116 Communication Networks
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The trailer provides a CRC-style Frame Check Sequence FCS. CSMACD imposes a minimum frame size which in turn translates to a minimum data field size.
Should the data field be shorter than required, it is padded with enough octets to achieve the required minimum.
Figure 9.101 CSMACD MAC frame structure.
Field Description
Preamble Special bit pattern for synchronization purposes.
Start Delimiter Marks the beginning of frame.
Addresses Source and destination addresses.
Length Denotes the length of the LLC data unit in octets.
Data Actual user data i.e., LLC PDU.
Padding Appended to the LLC data unit to ensure minimum length.
FCS Frame Check Sequence.
9.2.3. Token Bus
The token bus protocol is specified by the IEEE 802.4 and ISO 8802.4 standards. The logical ring is determined by the descending numeric order of the station
addresses. When a station takes possession of the token, it is given exclusive network access for a limited period of time. The station either transmits during this
time window or hands over the token to its successor. The token holder can also poll other stations in order to learn about their status.
The protocol provides for token-related fault handling. When a station passes the token to its successor, it listens for transmissions on the bus for a period of time. A
transmission would be a sign that the successor has successfully received the token and is either passing it on or is transmitting data. If the token sender establishes that
the token has not been received after two attempts, it will attempt to bypass the presumably faulty station. To do this, it polls other stations to find out who is the next
logical successor and will attempt to pass the token to that station.
The general structure of a token bus frame is shown in Figure 9.102. The beginning and end of the frame are marked by two delimiting octets. The Frame
Control field is used for building different types of frames e.g., token frame, polling frame. For some frames e.g., the token frame the Data field may be completely
empty. The Preamble, Addresses, and FCS fields are as in CSMACD.
Figure 9.102 Token bus MAC frame structure.
Field Description
Preamble Special bit pattern for synchronization purposes.
Start Delimiter Marks the beginning of frame.
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Frame Control field.
Addresses Source and destination addresses.
Data Actual user data e.g., LLC PDU.
FCS Frame Check Sequence.
End Delimiter Marks the end of frame.
9.2.4. Token Ring
The token ring protocol is specified by the IEEE 802.5 and ISO 8802.5 standards. The most well-known realization of this protocol is the IBM token ring product.
The token ring protocol operates largely as described in section 9.1.1. A time limit is imposed on each station for holding the token and transmitting on the ring. The
protocol includes a scheme for handling priority traffic, and for a station to assume a monitoring role to keep an eye on the status of the network. It is also possible to
bypass an inactive or faulty station without disrupting ring traffic.
The general structure of a token ring frame is shown in Figure 9.103. The shaded fields appear in data frames but not in a token frame. As before, the start and end of
the frame is delimited by two special octets. Addresses, Data, and FCS fields are as before. The Access Control field is an octet divided into four components:
•
A token bit flag to indicate if this is a token frame.
•
A monitor bit flag set by the monitor station during recovery.
•
A three-bit priority field which can indicate up to eight token priority levels. Only a station which has a frame of equal or higher priority can gain control of
the ring.
•
A three-bit reservation field is used for implementing a reservation scheme for handling priority traffic. When a frame i.e., busy token is passing through a
station which has a frame waiting to be transmitted, it can raise the reservation field value to the value of the priority of the frame. When the station finally gets
hold of the token and transmits its frame, it restores the previous reservation value. This scheme ensures that higher priority frames have a better chance of
being transmitted first, without totally denying lower-priority frames an opportunity to be also transmitted.
Figure 9.103 Token ring MAC frame structure. Field
Description
Start Delimiter Marks the beginning of frame.
Access Control Various access-related flags and values.
Frame Control Denotes the frame type.
