the remainders are identical. In order to make the comparison of the remainders, both the data block and the remainder must be sent over the transmission facility. The exact format for that transmission varies by the application. For T-1, for example, designated framing bits of the Extended SuperFrame are reserved for the CRC result. In other systems, such as local area networks, the CRC result may be appended to the end of the data block being sent as a trailer. There are several CRC standards, such as CRC-12, CRC-16, and CRC-CCITT. CRC 12 specifies a polynomial of degree 12 (i.e. x 12 + …..), and CRC-16 and CRC-CCITT specify polynomials of degree 16. With the proper selection of polynomials, undetected errors may be as low as 1 in 109.
  • Page Student Notes Global Wireless Education Consortium  Automatic Repeat Request (ARQ) Automatic Repeat Request is an error correction method typically used in data links. Typically, it works together with an error detection method. After a block of data has been sent, the receiving equipment must signal the transmitting equipment that it has either accepted the data or rejected it. If it has rejected the data, it requests a retransmission. Acceptance is generally indicated by the acronym ACK, and rejection by the acronym NAK. The examination of the data blocks may occur in one of two ways: stop and wait or continuous . With stop and wait, the transmitter must wait until each block of data is examined and either accepted or rejected before sending another block. This will decrease the transmission rate of the system. With continuous examination, transmission of data blocks continues until a NAK is sent back to the transmitter. At that point, one of two corrective measures may be taken. With selective ARQ, the transmitter will resend only the faulty block of data (the one which generated the NAK on the receiving end). In continuous transmission, ACK and NAK signals are sent on different channels than the actual data, and the system will allow a predetermined number of blocks of data to be transmitted without receiving an acknowledgment or rejection. Transmission of several valid data blocks may therefore occur prior to the retransmission of the faulty block. For selective ARQ to work, the receiver must therefore manage the data with buffers (storage) so that a retransmitted block is correctly reinserted into the data stream where the faulty block would have been. Go back N ARQ simplifies the management of the retransmitted data by having the transmitting end resend all blocks of data from the faulty block to the present block. Because several blocks of data may need to be resent, the transmitter must buffer (store) as many blocks as the receiver will allow to go unacknowledged. The need to retransmit multiple blocks of valid data when a faulty block is detected makes Go back N ARQ inefficient in noisy environments.
  • Page Student Notes Global Wireless Education Consortium  Forward Error Correction (FEC) The theory behind Forward Error Correction is to mathematically create a pattern of parity bits from a block of data and send those parity bits along with the data to the receiving end. On the receiving end, another mathematical operation is performed on the parity bits, which should result in a specific bit pattern. If the bit pattern which resulted from the receiving end’s mathematical operation is not as expected, individual bits within the erroneous bit pattern point to where the data bits are in error, and a correction can be applied. Notice that although this process works similarly to the Cyclic Redundancy Check, it goes one step further. Errors are not only detected, they are also corrected. There are several types of Forward Error Correction, and in general, the more complex the mathematical operation, the better the error correction. The mathematics of these methods is beyond the scope of this text. For example, the Hamming Code, one of the more common methods of Forward Error Correction, uses matrix algebra to generate the bit pattern which verifies the data. If an error has occurred, the result of the receiving end’s matrix multiplication will point out the particular data bit that was in error. The limitation of the Hamming Code is that only one error per word can be corrected.
  • Page Student Notes Global Wireless Education Consortium  The Bit Error Rate is probably the best-known and most widely used indicator of the performance of a digital circuit. The theory of this measurement is that when a known pattern of binary digits is sent on a line, it should be received, detected, and decoded without error on the receiving side of the transmission. To make this measurement valid, it is necessary to perform this test over a predetermined time period and use the statistical average of the errors as the measurement. This methodology compensates for temporary conditions that may cause a short-term high level of errors. There are three common bit pattern lengths: 63, 511, and 2047. The patterns can be all 1s (all mark), alternate mark-space patterns, or complex patterns that may not repeat for millions of bits. Bit Error Rate Testers used on high-speed digital lines can use patterns as long as there are 223 1-bits. In general, the more digits in the pattern, the more stringent the test for error. Manufacturers of digital equipment are free to set their own proprietary standards to measure Bit Error Rate. For example, a digital set top converter manufacturer may use a proprietary string of digits which are detected during the diagnostic phase of converter setup. In this scenario, the indication to the technician performing the setup may be minimal, perhaps only a pass-fail LED readout. More sophisticated test equipment will actually track and record Bit Error Rates over time, and provide that information on printed or graphically displayed outputs. Block Error Rate is similar to Bit Error Rate, in that a predetermined bit pattern is sent over the line. BLER provides more information, as it tracks data on a block basis, rather than just individual data bits. This type of tracking is helpful in detecting recurring patterns of errors. To be most effective, the test block size should be the same as the typical block of actual data. A measurement called effective information throughput is closely related to the BLER. If the BLER is 1%, then one block in every 100 will need to be retransmitted, resulting in an effective information throughput of 99%.
  • Page Student Notes Global Wireless Education Consortium  Errored Seconds and Severely Errored Seconds Any second containing one or more errors is called an error second. Any period of 1 second with a BER exceeding 10 3 is called a severely errored second.
  • Page Student Notes Global Wireless Education Consortium  BERT (Bit Error Rate Tester) is a beginning step in the troubleshooting process. It may be likened to the “idiot lights” that the automotive industry places on dashboards, as a BERT will tell you that you have a problem, but will not provide much more than an indication of how severe the problem is. At a critical BER, the subscriber will encounter severe impairments such as freeze frame on the received video. Bit errors may be due to line or data errors, as we have discussed earlier. To find out more details when the BERT reveals errors above specifications, other tests must be performed, such as the jitter measurements, eye and constellation diagrams, and power level measurements. BERT by itself can be useful as a field measurement, but is best used with another unit as well. A simple substitution of a second unit followed by another BERT may narrow the problem to the initial piece of equipment. This is probably the most practical course of troubleshooting for a field technician, since most of the other tests require specialized test equipment and the associated training. As mentioned earlier, BERT is the most commonly used measurement of digital errors. To perform a BERT, the system must be equipped with a standard signal source and a detector. In the slide above, a pseudo random bit sequence is generated by a feedback shift register driven by a very stable clock source. The data from the shift register is passed through an interface circuit to generate the correct code format and output level. At the receive end, the same type of interface circuit strips off the code and recovers a clock. This clock drives a reference pseudo random bit sequence generator which has an output that is compared to the received data. When the system is properly synchronized, all errors in the received signal are recorded in the error counter. This combination of signal source may be self contained in the system or be part of portable test equipment. In portable test equipment, BERT testing is often part of a larger set of tests that can be done on a system.
  • Page Student Notes Global Wireless Education Consortium 
  • Page Student Notes Global Wireless Education Consortium 

Basic Data Networks Basic Data Networks Presentation Transcript

  • Switch - Basic Data Networks (S-BDN)
  • S-BDN
    • © Copyright 2001 Global Wireless Education Consortium
    • All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws.
    • Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners .
  • S-BDN
    • Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039.
    • GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
  • Table of Contents
    • Overview 5
    • Learning Objectives 6
    • Data Networks 7
    • Backbone Networks 17
    • Error Detection And Correction 50
    • Summary 61
    • Contributors 62
  • Overview
    • In this module, you will learn about local area networks, access networks, and backbone networks.
    • Layered protocols, packet structure, addressing, and error detection and correction within the Public Switched Telephone Network are also discussed.
  • Learning Objectives
    • After completing this module, you will be able to:
    • Compare and contrast the alternative topologies of networks made up of hubs, routers, switches and gateways that exist in the basic Public Switched Telephone Network (PSTN).
    • Explain how errors are detected and how to correct them.
  • Data Networks
  • Bandwidth Narrowband 0 - 64 Kbps Wideband 64 Kbps - 45 Mbps Broadband 45 Mbps and Beyond X.25 Frame Relay ATM SMDS Token Ring x x T-1, PRI T- 3 SONET LAN Backbone IP Ethernet Network Hierarchy and Protocols Source: KnowledgeLink,Inc Network Access
  • Topology Definition: The physical and logical way the network elements are connected Network Topology Source: KnowledgeLink,Inc
  • Local Area Network (LAN) Topologies Source: KnowledgeLink,Inc Legend Device on LAN = Bus Star Ring
  • Example: Ethernet CSMA/CD: Carrier Sense Multiple Access/ Collision Detection Sends Data Units out unless you detect another data unit. a lready in progress on the bus. Bus Topology Source: KnowledgeLink,Inc Legend Device on LAN = Bus
  • Ring Example: Token Ring Send data units when you possess the token. . Token: Special Type of Data Unit Ring Topology Source: KnowledgeLink,Inc
  • LAN Standards Source: KnowledgeLink,Inc IEEE 802.1 INTERNETWORKING lOGICAL LINK CONTRO (LLC) OSI LAYERS 3-7 OSI LAYER 2 OSI LAYER 1 PHYSICAL MEDIUM ACCESS CONTROL (MAC) IEEE 802.2 TYPE 1 - UNACKNOWLEDGED CONNECTIONLESS SERVICE TYPE 2 - CONNECTION MODE SERVICE TYPE 3 - ACKNOWLEDGED CONNECTIONLESS SERVICE 802.3 CSMA/CD (ETHERNET) 802.4 TOKEN BUS 802.5 TOKEN RING 802.6 DQDB (MAN) BASEBAND COAXIAL AND UNSHIELDED TWISTED PAIR BROADBAND COAXIAL BROADBAND COAXIAL OPTICAL FIBER SHIELDED OR UNSHIELDED TWISTED PAIR OPTICAL FIBER OR COAXIAL CABLE
  • Entry to Wide Area Network (WAN) Enter via port on LAN router Access Networks Source: KnowledgeLink,Inc Customer Premises [ Bridge- ] Router Access Network
  • Protocol Layers Involved in Data Transfer 3 2 1 Switch A Switch C X.25 Network Switch B Data Flow in an X.25 Network Source: KnowledgeLink,Inc
  • Frame Relay Network Protocol Layers Involved in Data Transfer 3 2 1 Switch A Switch C Switch B Data Flow in a Frame Relay Network Source: KnowledgeLink,Inc
  • Backbone Networks
  • ATM Cell Structure Header Payload 48 Octets 5 Octets Backbone Example: Asynchronous Transfer Mode (ATM) Source: KnowledgeLink,Inc
  • ATM users receive as much bandwidth as they require Python with expandable skin (bandwidth) Asynchronous Transfer Mode (ATM) Source: KnowledgeLink,Inc
  • User 1 User 2 User 3 125  sec 250  sec 375  sec 500  sec Asynchronous means data units (cells) are placed on the network as the application requires User 1 User 1 User 1 User 2 User 2 User 2 User 3 Bandwidth on Demand Source: KnowledgeLink,Inc
  • Open Systems Interconnection (OSI) Model Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Network Data Link Data Link Network Node 7 6 5 4 3 2 1 7 6 5 4 3 2 1
  • Wireless OSI Model MESSAGES USE BOTTOM 3 LAYERS Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Network Data Link Data Link Network Node 7 6 5 4 3 2 1 7 6 5 4 3 2 1
  • The OSI Seven-Layer Model (E-Mail)? Source: KnowledgeLink,Inc
  • A Set of Rules Source: KnowledgeLink,Inc The OSI Model Application Presentation Session Transport Network Data Link Physical Header User Info 1 7 6 5 4 3 2
  • Layer 1 Details Network Data Link Physical Application Presentation Session Transport Transmits bits received from the Data Link Layer across the transmission medium
  • Layer 2 Details Data Link Physical Sequences messages and checks for errors between adjacent link stations Transmits bits received from the Data Link Layer across the transmission medium Network Application Presentation Session Transport
  • Layer 3 Details Network Data Link Physical Fragments or “packetizes” messages and routes them to the proper destination Sequences messages and checks for errors between adjacent link stations Transmits bits received from the Data Link Layer across the transmission medium Application Presentation Session Transport
  • Layer 4 Details Network Data Link Physical Transport Fragments or “packetizes” messages and routes them to the proper destination Sequences messages and checks for errors between adjacent link stations Transmits bits received from the Data Link Layer across the transmission medium Provides multiplexing, network connection management, quality of service, etc. Application Presentation Session
  • Packet Format Header Information Source: KnowledgeLink,Inc
    • Header
    • User data
    • Perhaps a trailer
    • Store and forward
    Packet Switched Routing Source: KnowledgeLink,Inc
  • Protocol Data Units Source: KnowledgeLink,Inc Header Information Packet Format Layer 2 Info Layer 3-5 Info Layer 2 Info Flag Flag Message Format Types of Protocol Data Unit Being Sent Through a Packet Switched Network
  • Protocol Data Unit Routing Fields Source: KnowledgeLink,Inc Header Trailer User Data
  • Destination Node Won’t Change Address of Next Node on Route The router stores packet, reads the destination information and forwards it to the next node Routers Routing Packets Final Path ROUTER NODE B DATA C C DATA B C NODE A NODE C
  • Packet Switched Network Routing
  • Data Switch Routing Example
    • Ethernet Switch Layer2 Operations
      • Receives the Ethernet frame
      • Looks up the 6-byte destination address in a forwarding table
      • Sends the packet out only to the port associated with the destination address
  • Ethernet Switch
  • Data Network Devices & the OSI Reference Model
    • Gateways
    • Routers
    • Switches
    • Hubs
    • Bridges
    • Repeaters
    Source: KnowledgeLink,Inc
  • Repeater
    • Layer 1 - Physical
    • I physically repeat and regenerate bits for my own LAN
    Source: KnowledgeLink,Inc
  • Bridge LAN 1 LAN 2
    • Layer 2 – Data Link Layer
    • I link data frames by bridging between LANs
    Source: KnowledgeLink,Inc
  • Hub USER USER USER USER
    • Layer 2 – Data Link Layer
    • I route packets between islands with common protocols
    Source: KnowledgeLink,Inc
  • Data Switch USER USER
    • Layer 2 – Data Link Layer
    • I route packets between ports
    • Packets are not sent to every port
    • (only to the destination port)
    Source: KnowledgeLink,Inc USER USER
  • Router
    • Layer 3 – Network Layer
    • I route packets between islands with common protocols
    Source: KnowledgeLink,Inc
  • Gateway
    • Layer 3+
    • I convert protocols between islands
    • Synchronous to asynchronous
    SYNC ASYNC Source: KnowledgeLink,Inc
  • Data Network Devices in Data Network Applications
  • Typical Packet Data Services Router Router Router Carrier Network Router Chicago Router LAN Hub Client Detroit Router LAN Hub Server Customer Network Customer Network Access Line Access Line
    • Routers replace Layer 2 frame headers & trailers so packet can travel end-to-end over many links. They provide gateways into and out of the PSTN.
    IP packets / TR frames IP packets / PPP frames Router or Gateway modifies frame IP packets / TR frames Packets, Routers, and the PSTN
  • Combined Network Source: KnowledgeLink,Inc CMTS MSC TELCO NIU NIU Host Digital Terminal SS7 SS7 Internet Intranet or Public Data Network Average Residence Power Residence Corporate A Corporate B Trunks to Telco Switch Gateway
  • IP Telephony Using a Computer as the Terminal Source: KnowledgeLink,Inc PSTN Internet CODEC Gateway Router Cable Modem Microphone
  • IP Telephony Generates Mobile-like Traffic Source: KnowledgeLink,Inc PSTN Internet CODEC Gateway Router Cable Modem Telephone Adapter
  • Detecting and Fixing Errors
  • Detecting and Fixing Errors
    • Committed Information Rate (CIR)
    • Data collisions
    • Protocol analyzers
    Source: KnowledgeLink,Inc
  • Block Parity Check Source: KnowledgeLink,Inc
  • New Terms
    • Longitudinal Redundancy Check (LRC)
    • Block Parity Check
    • Checksum
    • XMODEM checksum
    Source: KnowledgeLink,Inc
  • Cyclic Redundancy Check (CRC) Data Source: KnowledgeLink,Inc CRC Result
  • Automatic Repeat Request (ARQ)
    • Automatic Repeat Request (ARQ)
    • Acceptance (ACK)
    • Reject (NAK)
    Source: KnowledgeLink,Inc
  • Forward Error Correction
    • Mathematical creation of parity bits on the transmitting end and on the receiving end.
    • If the parity bits on each end match, there is no error.
    • If the parity bits do not match, the error has been detected.
    • Forward Error Correction can correct errors, CRC cannot.
    Source: KnowledgeLink,Inc
  • Types of Digital Error Measurement
    • Bit Error Rate (BER)
    • Block Error Rate (BLER)
    Source: KnowledgeLink,Inc
  • Digital Error Measurement
    • Errored Seconds
    • Severely Errored Seconds
    Source: KnowledgeLink,Inc
  • Bit Error Rate Tester
  • Industry Contributors
    • Lucent Technologies, Inc. ( http://www.lucent.com )
    • Telcordia Technologies, Inc ( http://www.telcordia.com )
    • KnowledgeLink, Inc ( http://www.knowledgelinkinc.com )
    The following companies provided materials and resource support for this module:
  • Individual Contributors
    • The following individuals and their industry or educational institutions provided materials, resources, and development input for this module:
    • Dr. Bruce Black
      • Rose-Hulman Institute of Technology
      • http://www.rose- hulman . edu /
    • Dr. Philip DiPiazza
      • Florida Institute of Technology
      • http://www.fit. edu /
    • Mr. Jay Junkus
      • KnowledgeLink, Inc
      • http://www.knowledgelinkinc.com
  • Individual Contributors, cont.
    • Mr. Ron Koziel
      • KnowledgeLink, Inc
      • http://www. knowledgelinkinc .com
    • Mr. Ken Robinson
      • Ericsson
      • http://www. ericsson .com