Better to know some
... than all
IEEE 802 Standards
The IEEE 802 series of recommendations provide a widely-accepted set of LAN standards. These recommendations are formulated by nine subcommittees.
LAN recommendations. These are separately discussed below.
Logical Link Control
LLC is specified by the IEEE 802.2 and ISO 8802.2 standards. It provides link services to LAN users, independent of the MAC protocol involved. LLC offers three types of service:
* Unacknowledged connectionless service. This service must be provided by all 802.2 implementations. It is based on data being transferred in independent data units, the delivery of which is neither guaranteed, nor acknowledged. Furthermore, there are no provisions for ordered delivery of data units or for flow control. Obviously, a higher-level protocol is needed to make this service reliable.
* Connection-oriented service. This service is based on the use of logical connections. Data is transferred using ordered, acknowledged, and flow controlled data units. Transmission errors are detected and reported.
* Acknowledged connectionless service. Same as the unacknowledged connectionless service, except that the delivery of each data unit is acknowledged before the next data unit is sent. The LLC service is provided through a set of service primitives.
LLC also uses two primitives to communicate with the MAC layer. MA-UNITDATA is used to transfer data units between LLC and MAC. MA-DATA-UNITSTATUS is used for acknowledgments. The header (Source and Destination addresses and Control field) are appended to user data by LLC before the PDU is passed onto the MAC layer. The addresses identify peer LLC users.
The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol is specified by the IEEE 802.3 and ISO 8802.3 standards. CSMA/CD is based on the widely-publicized and well-adopted Ethernet specification, and offers data rates in order of 10 mbps using the baseband or the broadband technique.
It consists of a Data field (which is an LLC PDU) with a header and a trailer added at either end. The header provides synchronization, addressing, and length information. The trailer provides a CRC-style Frame Check Sequence (FCS). CSMA/CD 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.
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 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 CSMA/CD.
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. 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 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.
The Frame Control field specifies the frame type (i.e., whether it is an LLC PDU or some kind of MAC control frame). The Frame Status field is used by the receiving station to indicate that it has successfully received the frame and extracted its data, so that when the frame reaches the originator, it can be disposed of.