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Each frame (not token) contains a destination address (DA) field and a source address (SA) field. Depending on the token ring LAN system, the address fields may be 16 bits or 48 bits in length. In either case, the first bit indicates whether the frame is directed to an individual station (0) or to a group of stations (1).

The INFO field carries zero, one, or more octets of user data intended for the MAC, NMT (network management), or LLC. Although there is no maximum length specified for the INFO field, the time required to transmit a frame may be no greater than the token-holding period that has been established for the station. For LLC frames, the format of the information field is not specified in the referenced standard (Ref. 5). However, in order to promote interworking among stations, all stations should be capable of receiving frames whose information field is up to and including 133 octets in length.

The frame check sequence is a 32-bit sequence based on the standard CRC generator polynomial. The error-detection process utilizing a CRC was described in Section 10.5.3.1 using a 16-bit generator polynomial. The 32-bit polynomial was introduced in Section 11.6.2.1. As the order of the polynomial increases, the error-detection capability increases markedly.

The end delimiter (ED) is one octet in length and is transmitted as the sequence JK1JK1IE. The transmitting station transmits the delimiter as shown. Receiving stations consider the ED valid if the first six symbols JK1JK1 are received correctly. The I is the intermediate frame bit and is used to indicate whether a frame transmitted is a singular frame or whether it is a multiple frame transmission. The I bit is set at 0 for the singular frame case. The E bit is the error-detected bit. The E bit is transmitted as 0 by the station that originates the token, abort sequence, or frame. All stations on the ring check tokens and frames for errors such as FCS errors and nondata symbols. The E bit of tokens and frames that are repeated is set to 1 when a frame with an error is detected; otherwise the E bit is repeated as received.

The last field in the frame is the frame status (FS) field. It consists of one octet of the sequence ACrrACrr. The r bits are reserved for future standardization and are transmitted as 0s, and their value is ignored by the receiver. The A bit is the addressrecognized bit, and the C bit is the frame-copied bit. These two bits are transmitted as 0 by the frame originator. The A bit is changed to 1 if another station recognizes the destination address as its own or relevant group address. If it copies the frame into its buffer, it then sets the C bit to 1. When the frame reaches the originator again, it may differentiate among three conditions:

1. Station nonexistent or nonactive on the ring;

2. Station exists but frame was not copied; or

3. Frame copied.

Fill is used when a token holder is transmitting preceding or following frames, tokens, or abort sequences to avoid what would otherwise be an inactive or indeterminate transmitter state. Fill can be either 1s or 0s or any combination thereof and can be any number of bits in length within the constraints of the token-holding timer.

IEEE Std 802.5 describes a true baseband-transmitting waveform using differential Manchester coding. It is characterized by the transmission of two line signal elements per symbol. An example of this coding is shown in Figure 11.16. The figure shows only the data symbols 1 and 0 where a signal element of one polarity is transmitted for one-half the duration of the symbol (bit) to be transmitted, followed by the contiguous transmission of a signal element of the opposite polarity for the remainder

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Figure 11.16 Differential Manchester coding format for symbols 1, 0, nondata J, and nondata K. (From IEEE Std. 802.5, [Ref. 5], reprinted with permission.)

of the symbol duration.9 The following advantages accrue for using this type of coding:

•The transmitted signal has no dc component and can be inductively or capacitively coupled.

•The forced midsymbol transition provides inherent timing information on the channel.

All stations on the LAN ring are slaved to the active monitor station. They extract timing from the received data by means of a phase-locked loop. Latency is the time, expressed in number of bits transmitted, for a signal element to proceed around the entire ring. In order for the token to circulate continuously around the ring when all stations are in the repeat mode, the ring must have a latency of at least the number of bits in the token sequence—that is, 24. Since the latency of the ring varies from one system to another and no a priori knowledge is available, a delay of at least 24 bits should be provided by the active monitor.

A LAN station provides an output with an error rate of less than or equal to 1 × 10−9 when the signal-to-noise ratio at the output of the equalizer, specified in paragraph 7.5.2 of the reference document (Ref. 5), is 22 dB.

11.6.4 Fiber Distributed Data Interface (FDDI)

11.6.4.1 Overview. A fiber distributed data interface (FDDI) network consists of a set of nodes (e.g., LAN stations) connected by an optical transmission medium (or other

9Polarity refers to whether a signal is a positive-going or negative-going voltage or simply a positive voltage or a negative voltage.

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Figure 11.17 FDDI token ring: example of logical configuration.

medium) into one or more logical rings. A logical ring consists of a set of stations connected as an alternating series of nodes and transmission medium to form a closed loop. This is shown in Figure 11.17. Information is transmitted as a stream of suitably encoded symbols from one active node to the next. Each active node regenerates and repeats each symbol and serves as a means for attaching one or more devices to the ring for the purpose of communicating with other devices on the ring.

FDDI provides equivalent bandwidth to support a peak data rate of 100 Mbps and a sustained data transfer rate of at least 80 Mbps. With equivalent 4B/ 5B coding, FDDI line transmission rate is 125 Mbaud (peak). It provides connectivity for many nodes over distances of many kilometers in extent. Certain default parameter values for FDDI (such as timer settings) are calculated on the basis of up to 1000 transmission links or up to 200 km total fiber path length (typically corresponding to 500 nodes and 100 km of dual-fiber cable, respectively). However, the FDDI protocols can support much larger networks by increasing these parameter values.

Two kinds of data service can be provided in a logical ring: packet service and circuit service. With packet service, a given station that holds the token transmits information on the ring as a series of data packets, where each packet circulates from one station to the next. The stations that are addressed copy the packets as they pass. Finally, the station that transmitted the packets effectively removes them from the ring.

In the case of circuit service, some of the logical ring bandwidth is allocated to independent channels. Two or more stations can simultaneously communicate via each channel. The structure of the information stream within each channel is determined by the stations sharing the channel.

Conventional FDDI provides packet service via a token ring. A station gains the right to transmit its information on to the medium when it detects a token passing on the medium. The token is a control signal comprising a unique symbol sequence that circulates on the medium following each series of transmitted packets. Any active station, upon detection of a token, may capture the token by removing it from the ring. The station may then transmit one or more data packets. After transmitting its packets, the station issues a new token, which provides other stations the opportunity to gain access to the ring.

Each station has a token-holding timer (or equivalent means) incorporated, which

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limits the length of time a station may occupy the medium before passing the token onwards.

FDDI provides multiple levels of priority for independent and dynamic assignment depending on the relative class of service required. The classes of service may be synchronous, which may be typically used for applications such as real-time packet voice; asynchronous, which is typically used for interactive applications; or immediate, which is used for extraordinary applications such as ring recovery. The allocation of ring bandwidth occurs by mutual agreement among users of the ring.

Error-detection and recovery mechanisms are provided to restore ring operation in the event that transmission errors or medium transients (e.g., those resulting from station insertion or deletion) cause the access method to deviate the normal operation. Detection and recovery of these cases utilize a recovery function that is distributed among the stations attached to the ring.

One of the more common topologies of FDDI is the counterrotating ring. Here two classes of station may be defined: dual (attachment) and single (attachment). FDDI trunk rings may be composed only of dual-attachment stations which have two PMD (physical layer medium dependent) entities (and associated PHY entities) to accommodate the dual ring. Concentrators provide additional PMD entities beyond those required for their own attachment to the FDDI network, for the attachment of single-attachment stations which have only one PMD and thus cannot directly attach to the FDDI trunk ring. A dual-attachment station, or one-half of it, may be substituted for a single-attach- ment station in attaching to a concentrator. The FDDI network consists of all attached stations.

The example illustrated in Figure 11.18 shows the concept of multiple physical connections used to create logical rings. As shown in the figure, the logical sequence of MAC connections is stations, 1, 3, 5, 8, 9, 10, and 11. Station 2, 3, 4, and 6 form an FDDI trunk ring. Stations 1, 5, 7, 10, and 11 are attached to the ring by lobes branching out from stations that form it. Stations 8 and 9, in turn, are attached by lobes branching out from station 7. Stations 2, 4, 6, and 7 are concentrators, serving as the means of attaching MAC entities and station functionality. The concentrator examples of Figure 11.18 do not show any MACs, although their presence is implied by the designation of these concentrators as stations.

Symbol set. A symbol is the smallest signaling element used by the FDDI MAC. Symbols can be used to convey three types of information:

1. Line states such as halt (H), quiet (Q), and idle (I);

2. Control sequences such as the starting delimiter (SD), ending delimiter (ED) symbol (T), initial SD symbol (J), and final symbol (K); and

3. Data quartets, each representing a group of four ordered data bits.

Peer MAC entities on the ring communicate via a set of fixed-length symbols. These symbols are passed across the MAC-to-PHY interface via defined primitives. The MAC generates PDUs as matched pairs of symbols in accordance with the referenced FDDI standards.

11.6.4.2 FDDI Protocol Data Units. Two types of PDUs are used by the MAC: tokens and frames. In the figures that follow, formats are depicted of PDUs in the order of transmission on the medium, with the leftmost symbol transmitted first. Figure 11.19 illustrates the token format. The token is the means by which the right to transmit MAC

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Figure 11.18 FDDI topology example. (From Ref. 6.)

SDUs (service data units) (as opposed to the normal process of repeating) is passed from one MAC to another.

FDDI frame. Figure 11.20 shows the FDDI frame format. The frame format is used for transmitting both MAC recovery information and MAC SDUs between peer MAC entities. A frame may or may not have an information field.

Frame length. The maximum frame length is 9000 symbols or 4500 octets. Destination and source addrsses. The approach is very similar to token ring.

Addresses may be either 16 bits or 48 bits in length.

Frame check sequence. The FDDI FCS is identical to the FCS used in CSMA/ CD and token ring.

Figure 11.19 The FDDI token format.

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Figure 11.20 The FDDI frame format (Ref. 7).

11.6.4.3 FDDI Timers. Each MAC maintains three timers to regulate the operation of the ring. These timers are: (1) token holding timer (THT), (2) valid transmission timer (TVT), and (3) token rotation timer (TRT).

11.6.4.4 FDDI Operation. Access to the physical medium (i.e., the ring) is controlled by passing a token around the ring. The token gives the downstream MAC (receiving relative to the MAC passing the token) the opportunity to transmit a frame or sequence of frames. The time a MAC may hold a token is limited by the token-holding timer. If a MAC wants to transmit, it strips the token from the ring before the frame control field of the token is repeated. After the token is completely received, the MAC begins transmitting its eligible queued frames. After transmission, the MAC issues a new token for use by a downstream MAC.

MACs that are not transmitting repeat the incoming symbol stream. While repeating the incoming symbol stream, the MAC determines whether frames are intended for this MAC. This is done by matching the DA to its own address or a relevant group address. If a match occurs, the frame is processed by the MAC or sent to SMT (station management) or LLC.

Frame transmission. Upon request for SDU transmission, the MAC constructs the PDU or frame from the SDU by placing the SDU in the INFO field of the frame. The SDU remains queued by the requesting entity awaiting the receipt of a token that may be used to transmit it. Upon reception and capture of an appropriate token, the MAC begins transmitting its queued frame(s) in accordance with the rules of token holding. During transmission, the FCS for each frame is generated and appended to the end of the PDU. After transmission of the frame(s) is completed, the MAC immediately transmits a new token.

Frame stripping. Each transmitting station is responsible for stripping from the ring the frames that it originated. A MAC strips each frame that it transmits beginning not later than the seventh symbol after the end of the SA field. Normally, this is accomplished by stripping the remainder of each frame whose source address matches the MAC’s address from the ring and replacing it with idle symbols.

The process of stripping leaves remnants of frames, consisting at most of PA, SD, FC, DA, and SA and six symbols after the SA field, followed by idle symbols. These remnants exist because the decision to strip a frame is normally based upon recognition of the MAC’s address in the SA field, which cannot occur until after the initial part of the frame has already been repeated. These remnants are not recognized as frames because they lack an ending delimiter (ED). The limit of remnant length also prevents remnants from satisfying the minimum frame length criteria. To the level of accuracy required for statistical purposes, they can be distinguished from errored or lost frames

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because they are always followed by the idle symbol. Remnants are removed from the ring when they encounter a transmitting MAC. Remnants may also be removed by the smoothing function of PHY.

Ring scheduling. Transmission of normal PDUs (i.e., PDUs formed from SDUs) on the ring is controlled by a timed token-rotation protocol. This protocol supports two major classes of service:

1. Synchronous—guaranteed bandwidth and response time; and 2. Asynchronous—dynamic bandwidth sharing.

The synchronous class of service is used for those applications whose bandwidth and response limits are predictable in advance, permitting them to be preallocated (via the SMT). The asynchronous class of service is used for those applications whose bandwidth requirements are less critical (e.g., bursty or potentially unlimited) or whose response time requirements are less critical. Asynchronous bandwidth in FDDI is instantaneously allocated from the pool of remaining bandwidth that is unallocated, unused, or both. Section 11.6.4 is based on ANSI X3.231-1994 (Ref. 7) and ANSI X3.239-1994 (Ref. 8).

11.7 LAN INTERWORKING VIA SPANNING DEVICES

11.7.1 Repeaters

A repeater is nothing more than a regenerative repeater (see Section 6.6). It extends a LAN. It does not provide any kind of segmentation of a LAN, except the physical regeneration of the signal. Multiple LANs with common protocols can be interconnected with repeaters, in effect making just one large segment. A network using repeaters must avoid multiple paths, as any kind of loop would cause data to circulate indefinitely and could ultimately make the network crash. The multiple path concept is shown in Figure 11.21.

The following example shows how a loop can be formed. Suppose two repeaters connect CSMA/ CD LAN segments as shown in Figure 11.21. Station #1 initiates an interchange with station #3, both on the same segment (upper in the figure). As data packets or frames are transmitted on the upper segment, each repeater will transmit them unnecessarily to the lower segment. Each repeater will receive the repeated packet on the lower segment and retransmit it once again on the upper segment. As one can see, any traffic introduced into this network will circulate indefinitely around the loop created by the two repeaters. On larger networks the effects can be devastating, although perhaps less apparent (Ref. 9).

11.7.2 LAN Bridges

Whereas repeaters have no intelligence, bridges do. Bridges can connect two LANs, at the data-link or MAC protocol level. There are several varieties of bridges, depending on the intelligence incorporated.

There is the transparent bridge that builds a list of nodes the bridge sees transmitting on either side. It isolates traffic and will not forward traffic that it knows is destined to another station on the same side of the bridge as the sending station. The bridge is able to isolate traffic according to the MAC source and destination address(es) of each individual data frame. MAC-level broadcasts, however, are propagated through

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	1		3

LAN segment #1

repeater

repeater

LAN segment #2

Figure 11.21 Repeaters in multiple paths. (Courtesy of Hewlett-Packard Co., Ref. 9.)

the network by the bridges. A bridge can be used for segmenting and extending LAN coverage. Thus it lowers traffic volume for each segment. A transparent bridge does not modify any part of a message that it forwards.

The second bridge is the translation bridge. It is used to connect two dissimilar LANS, such as a token ring to CSMA/ CD. In order to do this it must modify the MAC-level header and FCS of each frame it fowards in order to make it compatible with the receiving LAN segment. The MAC addresses and the rest of the data frame are unchanged. Translation bridges are far less common than transparent bridges.

The third type of bridge, as shown in Figure 11.22, is the encapsulation bridge. It is also used to connect LANs of dissimilar protocols. But rather than translate the MAC header and FCS fields, it simply appends a second MAC layer protocol around the original frame for transport over the intermediate LAN with a different protocol. There is the destination bridge which strips off this additional layer and extracts the original frame for delivery to the destination network segment.

The fourth type of bridge is a source routing bridge. It is commonly used in token ring networks. With source routing bridges, each frame carries within it a route identifier (RI) field, which specifies the path which that frame is to take through the network. This concept is illustrated in Figure 11.22 (bottom).

Up to this point we have been discussing local bridges. A local bridge spans LANs in the same geographical location. A remote bridge spans LANs in different geographic locations. In this case, an intervening WAN (wide area network) is required. The remote bridge consists of two separate devices that are connected by a WAN, affording transport of data frames between the two. This concept is shown in Figure 11.23.

As illustrated in Figure 11.23, the LAN data packet/ frame is encapsulated by the remote bridge adding the appropriate WAN header and trailer. The WAN transports the data packet/ frame to the distant-end remote bridge, which strips the WAN header and trailer, and delivers the data packet/ frame to the far-end LAN. Remote bridges typically use proprietary protocols such that, in most cases, remote bridges from different vendors do not interoperate.

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B B

Encapsulation

A second MAC header is placed on the frame for transport over a different topology.

B B

Source Route

Route identifier (RI) field in each frame describes the path.

Figure 11.22 The concept of bridging. Top: encapsulation bridge; bottom: source routing bridge. (Courtesy of Hewlett-Packard Co., Ref. 9.)

Bridges are good devices to segment LANS, particularly CSMA/ CD LANs. Segmenting breaks up a LAN into user families. It is expected that there is a high community of interest among members of a family, but a low community of interest among different families. There will be large traffic volumes intrasegment and low traffic volumes intersegment. It should be pointed out that routers are more efficient at segmenting than bridges (Ref. 9).

			The WAN link is a pipe that
	B	B	is used to carry the LAN
			packet.
			There are no changes to the
			LAN packet.

	Preamble	Ethernet	Network Protocol	Data	FCS	LAN
						Packet
						WAN
	WAN	Ethernet	Network Protocol	Data	WAN
						Frame
					FCS

Figure 11.23 The concept of remote bridging. The LAN frame/ packet is encapsulated in a WAN frame. (Courtesy of Hewlett-Packard Co., Ref. 9.)

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A major limitation of bridges is the inability to balance traffic across two or more redundant routes in a network. The existence of multiple paths in a bridged network can prove to be a bad problem. In such a case, we are again faced with the endless route situation as we were with repeaters. One way to avoid the problem is to use the spanning tree algorithm. This algorithm is implemented by having bridges communicate with each other to establish a subset of the actual network topology that is loop-free (often called a tree). The idea, of course, is to eliminate duplicate paths connecting one LAN to another, or one segment to another. If there is only one path from one LAN to another, there can be no loop formed (Ref. 9).

11.7.3 Routers

Routers carry more intelligence than bridges. Like a bridge, a router forwards data packets/ frames. Routers make forwarding decisions based on the destination network layer address. Whereas a bridge worked on the data-link layer, a router operates at the network-layer level. Routers commonly connect disparate LANs such as CSMA/ CD to token ring and FDDI to CSMA/ CD.

Routers are addressable nodes in a network. They carry their own MAC address(es) as well as a network address for each protocol handled. Because routers are addressable, a station desiring the facility of a router must direct its packets/ frames to the router in question so that the traffic can be forwarded to the appropriate network. As one would expect, networking software at each station is more complex with a network using routers than one using bridges.

Routers handle only traffic addressed to them. They make decisions about forwarding data packets/ frames based on one or several criteria. The decisions may be based on the cost of the link, the number of hops on each path, and the time-to-live.

Routers change packets/ frames that pass through them such as MAC source and destination address; they may also modify the network protocol header of each frame (typically decrementing the time-to-live in the case of IP and other protocol fields).

Because routers have more intelligence than bridges, routers will typically have better network management agents installed. This enables them to be remotely configured, to be programmed to pass or not to pass data for security purposes, and to be monitored for performance, particularly error performance. Due to the additional processing performed at routers, they tend to be slower than bridges. Reference 9 suggests that some protocols do not lend themselves to routing, such as IBM’s SNA and NetBios, among others.

11.7.4 Hubs and Switching Hubs

A hub is a multiport device that allows centralization. A hub is usually mounted in a wiring closet or other central location. Signal leads are brought in from workstations/ PCs and other data devices, one for each hub port. Physical rings or buses are formed by internally configuring the hub ports. A typical hub may have 8 or 16 ports. Suppose we wished to incorporate 24 devices on our LAN using the hub. We can stack two hubs, one on top of the other (stackables), using one of the hub ports on each interconnection. In this case we would have a hub with a 30-port capacity (2 × 16 − 2).10 Hubs may also have a certain amount of intelligence, such as the incorporation of a network management capability. Also, each hub can include a repeater.

10 Two 16-port hubs are used for a total of 32 ports. However, two ports are required to connect one hub to the other. This leaves just 30 ports for equipment connections.