17.5 LAN STANDARDS

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There is a wide variety of LANs—different topologies, different transmission media, different data rates, and so forth. The Institute of Electrical and Electronics Engineers (IEEE) set up a committee known as the 802 committee to develop various LAN standards. These standards together are known as IEEE 802 standards. These standards address only the physical and datalink layers of LANs. They specify the protocols to be used in MAC and LLC sublayers, the physical layer specifications, and the physical medium to be used.

The IEEE 802 committee formulated various LAN standards. These standards address only the physical and datalink layers of LANs. Note that the standards also specify the physical medium to be used in different LANs.

17.5.1 IEEE 802.2 Standard

IEEE 802.2 standard specifies the LLC sublayer, which provides the following services:

Unacknowledged connectionless service (type 1 service): This is a datagram service. There will be no flow control and no error control. Higher layers have to take care of these issues.

The IEEE 802.2 standard specifies the LLC sublayer specifications. This sublayer specification is common to all IEEE standards-based LANs.

Connection-mode service (type 2 service): A logical connection will be set up, and flow control and error control are provided.

Acknowledged connectionless service (type 3 service): This is a datagram service, but with acknowledgements.

This LLC layer is common to all the other IEEE standards–based LANs.

17.5.2 IEEE 802.3 Standard

Based on the popularity of Ethernet, IEEE released a compatible LAN standard that is specified in IEEE 802.3. LANs based on the 802.3 standard have the following characteristics:

Topology: Bus, tree, or star
MAC sublayer: CSMA/CD
Physical layer can be one of the following:
- Baseband coaxial cable operating at 10Mbps
- Unshielded twisted pair operating at 10Mbps or 100Mbps
- Shielded twisted pair operating at 100Mbps
- Broadband coaxial cable operating at 10Mbps
- Optical fiber operating at 10Mbps

IEEE 802.3 operating at 10Mbps has six alternatives:

10 BASE 5: 10Mbps baseband 500 meter segment length
10 BASE 2: 10Mbps baseband, 200 meter segment length
10 BASE T: 10Mbps baseband, twisted pair
10 BROAD 36: 10Mbps broadband, 3600 meter end-to-end span (1800 meter segment)
10 BASE F: 10Mbps baseband, fiber
1 BASE T: 1Mbps baseband, twisted pair (now obsolete)

In addition, IEEE 802.3 specifies 100Mbps LAN (fast Ethernet) (known as 100 BASET).

The format of the MAC frame in IEEE 802.3 standard is slightly different from that of the Ethernet frame. The IEEE 802.3 MAC frame format is shown in Figure 17.6.

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Figure 17.6: Frame for IEEE 802.3 standard.

Preamble (7 bytes): The bit pattern 010101… is sent for the receiver to establish synchronization.

SFD (1 byte): Start frame delimiter 10101011 to indicate the actual start of the frame. This enables the receiver to locate the first bit of the rest of the frame.

The IEEE 802.3 standard is based on the popular Ethernet LAN. The MAC frame formats of Ethernet and IEEE 802.3 are slightly different.

DA (2 or 6 bytes): Destination address. 48 bits or 16 bits (must be the same for a particular LAN). It can be a node address, group address, or global address.

SA (2 or 6 bytes): Source address—address of the node that sent the frame.

Length (2 bytes): Length of the LLC data field.

LLC data (variable): Data from the LLC layer.

Pad (variable): Bytes added to ensure that frame is long enough for proper operation of the collision detection scheme.

FCS (4 bytes): Frame check sequence is calculated based on all the bits except the preamble, SFD, and FCS (32 bits).

17.5.3 IEEE 802.4 Standard

IEEE 802.4 standard–based LANs have the following characteristics:

Topology: Bus, tree, or star
MAC sublayer: Token bus
The physical layer can be one of the following:
- Broadband coaxial cable at 1, 5 or 10Mbps
- Carrier band coaxial cable at 1, 5 or 10Mbps
- Optical fiber at 5, 10, or 20 Mbps

17.5.4 IEEE 802.5 Standard

IEEE 802.5–based LANs have the following characteristics:

Topology: Ring
MAC protocol: Token Ring
The physical layer can be one of the following:
- Shielded twisted pair at 4 or 16Mbps, maximum number of repeaters is 250
- Unshielded twisted pair at 4Mbps, maximum number of repeaters is 72

Because the topology of this LAN is ring, Token Ring protocol is used for MAC.

IEEE 802.5 MAC protocol: A small frame called a token circulates when all the nodes are idle. The node wishing to transmit seizes the token by changing one bit in the token and transforming it into a start-of-frame sequence for a data frame. The node appends the data to construct the data frame. Since there is no token on the ring, all other nodes only listen. The data frame transmitted by the node makes a round trip and returns to the originating node. The node will insert a new token on the ring when it has completed transmission of the frame or the leading edge of the transmitted frame has returned to the node.

Advantages of this MAC protocol are that it provides a flexible control to access the medium and is efficient under heavy load conditions. However, the disadvantages are that maintenance of the token is a problem: if the token is lost, the ring does not operate—so one node acts as monitor. This protocol is inefficient for light load conditions. The following improvements can be made to the Token Ring protocol:

Token Ring priority: Optional priority field and reservation fields in data frame and token (three bits and hence eight levels) are included. A node can transmit if its token priority is higher than the received one (set in the previous data frame). To avoid one or more nodes to having the highest priority all the time, the node that raises its priority in one token must lower its priority subsequently.

Early token release: For efficient ring utilization, a transmitting node can release a token as soon as it completes frame transmission, even if the frame header has not returned to the node.

An IEEE 802.5 LAN is based on ring topology. Hence, the MAC protocol is the Token Ring protocol. A small frame called a token circulates around the ring when the nodes are idle. A node wishing to transmit will seize the token and transmit its data. Since there is no token on the ring, all other nodes cannot transmit.

17.5.5 IEEE 802.12 Standard

IEEE 802.12–based LANs have the following characteristics:

Topology: Ring
MAC: Round-robin priority
Physical layer: Unshielded twisted pair operating at 100Mbps

17.5.6 FDDI LAN

FDDI (fiber distributed data interface)-based LANs have the following characteristics:

Topology: Dual bus
MAC Protocol: Token Ring
Physical layer can be one of the following:
Optical fiber operating at 100Mbps, maximum number of repeaters is 100, and the maximum distance between repeaters is 2 km.
Unshielded twisted pair at 100Mbps, maximum number of repeaters is 100, and maximum distance between repeaters is 100 meters.

In a fiber distributed data Interface (FDDI) LAN, the MAC protocol is similar to that of IEEE 802.5 except that early token release strategy is followed.

FDDI MAC protocol: It has the same functionality as the IEEE 802.5 MAC protocol except that in 802.5, a bit in the token is reserved to convert it into a data frame. In FDDI, once a token is recognized, it is seized, and the data frame is transmitted. This is done to achieve high data rate support. In FDDI, early token release is followed: the token is released after transmitting the data frame without waiting to receive the leading bit of the data frame.

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