Other types of LAN (token ring and token bus)

There have been a number of different LAN technologies developed over the years, all of which are tending to be replaced by ethernet, fast ethernet or Gigabit ethernet technology.

Three other technologies (token ring LAN, FDDI [fibre distributed data interface] andtoken bus) were, like ethernet, made into official IEEE 802-series standards and are still to be found deployed in corporate networks. For this reason, they deserve mention here. Given the large number of IBM computers deployed with token ring LAN networks, token ring LANs may live on for a while yet. Indeed, there is still a level of ongoing standardisation effort looking to upgrade token ring to encompass 100 Mbit/s and Gigabit versions.

Wireless LANs

In recent years, wireless LANs (using radio transmission) have become popular. ETSI (Euro-pean Telecommunications Standards Institute) developed a system called HIPERLAN (HIgh PErformance Radio LAN), but the most popular version looks likely to be that based on the IEEE 802.11 standard. IEEE 802.11wireless LANs (WLANs)are, in effect, wireless versions of ethernet LANs — as explained in Appendix 6.

Token ring LAN (IEEE 802.5)

Thetoken ring LAN standard (defined by IEEE 802.5-series standards) employs atoken(passed between each of the terminals connected to the ring topology (Figure 4.1) in turn) to assign the

‘right to transmit data’ on the LAN. The manner in which the token is passed is as follows: the token itself is used to carry the packet of data. The transmitting terminal sets the token’sflag, putting the destination address in theheaderto indicate that the token is full. The token is then passed around the ring from one terminal to the next. Each terminal checks whether the data is intended for it, and passes it on. Sooner or later the token reaches the destination terminal where the data is read. Receipt of the data is confirmed to the transmitter by changing a bit value in the token’s flag. When the token gets back to the transmitting terminal, the terminal is obliged to empty the token and pass it to the next terminal in the ring.

One of the beneficial features of IEEE 802.5 MAC protocol is its ability to establish priorities among the ring terminals. This it does through a set of priority indicators in the token. As the token is passed around the ring, any terminal may request its use on the next pass by putting arequest of a given priority in the reservation field. Provided no other station makes a higher priority request, then access to the token is given next time around. The reservation field therefore gives a means of determining demand on the LAN at any moment by counting the number of requests in the flag. In addition, the system of prioritisation ensures that terminals with the highest pre-assigned authority have the first turn. High speed operation of certain pre-determined, time-critical devices is likely to be crucial to the operation of the network as a whole, but they are unlikely to need the token on every pass, so that lower priority terminals get a chance to use the ring when the higher priority stations are not active.

Token ring was developed by IBM, and is most common in office installations where large IBM mainframe and mid-range computers (particularly AS400) are in use. The original form required specialised cabling (IBM type 1) and operated at 4 Mbit/s. The idea was that a single cable loop could be laid through all the offices on a floor or in a building and devices added on demand. To avoid the disturbances and complications which might arise when connecting new devices to the ring (any break in the ring renders the LAN inoperative), IBM developed a sophisticated cabling system, including the various IBM special cables. The cable loop was pre-fitted with a number of sockets at all possible user device locations. The sockets ensured that when no device was connected, the ring was through-connected. But on plugging in a new device, the ring is diverted through that device (Figure 4.13). Thebaluns(special socket) for early token ring networks thus catered not only for correct impedance matching, but also for the ring continuity.

Token ring network interface cards (NICs) in the individual end user computer devices connected to token ring LANs also have to be designed in such a way as to ensure ring

Figure 4.13 Socket design in Token Ring LANs to ensure ring continuity.

Other types of LAN (token ring and token bus) 151 continuity in the case that the device is switched off. Thus the card reverts to a ‘switched-through’ state when no power is applied, so that even though the end device itself plays no active part in token-passing while switched off, the tokens nonetheless still have a complete ring available.

The further development of the token ring technology (mainly by IBM) has brought about the capability of use of twisted pair cabling, and the emergence of a 16 Mbit/s as well as the original 4 Mbit/s version. In the 16 Mbit/s version (IEEE 802.5f and 802.5n), higher quality cabling (typically category 5 cable) is required. There is also a 100 Mbit/s version (IEEE 802.5t).

Token ring LAN hubs have also developed alongside ethernet hubs, and allow for similar collapsed backbone topologies in conjunction with structured cabling systems. The ring topol-ogy is collapsed into the hub itself, while two sets of wires to each individual user station allows for the extension of the ring to each user device. The switch-through function previ-ously performed by the socket is undertaken at the hub, thus reducing the complexity and cost of individual sockets, so that standard category 5 structured cabling and the associated RJ-45 sockets may be used. Thetoken ring LAN may differ from theethernet LAN only in the port cards used within the hub and the LAN cards used in the individual PCs. Otherwise cabling, wiring cabinet and LAN hub unit may be identical. Indeed, in some companies, ethernet and token ring LANs exist alongside one another, without the user being aware to which type of LAN he or she is connected.

Token ring LANs, like ethernet LANs, are common in office environments, linking personal computers for the purpose of data file transfer, electronic messaging, mainframe computer interaction or file sharing. Some LAN administrators used to be enthusiastic about whether ethernet or token ring offered the best solution, but in reality, for most office users, there was little to choose between them. Token ring LANs perform better than ethernets at near full capacity or during overload but can be more difficult and costly to install — especially when only a small number of users are involved. In most cases, the choice between ethernet and token ring comes down to the recommendation of a user’s computer supplier, since hardware and software of a particular computer type may have been developed with one or other type of LAN in mind. Thus token ring remains the recommendation of the IBM company, while in all other environments, ethernet has gained the upper hand.

Should the ring of a token ring LAN be broken (i.e., lose its continuity), then the LAN ceases to operate, since the token can no longer be returned to its sender and passed on, as required by the protocol. This made it unpopular with some LAN administrators. To provide for a ringcontinuity check, the layer 2 addressing scheme (i.e., the MAC addressing scheme) of token ring allocates a specialloopback address(also called theno station address). When the no station address (a string of all 0s) is set as the MAC address, then each terminal is expected to ignore the packet and token, merely forwarding it around the ring. Provided the ring is complete, the packet and token return around the ring (i.e., arelooped back) to their origin. In contrast to theno station address (binary string of all 0’s), thebroadcast address is set as a binary string of all 1s). When the broadcast address is set in the token, each of the stations in the terminal will receive the same broadcast message. Such messages can be useful for simultaneous reconfiguration of the LAN.

FDDI (fibre distributed data interface)

Thefibre distributed data interface (FDDI)is a 100 Mbit/s token ring network defined by ANSI (American National Standards Institute) X3. FDDI is ametropolitan area network (MAN) tech-nology which 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 was most used as an optimum ‘backbone’ transmission system for

campus-wide wiring schemes, especially where network management and fault recovery were required. In particular, FDDI became popular in association with the very first optical fibre building cabling schemes, since 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. Due to its expensive nature and the rapid development of alternative technologies (includingATM — asynchronous transfer mode; and later, 100 Mbit/s fast ethernet), FDDI fell into decline, no longer being recommended or further developed by most LAN and computer manufacturers. Nonetheless, some of the physical layer standards developed for FDDI live on as fast ethernet.

A second generation version of FDDI, FDDI-2 was developed to include a capability similar tocircuit-switching to allow voice and video to be carried reliably in addition to packet data.

But these capabilities were never widely used. Nor was the copper cable version: CDDI (copper distributed data interface).

The FDDI standard is basically aphysical layerandMAC (medium access control)standard, defined in four parts, and to be used in conjunction with the standard logical link control (LLC) defined by IEEE 802.2:

• media access control (MAC)defines the rules for token passing and packet framing;

• physical layer protocol (PHY)defines the data encoding and decoding;

• physical media dependent (PMD) defines drivers for the fibre optic components; and

• 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 4.14a shows. The fault need only be recognised by theCMTs (connection 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 4.14b).

When configured as a ring, each of the stations is said to be indual-attached connection.

Alternatively, a fibre star connection can be formed using single-attached stations with a

Figure 4.14 Fibre distributed data interface (FDDI).

Other types of LAN (token ring and token bus) 153 multiport concentrator at the hub (itself a double attached station — Figure 4.14b). Single-attached stations (SASs)do not share the same capability for fault recovery as dual-attached stations (DASs)on a dual ring.

FDDI-2, the second generation of FDDI has a maximum ring length of 100 km and a capability to support around 500 stations including telephone and packet data terminals. 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 interworking with theintegrated voice data (IVD)LAN standard defined in IEEE 802.9.

Fibre channel

Another alternative medium for high speed data transfer based on a switched point-to-point LAN topology is the fibre channel (FC) — as standardised by ANSI (American National Stan-dards Institute) and the FCA (Fibre Channel Association).

Switched multimegabit digital service (SMDS)/DQDB (dual queue dual bus) MANs

SMDS (switched multimegabit digital service)networks aremetropolitan area networks (MANs) which conform to IEEE 802.6 and use a protocol calleddistributed 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 broadbandmetropolitan area interconnection of networks (like a LAN, but on a larger geographical scale), suitable for simultaneous transmission of not only data, but also voice and video signals. As a public data communications service, the switched multimegabit digital service became available in the United States in 1991. Like FDDI, the technology was too expensive, and it has fallen into disuse, but it also established some important principles for later communications and protocol design, which are worthy of discussion.

The DQDB protocol uses twoslotted buses of bit rates up to 155 Mbit/s for transporting segments of information (the DQDB name for the basic unit of user data carried by the network) between communicating broadband devices. Segments are 48 byte frames of user data information.

Figure 4.15 illustrates the structure of a switched multimegabit digital service (SMDS) network using the DQDB protocol. Two unidirectional high speed buses run out frommaster 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 for sending and receiving data.

The role of theframe generators is to structure the bit stream carried along the buses into fixed length, 57 byte,slots. Slots are really dataframes, but of a fixed length. They are filled by nodes wishing to send user information and are then carried downstream along the bus.

The relevant receiving node reads information 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 themedium 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.

Figure 4.15 Bus structure of DQDB.

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 to request bandwidth. For example, say node 2 of Figure 4.15 wished to transmit to node 5, then it would send a request on bus B. This advises theupstream nodes of bus A (i.e., node 1 in our case) that node 2 requires capacity on bus A. Node 2 must then wait until all other previouslypending 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 downstreamnode (i.e., node 3, 4 or 5).

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 decentralised, 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 itsrequest counter.

Each time a request passes on bus B, the request counter is incremented, 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 isavailable 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 awaiting 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 ‘0’, 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 supple-mented with a 4 bytesegment header, a 1 byteaccess control field and a 4 byteslot header as shown in Figure 4.16, so that the total length of a slot is 57 bytes. The frame structure of DQDB, and the functioning of the protocol is designed to make it compatible with asyn-chronous transfer mode (ATM),⁸an important WAN technology. The 53-bytecell comprised within each slot (Figure 4.16) is equivalent to the ATM cell.

The DQDBslot header carries a 2 bytedelimiter field and 2 bytes of control information used by the physical layer for the layer management protocol. Theaccess control field 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 identifier, like the logical channel number of X.25. This identifies the cells corresponding to a particular connection to the appropriate receiving node.

Data blocks to be carried by DQDB are formatted in the standard LLC (layer 2 protocol) manner of frameheader, followed byuser data block and the frametrailer (Figure 4.17). The frame header contains the address of the originating and destination nodes. The user data block

8See Appendix 10.

Other types of LAN (token ring and token bus) 155

Figure 4.16 Slot and segment structure of DQDB.

Figure 4.17 Segmentation and reassembly (SAR) of a data block for transmission using DQDB or ATM.

is the data frame to be carried which may be up to 9188 bytes in length (192 segments), and the trailer includes theframe check sequence. But before being transmitted across the physical medium, the SMDS protocols break down these data blocks (LLC frames) into individual seg-ments(i.e., 48 byte chunks), each of which is formatted asslot for transmission. This process is called segmentation. If necessary, the last segment is filled with padding (Figure 4.17).

At the receiving end, the slots are reassembled into the original LLC frame. The protocol sublayer which performs these functions is called thesegmentation and reassembly sublayer.

It is common to both SMDS and ATM. The ATM adaptation layer (AAL— the data frame carrying variants are AAL 3/4 and AAL5) operates in a similar manner.⁹

Segmentation and reassembly (SAR) of data blocks into fixed length cells or slots for transmission across a physical medium is one of the methods frequently chosen by protocol designers attempting to build networks suitable for both real-time signals like voice and video as well as carriage of data files. The use of slots allows the transmission of a large data frame to be interrupted temporarily when a high priority signal (such as real-time voice or video) needs to be carried. By so doing, we minimise the possibility of unacceptably long or variable

9See Appendix 10.

Figure 4.18 DQDB or SMDS configured in alooped bus topology.

propagation delays. Such delays lead to perceptible quality impairment of voice and video connections.

As well as in a straight forward bus configuration, SMDS (switched multimegabit digital service) networks which use 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 of Figure 4.15 are contained in the same node. This node also contains two end of bus devices as shown in Figure 4.18. In real terms, the network is still two independent buses, but there may be a practical advantage in not needing two separateframe generator nodes.

As well as in a straight forward bus configuration, SMDS (switched multimegabit digital service) networks which use 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 of Figure 4.15 are contained in the same node. This node also contains two end of bus devices as shown in Figure 4.18. In real terms, the network is still two independent buses, but there may be a practical advantage in not needing two separateframe generator nodes.