21 Campus and Metropolitan Area Networks (MANs) Metropolitan area networks (MANs) are network technologies similar in nature to local area networks (LANs), but with the capability to extend the reach of the LAN across whole cities or metropolitan areas, rather than being limited to, say, 100-200 metres of cabling. MANS have evolved because of the desire of companies to extend LANs throughout company office buildings spread across a campus or a number of different locations in a particular city. They provide for high speed data transport (at over lOOMbit/s) and are ideal for the interconnection of LANs. There was some effort to extend MAN capabilities to include the carriage of telephone and video signals as an ‘integrated’ network, but this work has largely been overtaken by ATM (asynchronous transfer mode), so that the MAN technologies themselves are already obsolescent. We review here, but only briefly, the most important MAN techniques, FDDI (fibre distributed data interface), and SMDS (switched multimegabit digital service) which is based on the DQDB (distributed queue dual bus) technique. 21.1 FIBRE DISTRIBUTED DATA INTERFACE The jibre distributed data interface (FDDI) is a 100 Mbit/s token ring network. It is defined in IEEE 802.8 and IS0 8802.8. FDDI can be used to interconnect LANs over an area spanning up to 100 km, allowing high speed data transfer. Originally conceived as a high speed link for the needs of broadband terminal devices, FDDI is now per- ceived as the optimum backbone transmission system for campus-wide wiring schemes, especially where network management and fault recovery are required. In particular, FDDI became popular in association with the very first optical fibre building cabling schemes, because it provided one of the first means to connect LANs on different floors of a building or in different buildings on a campus via optical fibre. Unfortunately, due to its expensive nature and the rapid development of ATM (asynchronous transfer mode, see Chapter 26) as well as alternative building cabling schemes, FDDI has fallen into decline, no longer being recommended or further developed by most LAN and computer manufacturers. 391 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 392 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS) A second generation version of FDDI, FDDI-2, was developed to include a capability similar to circuit-switching to allow voice and video to be carried reliably in addition to packet data, but these capabilities were never widely used. The FDDI standard is defined in four parts 0 media access control (MAC), like IEEE 802.3 and 802.5 (see Chapter 19) defines the rules for token passing and packet framing e physical layer protocol (PHY) defines the data encoding and decoding e physical media dependent (PMD) defines drivers for the fibre optic components e station management (SMT) defines a multi-layered network management scheme which controls MAC, PHY and PMD The ring of an FDDI is composed of dual optical fibres interconnecting all stations. The dual ring allows for fault recovery even if a link is broken by reversion to a single ring, as Figure 21.l(a) shows. The fault need only be recognized by the CMTs (connec- tion management mechanisms) of the station immediately on either side of the break. To all other stations the ring will appear still to be in its normal contra-rotating state (Figure 21.l(b)). When configured as a ring, each of the stations is said to be in dual-attached connection. Alternatively, a fibre star connection can be formed using single-attached stations with a multiport concentrator at the hub (Figure 21.2). Single-attachedstations (SASs) do not share the same capability for fault recovery as double-attached stations (DASs) on a dual ring. DAS DAS DAS 'Looped' [a) Failed link-ring conf igured as single logical loop (b) Normal dual contra - rotating fibre rings Figure 21.1 The fibre distributed data interface (FDDI) fault recovery mechanism for double attached stations. DAS, double attached station FIBRE DISTRIBUTED DATA INTERFACE 393 Net work connect ion S J / AS Bridge and / multi ort concenl'rator SAS Figure 21.2 Star configuration of FDDI. SAS, single attached station Like token ring LANs (IEEE 802.5) and ethernet LANs (IEEE 802.3), FDDI is essentially only a physical layer (OS1 layer 1) and data-link layer (OS1 layer 2) standard. At layers 3 and above, protocols such as X.25, TCP/IP may be used. FDDI-2, the second generation of FDDI (Figure 21.3) has a maximum ring length of 100 km and a capability to support around 500 stations including telephone and packet data terminals. Because of this, it was intended to support entire company telecom- munications requirements. ATM, however, has proved a more popular prospect for Building 1 Public network X25 gateway PA BX Bridge A Building 2 Figure 21.3 The fibre distributed data interface-2 (FDDI-2). AU, access unit 394 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS) providing these capabilities, and is now widely available from network and computer equipment manufacturers. The FDDI-2 ring is controlled by one of the stations, called the cycle master. The cycle master maintains a rigid structure of cycles (which are like packets or data slots) on the ring. Within each cycle a certain bandwidth is reserved for circuit-switched traffic (e.g. voice and data). This guarantees bandwidth for established connections and ensures adequate delay performance. Remaining bandwidth within the cycle is available for packet data use. The voice and video carriage capability of FDDI-2 is possible because of its inter- working with the integrated voice data (ZVD) LAN standard defined in IEEE 802.9. 21.2 SWITCHED MULTIMEGABIT DIGITAL SERVICE (SMDS) SMDS (switched multimegabit digital service) networks conform to IEEE 802.6 and use a protocol called distributed queue dual bus (DQDB). DQDB was co-developed by Telecom Australia, the University of Western Australia and their joint company, QPSX Communications Limited. It was designed to provide a basis for initial broadband metropolitan area interconnection of networks, but also give a possible migration path to B-ISDN (Chapter 25), for which it is now an optional access protocol. As a public data communication service, the switched multimegabit digital service (SMDS) became available in the United States in 1991. The DQDB protocol uses two slotted buses of bitrates up to 155 Mbit/s to transport segments of information between communicating broadband devices. Segments are 48 byte frames of user data information. Figure 21.4 illustrates the structure of a network using the DQDB protocol. Two unidirectional high speed buses run out from master and slave frame generators at opposite ends of the ribbon topology. Each of the devices (nodes) connected to the network are connected to both buses to send and receive data. The role of the frame generators is to structure the bit stream carried along the buses into 53 byte slots. These slots are filled by nodes wishing to send user information and unidirectional bus A W master frame it +t generator node 5 node4 node 3 node2 node 1 slave frame generator 4 unidirectional bus B Figure 21.4 Bus structure of DQDB SWITCHED MULTIMEGABIT DIGITAL SERVICE 395 are then carried downstream along the bus. The relevant receiving node reads informa- tion out of the slot being sent to it, but does not delete the slot contents. The slot thus remains on the bus, travelling further downstream until it falls off the end. When a node wishes to send information it may do so in the first available empty slot, but in doing so must follow the procedure set out in the medium access control (MAC) protocol. The MAC protocol is intended to ensure a fair use of the available bandwidth of the buses between all the devices wishing to send information. Before sending information, a sending node must know the relative position of the receiving node on the bus. It then sends a request in the opposite direction of the receiving node on the relevant bus. For example, say node 2 of Figure 21.4 wished to transmit to node 5, then it would send a request on bus B. This advises the upstream nodes of bus A (i.e. node l in our case) that it requires capacity on bus A. Node 2 must then wait until all other previously pending requests from other downstream nodes on bus A have been cleared. Once these are cleared, it may send in any free slot, and may continue to fill slots until a further slot request appears from a downstream node. It is a simple and yet very effective medium access control. Requests for use of bus A are sent on bus B. Meanwhile the use of bus B is governed by the requests on bus A. The control of the use of the network is decentralized, so that each node may independently determine when it may transmit information, but must be capable of keeping track of the pending requests. When a node is not communicating on one of the buses (say bus A), it monitors the requests for use of the bus, keeping a running total of the outstanding requests using its request counter. Each time a request passes on bus B, the request counter is incre- mented, and when a free slot goes by on bus A the counter is decremented. In this way it can keep track of whether a free slot on bus A is available to it or not. The request counter is never decremented to a value less than zero. Each time a node has a segment it wishes to send on bus A, it generates a waiting counter. The initial value copied into the waiting counter is that currently held in the request counter. The waiting counter is decremented each time a free slot passes on bus A until the value reaches ‘O’, when the segment may be sent in the next free slot. When transmitted onto one of the buses the 48 byte segment of user information is supplemented with a 4 byte segment header, a 1 byte access controlfield and a 4 byte slot header as shown in Figure 21.5, so that the total length of a slot is 57 bytes. I segment b 4 bvtes slot segment header header segmenf of user data (48 bytes) t 1 byte access control field Figure 21.5 Slot and segment structure of DQDB 3% CAMPUS AND METROPOLITAN AREA NETWORKS (MANS) header I user data block trailer -1 segment 3 1 l I Figure 21.6 Segmenting a data block for transmission using DQDB The slot header carries a 2 byte delimiter field and 2 bytes of control information used by the physical layer for the layer management protocol. The access controlfield may be written to by any of the nodes on the bus. This is the field in which the slot requests are transmitted. The segment header carries a 20-bit virtual channel identij?er, like the logical channel number (OS1 layer 2 address) of HDLC. This identifies the cells to the appropriate receiving node. Data blocks to be carried by DQDB are formatted in the standard manner of frame header, the user data block and the frame trailer. The frame header contains the address of the originating and destination nodes. The user data block is the data frame to be carried which may be up to 9188 bytes in length (192 segments), and the trailer includes the frame check sequence. Data blocks must be broken down into individual segments and then formatted as slots for transmission. If necessary, the last segment is filled with padding (Figure 21.6). FG = frame generator end of bus Figure 21.7 DQDB or SMDS configured in a looped bus topology SWITCHED MULTIMEGABIT DIGITAL SERVICE 397 Networks using the DQDB protocol may also be configured in a looped-bus topology. In this case the bus is looped so that the two frame generators (Figure 21.4) are contained in the same node. This node also contains two ends ofbus devices (Figure 21.7). In real terms, the network is still two independent buses, but there may be a practical advantage in not needing two separate frame generator nodes. When offered as a public network service, SMDS is usually configured as shown in Figure 21.8, the public network node acting as the master frame generator and access point for a wide area broadband network which may use a protocol other than DQDB for wide area transport of information. In this way SMDS may provide an access network protocol for a broadband network based upon ATM (Chapter 26). As you may note from comparing the two technologies, they have a number of features in common (cell size of 53 bytes, virtual channel identlJcation of individual channels, etc.) Although the DQDB protocol has the charm of being a very simple and purportedly ‘fair’ protocol, one of the debates that dogs its wider acceptance is the doubt which exists over its ‘fairness’. The slot request procedure used in the MAC does indeed help to share out the bandwidth resources between all the competing nodes, but it does not work well when many of the nodes wish to send at a bitrate close to that of the line. Let us return to Figure 21.4 and assume that the network has been idle, but that now both nodes 1 and 4 wish to transmit to node 5, both at the maximum bitrate. Node 1 starts sending immediately on bus A in every slot. Node 4, meanwhile, must first lodge a slot request on bus B. The request takes a little time to propagate along bus B until reaching node 1, whence node 1 must leave a free slot on bus A. It then goes on to use all subsequent free slots. As node 4 is only allowed to have one outstanding slot request, it must wait until this request is used up before generating the next one. Meanwhile node 1 is hogging all the slots. The ‘fairness’ problem is particularly acute when a very long bus is used, because an entire slot is only about 900 metres long at a bitrate of 155 Mbit/s (57 X 8 [bits per slot] X 3 X 10’ (speed of propagation in m/s/155 X 106 bits/s)). Thus for a lOkm bus there will always be 11 slots between nodes 1 and 4, always with one reserved for use of node 4 and the other ten in use by node 1. public network customer premises FG I SNI SNI = subscriber network interface FG = frame generator end of bus Figure 21.8 SMDS subscriber network interface 398 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS) 21.3 THE DEMISE OF MANS Because of the emergence of A TM (asynchronous transfer mode) as a universal network technology for the carriage of all types of voice, video and data information in local, metropolitan and wide area networks, the MAN technologies are already obsolescent. This is strong evidence of the rapid pace of development of modern technology, but a chilling reminder of the costs and risks involved in investing in the development of or purchase of new equipment.