`
`EXHIBIT
`
`EXHIBIT
`1011 – part 2
`1011 — part 2
`
`
`
`
`

`

`TABLE 13.1
`
`IEEE 802.3 10-Mbps physical layer medium alternatives.
`
`13.1 / ETHERNET AND FAST ETHERNET (CSMA/CD) 409
`
`10BASE5
`
`IOBASEZ
`
`10BASE-T
`
`1OBROAD36
`
`10BASE—FP
`
`Coaxial Cable
`(50 ohm)
`
`Coaxial Cable
`(50 ohm)
`
`Unshielded
`twisted pair
`
`Coaxial Cable
`(75 ohm)
`
`850-nm optical
`fiber pair
`
`Baseband
`(Manchester)
`
`Baseband
`(Manchester)
`
`Baseband
`(Manchester)
`
`Broadband
`(DPSK)
`
`Manchester/
`On—off
`
`Bus
`
`185
`
`30
`
`5
`
`Star
`
`100
`
`Busl Tree
`
`1800
`
`Star
`
`500
`
`—
`
`——
`
`33
`
`0.4-0.6
`
`0.4-1.0
`
`62.5/125 um
`
`Transmission_
`medium
`
`Signaling
`technique
`
`Topology
`
`Maximum
`segment
`length (m)
`
`Nodes per
`segment
`
`Cable
`
`Bus
`
`500
`
`100
`
`diameter (mm)
`
`10
`
`maximum of four repeaters in the path between any two stations, thereby extend~
`ing the effective length of the medium to 2.5 kilometers.
`
`10BASE2 Medium Specification
`
`To provide a lower-cost system than IOBASES for personal computer LANs,
`10BASE2 was added. As with 10BASE5, this specification uses 50—ohm coaxial
`cable and Manchester signaling. The key difference is that 10BASE2 uses a thinner
`cable, which supports fewer taps over a shorter distance than the 10BASE5 cable.
`Because they have the same data rate, it is possible to combine 10BASE5 and
`10BASE2 segments in the same network, by using a repeater that conforms to
`10BASE5 on one side and 10BASE2 on the other side. The only restriction is that
`a 10BASE2 segment should not be used to bridge two 10BASE5 segments, because
`a “backbone” segment should be as resistant to noise as the segments it connects.
`
`10BASE-T Medium Specification
`
`By sacrificing some distance, it is possible to develop a 10—Mbps LAN using the
`unshielded twisted pair medium. Such wire is often found prewired in office build-
`ings as excess telephone cable, and can be used for LANs. Such an approach is spec-
`ified in the 10BASE-T specification. The 10BASE-T specification defines a star—
`shaped topology. A simple system consists of a number of stations connected to a
`central point, referred to as a multiport repeater, via two twisted pairs. The central
`point accepts input on any one line and repeats it on all of the other lines.
`Stations attach to the multiport repeater via a point-to-point link. Ordinarily,
`the link consists of two unshielded twisted pairs. Because of the high data rate and
`the poor transmission qualities of unshielded twisted pair, the length of a link is lim—
`ited to 100 meters. As an alternative, an optical fiber link may be used. In this case,
`the maximum length is 500 m.
`
`Viptela, Inc. - Exhibit 1011 - Part 2
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`Page 1
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`
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`Viptela, Inc. - Exhibit 1011 - Part 2
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`410 CHAPTER 13 / LAN SYSTEMS
`
`10BROAD36 Medium Specification
`
`The IOBROAD36 specification is the only 802.3 specification for broadband. The
`medium employed is the standard 75-ohm CATV coaxial cable. Either a dual-cable
`or split-cable configuration is allowed. The maximum length of an individual seg-
`ment, emanating from the headend, is 1800 meters; this results in a maximum end-
`to-end span of 3600 meters.
`The signaling on the cable is differential phase—shift keying (DPSK). In ordi-
`nary PSK, a binary zero is represented by a carrier with a particular phase, and a
`. binary one is represent by a carrier with the opposite phase (180-degree difference).
`DPSK makes use of differential encoding, in which a change of phase occurs when
`a zero occurs, and there is no change of phase when a one occurs. The advantage of
`differential encoding is that it is easier for the receiver to detect a change in phase
`than to determine the phase itself.
`The characteristics of the modulation process are specified-so that the result-
`ing 10 Mbps signal fits into a 14 MHz bandwidth.
`
`IOBASE-F Medium Specification
`
`The IOBASE-F specification enables users to take advantage of the distance and
`transmission characteristics available with the use of optical fiber. The standard
`actually contains three specifications:
`
`o 10-BASE-FP (passive). A passive-star topology for interconnecting stations
`and repeaters with up to 1 km per segment.
`
`0 10-BASE-FL (link). Defines a point-to-point link that can be used to connect
`stations or repeaters at up to 2 km.
`
`0 10-BASE—FB (backbone). Defines a point-to-point link that can be used to
`connect repeaters at up to 2 km.
`
`All three of these specifications make use of a pair of optical fibers for each
`transmission link, one for transmission in each direction. In all cases, the signaling
`scheme involves the use of Manchester encoding. Each Manchester signal element
`is then converted to an optical signal element, with the presence of light corre-
`sponding to high and the absence of light corresponding to low. Thus, a 10-Mbps
`Manchester bit stream actually requires 20 Mbps on the fiber.
`The 10-BASE-FP defines a passive star system that can support up to" 33 sta-
`tions attached to a central passive star, of the type described in Chapter 3. 10-‘
`BASE—FL and 10-BASE-FP define point-to-point connections that can be used to
`extend the length of a network; the key difference between the two is that 10-
`BASE-FP makes use of synchronous retransmission. With synchronous signaling,
`an optical signal coming into a repeater is retimed with a local clock and retrans-
`mitted. With conventional asynchronous signaling, used with 10-BASE-FL, no such
`retiming takes place, so that any timing distortions are propagated through a series
`of repeaters. As a result, 10BASE-FB can be used to cascade up to 15 repeaters in
`sequence to achieve greater length.
`
`Viptela, Inc. - Exhibit 1011 - Part 2
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`Page 2
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`

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`13.1 / ETHERNET AND FAST ETHERNET (CSMA/CD) 411
`
`IEEE 802.3 100-Mbps Specifications (Fast Ethernet)
`
`Fast Ethernet refers to a set of specifications developed by the IEEE 802.3 com-
`mittee to provide a low—cost, Ethernet—compatible LAN operating at 100 Mbps. The
`blanket designation for these. standards is 100BASE-T. The committee defined a
`number of alternatives to be used with different transmission media.
`
`Figure 13.4 shows the terminology used in labeling the specifications and indi-
`cates the media used. All of the 10OBASE-T options use the IEEE 802.3 MAC pro-
`tocol and frame format. IOOBASE-X refers to a set of options that use the physical
`medium specifications originally defined for Fiber Distributed Data Interface
`(FDDI; covered in the next section). All of the 100BASE—X schemes use two phys-
`ical links between nodes: one for transmission and one for reception. IOOBASE-TX
`makes use of shielded twisted pair (STP) or high-quality (Category 5) unshielded
`twisted pair (UTP). 100BASE-FX uses optical fiber.
`In many buildings, each of the IOOBASE-X options requires the installation of
`new cable. For such cases, IOOBASE-T4 defines a lower-cost alternative that can
`use Category-3, voice grade UTP in addition to the higher-quality Category 5 UTP.“
`To achieve the 100-Mbps data rate over lower-quality cable, 100BASE-T4 dictates
`the use of four twisted pair lines between nodes, with the data transmission making
`use of three pairs in one direction at a time.
`For all of the IOOBASE-T options, the topology is similar to that of 10BASE-
`T, namely a star-wire topology.
`’
`Table 13.2 summarizes key characteristics of the IOOBASE-T options.
`
`100BASE-X
`For all of the transmission media specified under 100BASE—X, a unidirectional
`data rate of 100 Mbps is achieved by transmitting over a single link (single twisted
`pair, single optical fiber). For all of these media, an efficient and effective signal
`
`IEEE 802.3 (100-Mbps)
`
`/
`?ASE—X
`1W8?
`
`IOOBASE-FX
`
`10OBASE-T4
`
`2 Category 5 UTP
`
`2 STP
`
`2 Optical fiber
`
`4 Category 3 or Category 5 UTP
`
`FIGURE 13.4
`
`IEEE 802.3 IOOBASE-T options.
`
`4 See Chapter 3 for a discussion of Category 3 and Category 5 cable.
`
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`412 CHAPTER 13 / LAN SYSTEMS
`
`TABLE 13.2
`
`IEEE 8(Y2.‘3’100BASE\'-T physical layer medium alternatives.
`
`Transmission
`medium
`Signaling
`technique
`Data rate
`Maximum
`segment length
`Network span
`
`.
`
`IOOBASE-TX
`
`IOOBASE-FX
`
`100BASE—T4
`
`2 pair, STP
`
`4B5B, NRZI
`
`2 pair, Category
`5 UTP
`4B5B, NRZI
`
`2 optical fibers
`
`4B5B, NRZI
`
`4 pair, Category
`3, 4, or 5 UTP
`8B6T, NRZ
`
`100 Mbps
`100 in
`
`100 Mbps
`100 in
`
`100 Mbps
`100 m
`
`100 Mbps
`100 m
`
`200 m
`
`200 m
`
`400 m
`
`200 m
`
`encoding scheme is required. The one chosen was originally defined for FDDI, and
`can be referred to as 4B/5B-NRZI. See Appendix 13A for a description.
`The 100BASE-X designation includes two physical-medium specifications,
`one for twisted pair, known as IOOBASE-TX, and one for optical fiber, known as
`100-BASE-FX.
`
`IOOBASE-TX makes use of two pairs of twisted pair cable, one pair used for
`transmission and one for reception. Both STP and Category 5 UTP are allowed.
`The MTL-3 signaling scheme is used (described in Appendix 13A).
`10OBASE—FX makes use of two optical fiber cables, one for transmission and
`one for reception. With 10OBASEaFX, a means is needed to convert the 4B/5B-
`NRZI code groups stream into optical signals. The technique used is known as
`intensity modulation. A binary 1 is represented by a burst or pulse of light; a binary
`O is represented by either the absence of a light pulse or by a light pulse at very low
`intensity.
`
`100BASE-«T4
`
`IOOBASE-T4 is designed to produce a 100—Mbps data rate over lower-quality Cat-
`egory 3 cable, thus taking advantage of the large installed base of Category 3 cable
`in office buildings. The specification also indicates that the use of Category 5 cable
`is optional. IOOBASE-T4 does not transmit a continuous signal between packets,
`which makes it useful in battery-powered applications.
`For IOOBASE-T4 using voice-grade Category 3 cable, it is not reasonable to
`expect to achieve 100 Mbps on a single twisted pair. Instead, IOOBASE-T4 specifies
`that the data stream to be transmitted is split up into three separate data streams,
`each with an effective data rate of 33% Mbps. Four twisted pairs are used. Data are
`transmitted using three pairs and received using three pairs. Thus, two of the pairs
`must be configured for bidirectional transmission.
`As with 100BASE-X, a simple NRZ encoding scheme is not used for
`100BASE-T4; this would require a signaling rate of 33 Mbps on each twisted pair
`and does not provide synchronization. Instead, a ternary signaling scheme known as
`8B6T is used (described in Appendix 13A).
`
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`

`13.2 TOKEN RING AND FDDI
`
`13.2 / TOKEN RING AND FDDI 413
`
`Token ring is the most commonly used MAC protocol for ring-topology LANs. In
`this section, we examine two standard LANs that use token ring: IEEE 802.5 and
`FDDI.
`
`IEEE 802.5 Medium Access Control
`
`MAC Protocol
`
`The token ring technique is based on the use of a small frame, called a token, that
`circulates when all stations are idle. Astation wishing to transmit must wait until it
`detects a token passing by. It then seizes the token by changing one bit in the token,
`which transforms it from a token into a start—of—frame sequence for a data frame.
`The station then appends and transmits the remainder of the fields needed to con-
`struct a data frame.
`
`When a station seizes a token and begins to transmit a data frame, there is no
`token on the ring, so other stations wishing to transmit must wait. The frame on the
`ring will make a round trip and be absorbed by the transmitting station. The trans-
`mitting station will insert a new token on the ring when both of the following con-
`ditions have been met:
`
`- The station has completed transmission of its frame.
`
`- The leading edge of the transmitted frame has returned (after a complete cir-
`culation of the ring) to the station.
`
`If the bit length of the ring is less than the frame length, the first condition
`implies the second; if not, a station could release a free token after it has finished
`transmitting but before it begins to receive its own transmission. The second condi-
`tion is not strictly necessary, and is relaxed under certain circumstances. The advan-
`tage of imposing the second condition is that it ensures that only one data frame at
`a time may be on the ring and that only one station at a time may be transmitting,
`thereby simplifying error—recovery procedures.
`Once the new token has been inserted on the ring, the next station down-
`stream with data to send will be able to seize the token and transmit. Figure 13.5
`illustrates the technique. In the example, A sends a packet to C, which receives it
`and then sends its own packets to A and D.
`Note that under lightly loaded conditions, there is some inefficiency with
`token ring because a station must wait for the token to come around before trans-
`mitting. However, under heavy loads, which is when it matters, the ring functions in
`a round-robin fashion, which is both efficient and fair. To see this, consider the con-
`figuration in Figure 13.5. After station A transmits, it releases a token. The first sta-
`tion with an opportunity to transmit is D. If D transmits, it then releases a token and
`C has the next opportunity, and so on.
`The principal advantage of token ring is the flexible control over access that it
`
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`414 CHAPTER 13 / LAN SYSTEMS
`
`FIGURE 13.5 Token ring operation.
`
`provides. In the simple scheme just described, the access if fair. As we shall see,
`schemes can be used to regulate access to provide for priority and for guaranteed
`bandwidth services.
`
`The principal disadvantage of token ring is the requirement for token mainte-
`nance. Loss of the token prevents further utilization of the ring. Duplication of the
`token can also disrupt ring operation. One station must be selected as a monitor to
`ensure that exactly one token is on the ring and to ensure that a free token is rein-
`serted, if necessary.
`
`MAC Frame
`
`Figure 13.6 depicts the frame format for the 802.5 protocol. It consists of the fol-
`lowing fields:
`
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`Octets
`
`1
`
`1
`
`1
`
`2 or 6
`
`2 or 6
`
`2 0
`
`4
`
`1
`
`1
`
`13.2 / TOKEN RING AND FDDI 415
`
`SD 2 Starting delimiter
`AC 2 Access control
`FC 2 Frame control
`
`DA 2 Destination address
`SA 2 Source address
`FCS 2 Frame~check sequence
`
`(a) General frame format
`
`FCS
`ED 2 Ending delimiter
`FS 2 Frame status
`
`(b) Toke" frame format
`
`J, K 2 Nondata bits
`I
`2 Intermediate—frame bits
`E
`2 Error—detected bits
`
`TM ‘e>E"di“gde‘i"W“*='d
`PPP
`2 Priority bits M
`2 Monitor bit
`T
`2 Token bit
`RRR 2 Reservation bits
`'
`
`(c) Access control field
`
`-A-C-rr‘ -A-C‘rr
`A 2 Addressed recognized bit
`C 2 Copied bit
`
`FIGURE 13,6
`
`IEEE 802.5 frame format.
`
`Starting delimiter (SD). Indicates start of frame. The SD consists of signaling
`patterns that are distinguishable from data. It is coded as follows: JKOJKOOO,
`where J and K are nondata symbols. The actual form of a nondata symbol
`depends on the signal encoding on the medium.
`Access control (AC). Has the format PPPTMRRR, where PPP and RRR are
`3-bit priority and reservation variables, and M is the monitor bit; their use is
`explained below. T indicates whether this is a token or data frame. In the case
`of a token frame, the only remaining field is ED.
`Frame control (FC). Indicates whether this is an LLC data frame. If not, bits
`in this field control operation of the token ring MAC protocol.
`lDestination address (DA). As with 802.3.
`
`Source address (SA). As with 802.3.
`
`,Data unit. Contains LLC data unit.
`Frame check sequence (FCS). Ashwith 802.3.
`End delimiter (ED). Contains the e1‘ror—detection bit (E), which is set if
`any repeater detects an error, and the intermediate bit (I), which is used to
`indicate that this is a frame other than the final one of a multiple—frame
`transmission.
`
`Frame status (FS). Contains the address recognized (A) and frame-copied
`(C) bits, whose use is explained below. Because the A and C bits are outside
`the scope of the FCS, they are duplicated to provide a redundancy check to
`detect erroneous settings.
`
`We can now restate the token ring algorithm for the case when a single prior-
`ity is used. In this case, the priority and reservation bits are set to O. A station wishing
`
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`416 CHAPTER 13 / LAN SYSTEMS
`
`to transmit waits until «a token goes by, as indicated by a token bit of 0 in the AC
`field. The station seizes the token by setting the token bit to 1. The SD and AC
`fields of the received token now function as the first two fields of the outgoing
`frame. The station transmits one or more frames, continuing until either its supply
`of frames is exhausted or a token—holding timer expires. When the AC field of the
`last transmitted frame returns, the station sets the token bit to 0 and appends an ED
`field, resulting in the insertion of a new token on the ring.
`a
`Stations in the receive mode listen to the ring. Each station can check passing
`‘frames for errors and can set the E bit to 1 if an error is detected. If a station detects
`its own MAC address, it sets the A bit to 1; it may also copy the frame, setting the
`C bit to 1. This allows the originating station to differentiate three results of a frame
`transmission:
`
`0 Destination station nonexistent or not active (A = O, C = 0)
`- Destination station exists but frame not copied (A = 1, C = 0)
`- Frame received (A = 1, C = 1)
`
`Token Ring Priority
`
`The 802.5 standard includes a specification for an optional priority mechanism.
`Eight levels of priority are supported by providing two 3-bit fields in each data
`frame and token: a priority field and a reservation field. To explain the algorithm,
`let us define the following variables:
`
`Pf = priority of frame to be transmitted by station
`P, = service priority: priority of current token
`
`P, = value of P, as contained in the last token received by this station
`R, = reservation value in current token
`
`R, = highest reservation value in the frames received by this station during
`the last token rotation
`
`The scheme works as follows:
`
`1. A station wishing to transmit must wait for a token with P, S Pf.
`2. While waiting, a station may reserve a future token at its priority level (Pf).
`If a data frame goes by, and if the reservation field is less than its priority
`(R, < Pf), then the station may set the reservation field of the frame to its
`priority (R, <— Pf).,If a token frame goes by, and if (R, < Pf AND Pf < PS),
`then the station sets the reservation field of the frame to its priority (R, <-— Pf).
`This setting has the effect of preempting any lower-priority reservation.
`3. When a station seizes a token, it sets the token bit to 1 to start a data frame,
`sets the reservation field of the data frame to O, and leaves the priority field
`unchanged (the same as that of the incoming token frame).
`4. Following transmission of one or more data frames, a station issues a new
`token with the priority and reservation fields set as indicated inTable 13.3.
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`13.2,/ TOKEN RING AND PDDI
`
`417
`
`TABLE 13.3 Actions performed by the token holder to
`implement the priority scheme [based on VALE92].
`
`
`
` Conditions Actions
`
`Frame available AND P, S Pf
`
`Send frame
`
`(Frame not available OR TI-IT expired)
`ANDP,2MAX [R,, Pf]
`
`(Frame not available OR THT expired)
`AND P, < MAX [R,, 10,]
`AND P, > S,
`
`(Frame not available OR 'I'I-IT expired)
`AND P, < MAX [R,, Pf]
`AND P, = S,
`
`(Frame not available OR
`(Frame available and P, < S,,))
`AND P, = S,
`.
`AND R, > S,
`
`(Frame not available OR
`(Frame available and Pf < S,))
`AND P, = S,
`AND R, S S,
`
`Send token with:
`P,<—P,
`R, <—— MAX [R,, 10,}
`
`Send token with:
`
`P, <-—- MAX [R,, Pf]
`R, <-— 0
`Push S, <—— P,
`Push S, <— P,
`
`Send token with:
`
`P, <— MAX [R,, P,]
`R, <— 0
`Pop s,
`Push S, <— P,
`
`Send token with:
`P, <-— R,
`R, <-— 0
`Pop S,
`Push S, <-— P,
`
`Send token with:
`P; <- R;
`R, <— 0
`Pop S,
`Pop S,
`
`The effect of the above steps is to sort the competing claims and to allow the
`waiting transmission of highest priority to seize the token as soon as possible. A
`moment’s reflection reveals that, as stated, the algorithm has a ratchet effect on pri-
`ority, driving it to the highest used level and keeping it there. To avoid this, a sta-
`tion that raises the priority (issues a token that has a higher priority than the token
`that it received) has the responsibility of later lowering the priority to its previous
`level. Therefore, a station that raises priority must remember both the old and the
`new priorities and must downgrade the priority of the token at the appropriate
`time. In essence, each station is responsible for assuring that no token circulates
`indefinitely because its priority is too high. By remembering the priority of earlier
`transmissions, a station can detect this condition and downgrade the priority to a
`previous, lower priority or reservation.
`To implement the downgrading‘ mechanism, two stacks are maintained by
`each station, one for reservations and one for priorities:
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`418 CHAPTER 13 / LAN SYSTEMS
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`S, = stack used to store new values of token priority
`S, = stack used to store old values of token priority
`
`The reason that stacks rather than scalar variables are required is that the pri-
`ority can be raised a number of times by one or more stations. The successive raises
`must be unwound in the reverse order.
`
`To summarize, a station having a higher priority frame to transmit than the
`current frame can reserve the next token for its priority level as the frame passes by.
`When the next token is issued, it will be at the reserved priority level. Stations of
`lower priority cannot seize the token, so it passes to the reserving station or an inter-
`mediate station with data to send of equal or higher priority level than the reserved
`priority level. The station that upgraded the priority level is responsible for down-
`grading it to its former level when all higher-priority stations are finished. When
`that station sees a token at the higher priority after it has transmitted, it can assume
`that there is no more higher-priority traffic waiting, and it downgrades the token
`before passing it on.
`Figure 13.7 is an example. The following events occur:
`
`1. A is transmitting a data frame to B at priority 0. When the frame has com-
`pleted a circuit of the ring and returns to A, A will issue a token frame. How-
`ever, as the data frame passes D, D makes a reservation at priority 3 by set-
`ting the reservation field to 3.
`
`2. A issues a token with the priority field set to 3.
`3. If neither B nor C has data of priority 3 or greater to send, they cannot seize
`the token. The token circulates to D, which seizes the token and issues a data
`frame.
`
`4. After D’s data frame returns to D, D issues a new token at the same priority
`as the token that it received: priority 3.
`
`5. A sees a token at the priority level that it used to last issue a token; it there-
`fore seizes the token even if it has no data to send.
`
`6. A issues a token at the previous priority level: priority 0.
`
`Note that, after A has issued a priority 3 token, any station with data of pri-
`ority 3 or greater may seize the token. Suppose that at this point station C now has
`priority 4 data to send. C will seize the token, transmit its data frame, and reissue a
`priority 3 token, which is then seized by D. By the time that a priority 3 token
`arrives at A, all intervening stations with data of priority 3 or greater to send will
`have had the opportunity. It is now appropriate, therefore, for A to downgrade the
`token.
`
`Early Token Release
`
`When a station issues a frame, if the bit length of the ring is less than that of the
`frame, the leading edge of the transmitted frame will return to the transmitting sta-
`tion before it has completed transmission; in this case, the station may issue a token
`as soon as it has finished frame transmission. If the frame is shorter than the bit
`
`length of the ring, then after a station has completed transmission of a frame, it must
`
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`

`

`13.2 / TOKEN RING AND FDDI 419
`
`
`
`D reserves at
`higher level.
`
`4. D generates token
`at higher priority
`level.
`
`
`
`2. A generates higher
`priority token and
`remembers lower
`priority.
`
`5. A sees the high
`priority token and
`
` l. A is sending B;
`
`
`
`
`
`captures it.
`lower priority level.
`
`
`
`
`
`3. D uses higher
`P
`Y
`riorit
`token to
`send data to C.
`
`8
`6. A enerates token
`P
`P
`at the re-em led,
`
`‘ FIGURE 13.7
`
`IEEE token ring priority scheme.
`
`wait until the leading edge of the frame returns before issuing a token. In this latter
`case, some of the potential capacity of the ring is unused.
`To allow for more efficient ring utilization, an early token release (ETR)
`option has been added to the 802.5 standard. ETR allows a transmitting station to
`release a token as soon as it completes frame transmission, whether or not the
`frame header has returned to the station. The priority used for a token released
`prior to receipt of the previous frame header is the priority of the most recently
`received frame.
`
`One effect of ETR is that access delay for priority traffic may increase when
`the ring is heavily loaded with short frames. Because a station must issue a token
`before it can read the reservation bits of the frame it just transmitted, the station
`will not respond to reservations. Thus, the priority mechanism is at least partially
`disabled.
`
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`
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`

`420 CHAPTER 13 / LAN SYSTEMS
`
`Stations that implement ETR are compatible and interoperable with those
`that do not complete such implementation.
`
`IEEE 802.5 Physical Layer Specification
`
`The 802.5 standard (Table 13.4) specifies the use of shielded twisted pair with data
`rates of 4 and 16 Mbps using Differential Manchester encoding. An earlier specifi-
`cation of a 1-Mbps system has been dropped from the most recent edition of the
`standard.
`
`'
`
`A recent addition to the standard is the use of unshielded twisted pair at
`
`4 Mbps.
`
`TABLE 13.4
`
`802.5 physical layer medium alternatives.
`
`Transmission medium
`Data rate (Mbps)
`Signaling technique
`Maximum number of
`repeaters
`Maximum length
`between repeaters
`
`Shielded twisted pair
`4 or 16
`Differential Manchester
`250
`
`Unshielded twisted pair
`4
`Differential Manchester
`72
`
`Not specified
`
`Not specified
`
`FDDI Medium Access Control
`FDDI is a token ring scheme, similar to the IEEE 802.5 specification, that is designed
`for both LAN and MAN applications. There are several differences that are designed
`to accommodate the higher data rate (100 Mbps) of FDDI.
`
`MAC Frame
`
`Figure 13.8 depicts the frame format for the FDDI protocol. The standard defines the
`contents of this format in terms of symbols, with each data symbol corresponding to
`
`Bits
`
`64
`[PTembleLSDJ
`
`8
`FC
`
`16 or 48
`I DA
`
`16 or 48
`SA
`
`|
`
`2 0
`Info
`
`32
`FCS
`
`1
`
`4
`1
`ED 1 Es?
`
`(a) General frame format
`
`I
`FC 1 FS
`(b) Token frame format
`
`‘
`SA = Source address
`FCS = Frame-check sequence
`
`|£reambleL SD
`
`I
`
`LEGEND
`SD = Start-frame delimiter
`FC = Frame control
`DA = Destination address
`
`FIGURE 13.8 FDDI frame formats.
`
`ED = Ending delimiter
`FS
`= Frame status
`
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`

`13.2 / TOKEN RING AND FDDI 421
`
`4 data bits. Symbols are used because, at the physical layer, data are encoded in 4-
`bit chunks. However, MAC entities, in fact, must deal with individual bits, so the
`discussion that follows sometimes refers to 4-bit symbols and sometime to bits. A
`frame other than a token frame consists of the following fields:
`
`0 Preamble. Synchronizes the frame with each station’s clock. The originator of
`the frame uses a field of 16 idle symbols (64 bits); subsequent repeating sta-
`tions may change the length of the field, as consistent with clocking require-
`. ments. The idle symbol is a nondata fill pattern. The actual form of a nondata ,
`symbol depends on the signal encoding on the medium.
`
`Starting delimiter (SD). Indicates start of frame. It is coded as JK, where J and
`K are nondata symbols.
`
`Frame control (FC). Has the bit format CLFFZZZZ, where C indicates
`whether this is a synchronous or asynchronous frame (explained below); L
`indicates the use of 16- or 48-bit addresses; FF indicates whether this is an
`LLC, MAC control, or reserved frame. For a control frame, the remaining 4
`bits indicate the type of control frame.
`
`Destination address (DA). Specifies the station(s) for which the frame is
`intended. It may be a unique physical address, a multicast-group address, or a
`broadcast address. The ring may contain a mixture of 16- and 48-bit address
`lengths.
`
`Source address (SA). Specifies the station that sent the frame.
`
`Information. Contains an LLC data unit or information related to a control
`operation.
`
`Frame check sequence (FCS). A 32-bit cyclic redundancy check, based on the
`FC, DA, SA, and information fields.
`
`Ending delimiter (ED). Contains anondata symbol (T) and marks the end of
`the frame, except for the FS field.
`
`Frame Status (FS). Contains the error detected (E), address recognized (A),
`and frame copied (F) indicators. Each indicator is represented by a symbol,
`which is R for “reset” or “false” and S for “set” or “true.”
`
`A token frame consists of the following fields:
`
`Preamble. As above.
`
`Starting delimiter. As above.
`Frame control (FC). Has the bit format 10000000 or 11000000 to indicate that
`this is a token.
`
`Ending delimiter (ED). Contains a pair of nondata symbols (T) that termi-
`nate the token frame.
`
`A comparison with the 802.5 frame (Figure 13.6) shows that the two are quite
`similar. The FDDI frame includes a preamble to aid in clocking, which is more
`demanding at the higher data rate. Both 16- and 48-bit addresses are allowed in the
`same network with FDDI; this is more flexible than the scheme used on all the 802"
`
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`
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`

`

`422 CHAPTER 13 / LAN SYSTEMS
`
`standards. Finally, there are some differences in the control bits. For example,
`FDDI does not include priority and reservation bits; capacity allocation is handled
`in a different way, as described below.
`
`MAC Protocol
`
`The basic (without capacity allocation) FDDI MAC protocol is fundamentally the
`same as IEEE 802.5. There are two key differences:
`
`1. In FDDI, a station waiting for a token seizes the token by aborting (failing to
`repeat) the token transmission as soon as the token frame is recognized. After
`the captured token is completely received, the station begins transmitting one
`or more data frames. The 802.5 technique of flipping a bit to convert a token
`to the start of a data frame was considered impractical because of the high
`data rate of FDDI.
`
`2. In FDDI, a station that has been transmitting data frames releases a new
`token as soon as it completes data frame transmission, even if it has not begun
`to receive its own transmission. This.is the same technique as the early token
`release option of 802.5.. Again, because of the high data rate, it would be too
`inefficient to require the station to wait for its frame to return, as in normal
`802.5 operation.
`
`Figure 13.9 gives an example of ring operation. After station A has seized the
`token,
`it transmits frame F1, and immediately transmits a new token. F1 is
`addressed to station C, which copies it as it circulates past. The frame eventually
`returns to A, which absorbs it. Meanwhile, B seizes the token issued by A and trans-
`mits F2 followed by a token. This action could be repeated any number of times, so
`that, at any one time, there may be multiple frames circulating the ring. Each sta-
`tion is responsible for absorbing its own frames based on the source address field.
`A further word should be said about the frame status (FS) field. Each station
`can check passing bits for errors and can set the E indicator if an error is detected.
`If a station detects its own address, it sets the A indicator; it may also copy the
`frame, setting the C indicator; this allows the originating station, when it absorbs a
`frame that it previously transmitted, to differentiate among three conditions:
`
`0 Station nonexistent/nonactive
`
`0 Station active but frame not copied
`
`0 Frame copied
`
`When a frame is absorbed, the status indicators (E, A, C) in the FS field may be
`examined to determine the result of the transmission. However, if an error or fail—
`ure to receive condition is discovered, the MAC