Local Area Networks

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Local Area Networks

Chapter 8 – Token Ring LANs

635.412.71 Spring 05Class #5: Token Ring LANs & Fibre

Channel 2

Token Ring LANs and MANsIntroduction to Token Ring LANs

Although not as popular as they once were, token ring LANs are still important because of their large installed base

Two main Token Ring LAN implementations are found in widespread use: IEEE 802.5 Token Ring & FDDI

IEEE 802.5 Token Ring– Developed by IBM in the late 1970s & later standardized by the IEEE– The first implementations ran at 4-Mbps; by the late 1980s 16-Mbps

Token Ring became commercially available– By 1998 100-Mbps Token Ring was standardized– Switched versions of Token Ring at various speeds are also available

Fiber Data Distributed Interface (FDDI)– Developed by the ANSI ASC X3T9.5 committee in the mid 1980s– Uses a dual counter-rotating ring design running at 100-Mbps– Contains options making it suitable as a backbone technology – Can be used to develop a variety of complex network topologies


Channel 3

Token Ring LANs and MANs IEEE 802.5 Token Ring Medium Access Control

Definition of the Token Ring Topology– A token ring LAN connects stations in a closed loop with

unidirectional point-to-point links. – Data frames circulate around the ring from source to

destination– Permission to transmit is represented by a frame called a

token


Channel 4


MAC Protocol– Token ring operation centers around the token; any station

wishing to transmit must wait to capture the token As the token passes the station changes one bit to turn it

into a start-of-frame delimiter & then appends the rest of the data

Since the token has been ‘removed’ from the ring, the transmitting station creates a new token under these conditions:a. The station has finished transmission of the frameb. The leading edge of the transmitted frame makes a complete

circuitc. Condition (b) is sometimes relaxed on large rings to increase

efficiency; this complicates ring recovery procedures since multiple stations could have data frames in transit


Channel 5


MAC Protocol (continued)– Under lightly loaded conditions using a token ring results in

less than optimal efficiency because of latency – under heavy load token rings works well because it provides round-robin access to all stations

– The key advantage to token ring is flexible access control; not only is the basic scheme described above fair but priority and QoS schemes can be added

– The key disadvantage is the need for token maintenance and recovery procedures; without the token stations cannot communicate!

One station must be designated as the ring monitor to ensure one and only one token is on the ring at any time and to recover ring operation under abnormal conditions

A procedure for electing a ring monitor must be instituted


Channel 6

Token Ring LANs and MANs MAC Frame

There are two basic frame types in 802.5, a general frame and the token [Figure 8.2]


Channel 7


The general frame consists of the following fields:– Starting Delimiter: indicates the beginning of a frame; can be

identified by the use of physical-layer dependent non-data signals– Access Control: provides control bits for priority and reservation

as well as to denote a token frame– Frame Control: identifies the type of frame; at this point it would

either be a MAC frame or a LLC data frame– Source and Destination Address: addressing is the same as the

802.3 specification– Data payload: contains the LLC PDU– Frame Check Sequence: the same 32-bit CRC used in 802.3– End Delimiter: contains two control bits; one for error detection &

one to indicate that the frame is part of a multiframe transmission– Frame Status: contains two control bits used for

acknowledgement


Channel 8


MAC Frame Control Fields– The token consists of the SD, AC, and ED fields; it is turned into

a data frame by setting the token bit and appending the other data frame fields

– All stations in listen mode examine passing frames and will set certain bits in the frames if necessary

If a passing frame has an error the station will set the Error bit in the End Delimiter

If a station detects its own MAC address in a passing frame it will set the A (Address Recognized) bit

If a a passing frame is copied the C (Frame Copied) bit is set Such operation allows the transmitter to determine the status of

any data frame:– Destination nonexistent or not active (A=0, C=0)– Destination exists but frame not copied (A=1, C=0)– Frame successfully received (A=1, C=1)


Channel 9

Token Ring LANs and MANs Token Ring Priority

An optional priority mechanism is specified in the 802.5 standard to allow preferred access for designated stations

Eight levels of priority can be used via the 3 bit priority and reservation subfields in the Access Control

The priority scheme generally works as follows:– To transmit a frame at a given priority level a station must wait until

a token passes with a priority equal to or less than the frame priority– A station can make a ‘reservation’ for transmission of a data frame

by setting the reservation field in a passing data frame or a higher priority token

– When the station finally gets the token it resets the reservation field to zero so other stations can make reservations

– When finished the station issues a token with priority and reservation fields set as in Table 8.2


Channel 10

Token Ring LANs and MANs Token Ring Priority

Key aspects of the 802.5 priority scheme– The scheme assures that the station with the highest priority

frame is allowed to transmit as soon as possible– Because this algorithm will drive up the priority of the token, it

is the responsibility of the station raising the priority of the token to lower it when it is finished

A station can assume if it sees the token at the priority level it set than all stations with the priority at or above this level have had an opportunity to transmit

– If the priority is raised multiple times by different stations, the priority is lowered in reverse order (think of it working like a stack)


Channel 11

Token Ring LANs and MANs Token Ring Priority Example [Figure 8.3]


Channel 12

Token Ring LANs and MANs Token Maintenance

Ring Monitoring– To ensure proper token circulation and that the ring

recovers from error conditions, one station on the ring is designated as the Active Monitor

– All stations participate cooperatively in the election and monitoring of the ring; while one station is the Active Monitor all others are in Standby Monitor mode

Each station issues a Standby-Monitor-present frame that is absorbed by its downstream neighbor, stores the address of the upstream station, and then issues the same type frame to its downstream neighbor

The Standby-Monitor-present functionality can be useful in fault detection and isolation


Channel 13

Token Ring LANs and MANs Token Maintenance

Ring Monitoring (continued)– Active Monitor responsibilities:

Periodically issue an Active-Monitor-present frame to assure all other stations that an Active Monitor exists

Set a timer to a value just above the time required for a token to make one circuit of the ring; issue a new priority zero token if the timer expires without seeing a token

Set the monitor bit on any passing data frame and absorb any data frames that pass with the monitor bit set

– That means the transmitting station failed to absorb the frame – the monitor absorbs it & issues a priority zero token

– Should also happen if the monitor detects a circulating token with constant nonzero priority; this situation means the priority mechanism has failed

If another Active Monitor is detected, go into Standby Monitor mode


Channel 14

Token Ring LANs and MANs Early Token Release

On networks where the frame length > the bit length of the ring, transmission of a frame fully utilizes the ring

On networks where this is not the case the ring cannot be fully utilized unless there is a mechanism for early release of the token (allows multiple frames on the ring simultaneously)

An Early Token Release option has been added to the 802.5 specification; stations using ETR are backwards compatible

– ETR allows a station to issue a frame as soon as it finishes transmitting data without having received its frame header

– The priority mechanism is partially disabled with ETR: Station must issue a token before it receives back its frame with

possible reservations, so it will not accept reservations and issues a token with the priority of the last received data frame

Can cause delays for priority frames when ring is heavily loaded w/ short frames


Channel 15

Token Ring LANs and MANs Dedicated Token Ring

In 1998, another option was added to the 802.5 specification to allow for switched Token Ring LANs

Uses a physical star topology & central switching hub with ports capable of operating in two different modes:

– In Transmit Immediate protocol (TXI) mode no token protocol is used; a full-duplex dedicated link is used to connect the station to the switching matrix in the central hub

– In Token Passing protocol (TKP) mode the central hub port acts just like a classic 802.5 station participating in a token ring

– Smarter switches have ports that can automatically adjust from TKP to TKI (or vice versa) to accommodate topology changes

The central hub can use either cut-through or store-and-forward mode

Hubs & stations using either mode can be mixed


Channel 16

Token Ring LANs and MANs Dedicated Token Ring Topologies [Figure 8.4]


Channel 17

Token Ring LANs and MANs IEEE 802.5 Physical Layer

As with 802.3, a variety of physical layer options have been defined for 802.5

– Comparison of the physical layer alternatives [Table 8.3] At 4-Mbps and 16-Mbps differential Manchester signaling

is used and either the traditional or the switched mode can be used

At 100-Mbps the same physical layer specifications as Fast Ethernet are used; at this speed only switched mode is allowed

There is currently a task force working on specification of a 1-Gbps token ring that would use the 802.5 MAC while reusing the 802.3 Gigabit Ethernet physical layer specifications


Channel 18

Fiber Data Distributed Interface (FDDI)Introduction

Unlike the 802.3 and 802.5 specifications, the FDDI standards were developed by a different organization called the American National Standards Institute (ANSI) – specifically the X3T9.5 committee

FDDI was developed to provide a robust, reliable backbone technology for groups of lower speed Local Area Networks (the predominant LAN technologies at that time were 10-Mbps Ethernet and 4/16-Mbps Token Ring)

While FDDI is a token ring protocol developed on the basic concepts found in 802.5; there are differences specified later due to its higher speed and design as a backbone technology (see Table 8.4)


Channel 19

Fiber Data Distributed Interface (FDDI)MAC Frame

The FDDI MAC frame comes in two basic formats: 1. The general frame format2. The token


Channel 20


The general frame fields:– Preamble: needed to synchronize the frame to the station’s clock;

composed of 16 idle symbols (64 bits)– Start Delimiter: 2 nondata symbols (coded as JK) denotes the actual

start of the FDDI frame– Frame Control: 8 bits indicating the actual type of frame (token, data,

etc.)– Destination Address: a unicast, multicast, or broadcast address that

may be 16 or 48 bits in length– Source Address: unicast 16 or 48 bit address specifying the source of

the frame– Information: the variable length data payload, usually a LLC PDU– Frame Check Sequence: a 32 bit CRC protecting the Frame Control,

Destination Address, Source Address, & Information fields– End Delimiter: one 4 bit nondata symbol indicating end of frame– Frame Status: contains the error detected, frame copied, & address

recognized symbols


Channel 21


Token Frame Format: Preamble: same as above Start Delimiter: same as above Frame Control: contains either the bit string 10000000

(unrestricted token) or 11000000 (restricted token) End Delimiter: contains two nondata symbols


Channel 22


Although the two FDDI MAC frame formats are very similar to 802.5, note several key differences:

– A longer preamble is required with FDDI to allow proper synchronization at the higher data rate

– Two different addressing schemes are available (16-bit and 48-bit) as with the IEEE 802 protocols but the schemes can be mixed in the same network

– FDDI doesn’t use explicit fields for reservations and priority; this functionality is handled in a much different way explained in detail later


Channel 23

Fiber Data Distributed Interface (FDDI)MAC Protocol

The basic operation of the FDDI protocol is similar to 802.5 with two key differences:

– A station with data to transmit ‘captures’ the token by absorbing it completely once it recognizes the token; once it is absorbed it begins to transmit one or more data frames

– A new token is generated as soon as the station is finished transmitting data frame (early token release)

Multiple frames can be in transit on the FDDI ring; it is up to the transmitting station to absorb its own frames when they return

The three Frame Status fields work just like the 802.5 equivalents

If a frame error is reported by any station the destination will ignore the frame and reports the error to the LLC sublayer; it is the responsibility of the LLC sublayer to recover from the error


Channel 24

Fiber Data Distributed Interface (FDDI)FDDI MAC Protocol Operation [Figure 8.6]


Channel 25

Fiber Data Distributed Interface (FDDI)Capacity Allocation

Like 802.5 and the Early Token Release option, with FDDI the station will typically issue a new token before its transmitted frame returns – this makes the use of a reservation field ineffective

The FDDI scheme is intended to allow more granular capacity control than 802.5 than meets two objectives:

– Support for a mixture of stream and bursty traffic– Support for a multiframe dialog

The first objective recognizes FDDI’s role as a backbone technology; the second to provide good support for SANs and server farms


Channel 26


FDDI defines two traffic types to accommodate both stream & bursty traffic

– Synchronous: Each station can be allocated a fixed portion of the total ring bandwidth; frames transmitted during this period are synchronous frames

– Asynchronous: Any bandwidth that is not allocated or allocated but not used is available for the transmission of additional frames; frames transmitted during this period are asynchronous frames


Channel 27


Capacity Allocation Operation (Quality-of-Service)– Every station will be allocated bandwidth on every target token

rotation time (TTRT) which is defined & stored by each station– Each station may request a synchronous allocation (SA), i.e. for

every rotation cycle; when initialized each station has a SA=0 Stations request a nonzero SA using a station management frame Support for synchronous transmission is optional, i.e. if there is

residual capacity; a station may only send and support asynchronous frames

Assure the time between successive sightings of a token is on the order of TTRT or less

– The time allocations must satisfy the following inequality: Pring + Fmax + Ttoken + SAi <= TTRT

where Pring is ring propagation time, Fmax maximum frame transmission time, Fmax is token transmission time.

– In addition to TTRT & SA, each station tracks three more values: Token-rotation timer (TRT) Token-holding timer (THT) Late Counter (LC)


Channel 28

Fiber Data Distributed Interface (FDDI)Capacity Allocation Operation

Operation (continued)– At initialization the TRT = TTRT and LC = 0 and when the timer

is enabled TRT begins to count down– If (TRT = 0 and LC = 0), LC=1 and TRT is reset to TTRT– If (TRT = 0 and LC = 1) (expires for the second time) then LC =

2 and this station considers the token lost and initiates token recovery (the claim procedure)

– If a token is received before TRT = 0, the station transmission capability depends on whether or not it receives the token early (LC = 0 and TRT > 0)

Early Token: If the token is early then the station sets THT = TRT, set TRT = TTRT, and enables TRT again. At this point the station can transmit synchronous frames for time SAi and after that can transmit asynchronous frames for time THT

If the token is received late, the station resets LC = 0 and can only transmit synchronous frames for time SAi


Channel 29

Fiber Data Distributed Interface (FDDI)Capacity Allocation Operation

Operation (continued)– These procedures are intended to maintain an actual token

rotation time in the neighborhood of TTRT, though for short intervals the actual rotation time can exceed TTRT


Channel 30

Capacity Allocation Example


Channel 31

Fiber Data Distributed Interface (FDDI)Capacity Allocation Operational Example


Channel 32


More Capacity Allocation Details– Asynchronous Traffic Prioritization

Another eight priority levels can be defined which allow further prioritization of asynchronous traffic

A set of eight timer values Tpr(i); one for each priority value and each specifying the maximum time the token can take to rotate around the ring and still allow priority i frames to circulate

If the token arrives early then any asynchronous data of priority i can be transmitted as long as Tpr(i) > THT


Channel 33


More Capacity Allocation Details– Use of a Restricted Token

To allow stations to enter an extended data transfer (like a network write to a remote disk) after transmission of its first frame a station issues a restricted token

The restricted token essentially dedicates all asynchronous capacity to that station for an extended period

Such operation does not affect the transmission of synchronous frames using the previously defined rules


Channel 34

Fiber Data Distributed Interface (FDDI)Token Recovery and other Maintenance Procedures

Three maintenance processes are key to proper FDDI ring operation:– Claim token process– Initialization process– Beacon Process


Channel 35


Claim token process– A station enters this process when it sets it’s LC = 2 (i.e. -- it

believes the token has been lost)– At this time the station begins to issue Claim frames

These are used to decide which station initializes the ring & what value of TTRT all stations will use

– Each station begins to examine incoming Claim frames


Channel 36


Claim token process (continued)– Each station analyzes incoming Claim frames and determine

whether to continue to transmit (or defer) based on meeting the following criteria:

A Claim frame with a lower TTRT has precedence Claim frames with 48 bit addresses have precedence over ones

with 16 bit addresses Given equal TTRT values and address sizes, the Claim frame with

the highest address value has precedence If these criteria are met the station will continue to transmit its

Claim frame while absorbing incoming Claim frames– Eventually all but one station will defer and once that final

station receives back its Claim frame (telling it that it is the only station transmitting) it will initialize the ring. All stations will use the TTRT value advertised in the final Claim frame


Channel 37


• Initialization process• After ‘winning’ the Claim process the station will issue an

unrestricted token• The token cannot be captured on its first rotation; the first

appearance of the token signals that the ring is transitioning to the operational state and allows each station to reset it’s value of TRT


Channel 38


• Beacon process• The beacon process is used to isolate a major fault in the

ring (e.g. – a break)• If the Claim token process is initiated but does not come to

resolution (the ring is broken so the Claim process is not successful) then the station will enter the beacon process

• During the beacon process a station will begin to transmit beacon frames• A station will stop transmitting the beacon frames as soon as it

receives a beacon frame from its upstream neighbor• This process leads to only one station on the frame

transmitting beacon frames – the one immediately downstream from the fault


Channel 39

Fiber Data Distributed Interface (FDDI)FDDI Physical Layer Specifications

Physical Topology– Original design ran over multi-mode fiber optic links; variants for

operation over copper & single mode fiber have been added– The FDDI topology specifies a ring operating at 100-Mbps though

it actually has two counter-rotating rings; the second ring can be used as a backup ring or to double the potential bandwidth

Faults in the ring will not affect a majority of stations attached to the ring because the ring will ‘wrap’ itself around the fault

If the ring is cut multiple times each segment will reconfigure itself into smaller rings that continue to communicate

DAS

DAS

DAS

DAS

DAS

DAS

DAS

DAS

FDDIRing

FDDI RingFault


Channel 40

Fiber Data Distributed Interface (FDDI)FDDI Physical Layer specifications

FDDI has provisions for four different types of devices allowing construction of complex hierarchical topologies

– The ‘core’ of an FDDI has to be a ring; stations that connect fully to both counter-rotating rings are called Dual Attached Stations (DAS)

– Dual Attachment Concentrators (DAC) allow the connection of Single Attached Stations (SAS), which connect to only one of the two rings

– Tree hierarchies can be constructed off the ring using Single Attached Concentrators, allowing multiple levels of Single Attached Stations to connect to the core ring

– Dual homing can be done in the trees by using a DAC or a DAS in the tree structure; four different FDDI port types exist & help enforce which topologies are legal and which are not [Table 8.6 – fifth edition]


Channel 41


FDDI topology example

DAS

DAS

DAC

DAC

SAC

SAS SAC

SASSAC

SACSAC

SAC

SAC


Channel 42


Physical Layer Encoding– Over fiber optic cabling (usually multimode) FDDI uses the

4B/5B and NRZ-I encoding schemes described earlier for Fast Ethernet

– For FDDI over twisted pair (shielded or Category 5 unshielded) MLT-3 encoding is used


Channel 43

Practice Problem # 1

Q: Consider a CSMA/CD network running at 1 Gbps over a 1 km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?

A:– For a 1 km cable, the one-way propagation time is 5 msec or 2 = 10

msec. Shortest frame should take more than this time to transmit to allow the sender to identify any collisions in the worst case.

– At 1Gbps, the number of bits that should be transmitted during 10 msec = 10,000 bits = 1250 bytes.

– Thus, the frame should not be shorter than 1250 bytes.


Channel 44


Q:A 4-Mbps token ring has a token holding timer value of 10 msec. What is the longest frame that can be sent on this ring?

A:– At 4 Mbps, a station can transmit 40,000 bits or 5000 bytes in 10

msec. – This is an upper bound on frame length. – From this amount, some overhead bytes must be subtracted, giving

a slightly lower limit for the data portion.


Channel 45


Q:At a transmission rate of 5 Mbps and a propagation speed of 200 m/sec, to how many meters of cable is the 1-bit delay in a token ring interface equivalent?

A:– At 5 Mbps, a bit time is 200 nsec. – In 200 ns, the signal travels 40 m. – Thus, insertion of one new station adds as much delay as insertion of

40 meters of cable.


Channel 46


Q: A very heavily loaded 1-km long, 10 Mbps token ring has a propagation speed of 200 m/sec. There are 50 stations uniformly spaced along the ring. Data frames are 256 bits, including 32 bits of overhead. Acknowledgements are piggybacked onto the data frames are are thus included as spare bits within the data frames and are effectively free. The token is 8 bits. Is the effective data rate of this ring higher or lower than the effective data rate of 10 mbps CSDM/CD network?

A:– Measured from the time of token

capture, it takes 25.6 sec to transmit a packet.

– Additionally, a token must be transmitted, taking 0.8 sec

– Token must propagate 20 meters taking 0.1 sec.

– Thus we have sent 224 bits in 26.5 sec, which results in an effective data rate of 8.5 Mbps. This is more than the effective bandwidth for the Ethernet (4.7 Mbps(why?)) under the same parameters.


Channel 47


Q:Ethernet frame must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64 byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?

A:The maximum wire length in Fast Ethernet is 1/10 as long as in the regular Ethernet.


Channel 48


Q:A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring?

A:– With a rotation time of 40 msec and 100 stations, the time for the

token to move between stations is 40/100=0.4 msec. – A station may transmit for 10 msec, followed by a 0.4 msec gap

while the token moves to the next station. – The best case efficiency is then 10/10.4=96%.

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