Token Ring Frame Formats

There are two Token Ring frame formats defined in 802.5—a token frame with no data and a frame that contains data (that is, a busy token). A token frame with no data contains the following fields:

SD AC ED

A token data frame (a token carrying data) can contain the following fields:

SD AC FC DA SA RIF Data FCS ED FS

Notice that the Token Ring data frame format has more fields when compared to an Ethernet frame. This makes Token Ring a little more robust, as you can see in the following descriptions:

► SD (StartingDelimiter, 1 byte)—Indicates the start of the frame and is represented as JK0JK000 in hexadecimal. Don't worry too much about this field, but it merely is used to indicate any Manchester code violations.

► AC (Access Control, 1 byte)—Contains parameters that define the priority (1 bit that indicates whether the frame is a data or free token). A bit that indicates if the active monitor has seen the frame. The active monitor is a device on the ring that maintains the ring. The AC field is represented as PPPTMRRR, where the Pbits are priority bits, T"bits identify whether the frame is a token or data frame, Midentifies the monitor bit, and RRR specifies the reservation bits.

► ED (EndDelimiter, 1 byte)—Indicates the end of the frame. This field is set to JK1JK11E.

► FC (Frame Control, 1 byte)—Indicates which type of frame is arriving.

► DA (Destination Address, 48 bits)—Indicates the destination MAC address.

► SA (Source Address, 48 bits)—Indicates the source MAC address. If the first bit of the source address is set to 1, a routing information field (RIF) will be present.

► RIF (RoutingInformation Field)—Describes the routing information field, which can be up to 18 bytes in length.

► FCS (Frame Check Sequence, 32 bits)—Checks the FC, DA, SA, and Data fields.

► FS (Frame Status, 8 bits)—Indicates if the frame was recognized by another device and copied. This field is represented as AC00AC00, where A is set to 1 when the address is recognized and C is set to 1 when the frame is copied. The R bits are reserved. The A and C bits are copied, because there is no redundancy check or CRC made on this field.

An important fact is that Token Ring supports two broadcasts frame types—FF-FF-FF-FF-FF-FF and C0-00-FF-FF-FF-FF.

Figure 4.5 illustrates a variety of bit combinations and serves as a very handy diagram for troubleshooting Token Ring networks.

Token Ring has a number of built-in stations that monitor and maintain the ring. These stations enable the ring to recover from faults and error conditions, such as when there are no free tokens circulating a ring for an extended time. The stations ensure that the token is always available and report problems to network protocol analyzers, if they are present. Table 4.4 summarizes the functions performed by Token Ring stations.

Token Frame (No data)

Token Frame (No data)

• P bits indicate

• D bits are used

• If the E bit is set

• If the A bit is set

priority.

to identify

to 1, this frame

to 1, the address

• T bit indicates

whether the frame

will have a FCS

has been

whether the

is a LLC (01) or

error and must

recognized.

frame is a

MAC Frame (00).

be retransmitted

• C is set to 1 if

token (bit set

• The C bits indicate

by source device.

the frame has

to 0) or data

what type of

been copied.

frame if this

management frame

• Both A/C bits are

bit is set to 1.

the frame is:

duplicated,

• R bits are

0000 Express buffer

because the FCS

used for

0010 Beacon frame

does not cover

reservation

0011 Claim token

this field.

purposes.

0100 Ring purge

0101 Active monitor

0110 Standby

monitor present

Figure 4.5 Token Ring frame formats.

Table 4.4 Function performed by Token Ring stations.

Service

Description

Active Monitor

Can be any station on a ring. The main function is to provide timing information and maintenance functions. One of the main functions of the Active Monitor is to ensure that frames will not circulate the ring forever. A bit in the Token Ring frame called the monitor bit ensures that the frame will only circulate the ring once.

Standby Monitor

Can be any station on a ring. This station monitors the current active monitor and replaces it if it becomes available.

Ring Error Monitor

Typically a network analyzer. This monitor collects errors and other data seen on the Token Ring.

Beacon

A special frame that indicates a problem on the ring. A beacon is sent out by a by a station when a free token frame has not been seen for an amount of time.

Ring Purge

A recovery action performed by an active monitor in instances when a recovery of the ring is required.

Token Ring networks elect two devices called the active monitor and standby active monitor. Any device on the ring can perform this function. The function performed by the active monitor is basically to ensure the integrity of the ring and ensure no device is holding onto the token free frame forever. In case a token free frame is lost or corrupted, for example, the active monitor will clear or purge the ring and issue a new token. The standby active monitor waits for a failure on the active monitor.

If there's a token failure (such as a station waits for a token but does not see it for a specified frame or a faulty device does not release the token or continually sends data irrespective of other devices), a special frame advertises that there is no token available. This process is called beaconing. When beaconing occurs, the ring is down, and all stations will re-insert into the ring. Beaconing is not a desirable process for a network, because it indicates a possible hardware fault. During the beaconing process, a Token Ring station is unable to send data.

Let's now look at how a Token Ring station attaches, or inserts, itself into a ring using a procedure called the ring insertion process.

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