IEEE 802 Reference model
IEEE 802 Reference model is a basic model that gives an outline of how a LAN needs to be.
It basically has 4 components and 3 versions.
The components are:
- Logical Link Layer (LLC)
- Medium Access Layer (MAC)
- Physical Layer
- Medium
Logical Link Layer
Logical link layer or LLC is the upper sub-layer of the Data link layer of the seven-layer OSI model. It acts as an interface between the network layer and the MAC sub-layer.
Some of the important functions of LLC are:
- Error recovery
- Flow control
- User addressing
Media Access Layer
This is the bottom sub-layer of the Data link layer. This along with the LLC complete the Data link layer.
The basic function of this sub-layer is to control access to the medium, that is, it decides as to which device will access the medium when.
Some of its basic functions are:
- Error detection
- Control of access to media
- Addressing of the stations in a network
Physical Layer
This layer comprises of the physical components of any network. It deals with the physical connectivity of two devices.
It comprises of wiring, cables, equipment, frequencies, pulses, etc. needed to communicate in the network.
It provides services and the transferred data to the data link layer.
Versions of IEEE 802 reference model
Some of the versions of IEEE 802 reference model:
- IEEE 802.3 MAC sublayer
- IEEE 802.4 token bus
- IEEE 802.5 token ring
IEEE 802.3 MAC sublayer
This version deals completely with the sublayer. It is concerned with the frames that are sent for communication between the data link layers.
The frame has seven fields that are listed below.
Preamble
This indicates the beginning of the frame. It informs the receiver data link layer about an incoming frame. Its size is 7 bytes.
Start Frame Delimiter (SFD)
It is a 1-byte long field that indicates to the receiver that after these 8 bits, the frame will begin.
Destination address
It includes a 6-byte long MAC address of the receiver or the destination of the frame.
Source address
This field has the 6-byte MAC address of the sender of the frame for replying purpose.
Length
This field indicates the length of the data that the frame has. It is 2 byte in size.
Data
This is the main data that the frame was intended to carry. Its size can be up to 1500 bytes.
CRC
This 4-byte field is solely for the purpose of identification of errors, if any, in the frame.
IEEE 802.4 Token Bus
This is a LAN topology whose physical topology resembles that of bus topology, but its logical topology is ring topology.
In token bus, a token is held by any one station. That station alone can transmit frames and the rest of the stations need to receive to the station with the token.
The station can transmit any number of frames it wants, but, only till the time it has the token. This time is called token holding time, or THT and is generally 10 msec.
Once THT is over, the station needs to stop its transmission and pass on the token to the next station. This station is decided by the address of the station.
The station with the next address is passed the token, thus forming a logical ring.
IEEE 802.5 Token Ring
IEEE 802.5 token ring follows ring topology, both logically and physically.
In this, each station has attached to it a ring interface unit or RIU. This RIU is responsible for transmitting the message to the ring. That is, it simply receives the message from the station and retransmits it to the ring.
Its working is similar to that of the token bus, but the difference lies in the way of passing the token.
The token is passed to the station that is placed next to the station holding the token physically. It thus forms the physical ring and the addressing is also done in order, therefore forming a logical ring too.
