Ieee Local Area Network Standards; Ieee 802.4 Token Bus Local Area Networks; Token Bus Operation - Motorola MC68824 User Manual

1.2.2 IEEE Local Area Network Standards
The IEEE 802 standards body has specified a set of standards based on the lowest two layers of
the ISO/OSI model. In order to meet the diverse requirements of the user community, the 802
standards body has defined five different standards so far. These standards are:
1. IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMAlCD) on a Bus
Topology
2. IEEE 802.4 Token Bus
3. IEEE 802.5 Token Ring
4. IEEE 802.6 Metropolitan Area Network (MAN)
5. IEEE 802.9 Intergrated Voice Data LAN Access Method
The IEEE standards do not directly correspond to the ISO/OSI model as shown in Figure 1-1. IEEE
breaks the data link layer into the media access control (MAC) and the logical link control (LLC)
sublayers (see Figure 1-2). The MAC layers are specified by working groups 802.3, 802.4, 802.5,
802.6, and 802.9. The LLC sublayer is specified by 802.2. IEEE has other supporting working groups
which consist of network management and interworking (802.1), broadband advisory (802.7), and
fiber optic advisory (802.8).
802.1 MANAGEMENT
802.2 LLC
I
BBBBB
Figure 1·2. IEEE Standard Model
1.3 IEEE 802.4 TOKEN BUS LOCAL AREA NETWORKS
DATA LLC
LINK
LAYER MAC
PHYSICAL
LAYER
The following paragraphs briefly describe the token bus operation and the options outlined in
the IEEE 802.4 standard.
1.3.1 Token Bus Operation
The IEEE 802.4 token passing bus standard is a deterministic protocol. This means that the user
is guaranteed access to the network on or before certain predefined intervals. The predefined
intervals are dependent on network characteristics such as number of stations and token hold
times within each station. A station is only allowed to transmit onto the network when it holds
the token. A token is an 8·bit pattern in the frame control portion of the frame. There is only one
token on a network. This token is passed from the station with the highest address to the station
having the next highest address and so on, with the lowest addressed station passing the token
back to the highest addressed station. This circular passing of the token forms a logical ring.
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