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Nondeterministic Access

Nondeterministic media access control, places access control responsibilities on the individual stations. Also known as Carrier Sense Multiple Access (CSMA), it is most effective in low-traffic environments. There are two variations, CSMA/CD and CSMA/CA.

Carrier Sense Multiple Access (CSMA) is a decentralized, contentious media access control method used in Ethernet and other bus-oriented LANs. Each of multiple stations, or nodes, must sense the carrier to determine network availability before access to the medium to transmit data. Further, each station must monitor the network to determine if a collision has occurred. Collisions render the transmission invalid and require retransmission. In the event of a busy condition, the station will back off the network for a calculated random time interval before attempting subsequent access.

CSMA works much like an old telephone party line, where there are multiple subscribers with individual logical addresses (in the form of telephone numbers) connected to a single physical circuit. When placing an outgoing call, the subscriber must pick up the telephone to monitor the line for a short period of time in order to sense the level of activity. If there is no activity, a call can be placed. If there already is a call in progress, another call attempt cannot be made without causing interference (and hard feelings). Rather, the telephone must be hung up. Subsequent random monitoring of the circuit can take place in order to determine its availability. Incoming calls are addressed to each party on the party line by varying the number of rings to indicate the unique logical address of each party sharing the circuit. Only the target party could answer the call without creating ill will. In other words, a set of media access control protocols were developed to govern the manner in which the circuit was managed to the satisfaction of all parties.

The above CSMA approach is known as Nonpersistent CSMA. Variations include 1-Persistent CSMA and P-Persistent CSMA. CSMA is implemented in two standard means, CSMA/CD and CSMA/CA. In either case, latency and throughput degrade under heavy loads of traffic. For example, an Ethernet network running at a theoretical speed of 10 Mbps typically provides about 4 to 6 Mbps throughput. While it is less costly than Token Ring networking, it also delivers less efficient use of bandwidth [9-11].

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is the most common media access control method used in bus networks. In an Ethernet environment, for instance (see Figure 9.6), the transmitting station sends a data packet in both directions of the bus, appended with a preamble (8 octets) for purposes of synchronization, destination (6 octets) and source (8 octets) addresses, and a type field (2 octets) identifying the higher layer protocol used in the data field (46 to 1500 octets). A Frame Check Sequence (FCS) consisting of a 32-bit CRC serves for error control [9-12]. Each transceiver (transmitter/receiver) of each station along the way reads the address prepended to the data packet. Should the address match, the transceiver provides the packet to the target device. Should the address not match, the transceiver forwards the packet to the next transceiver. Should a node detect a data collision, that station sends a brief jamming signal over a subcarrier frequency of the network to advise all stations of the collision. At that point, all devices back off the network, calculating a random time interval before attempting a retransmission.


Figure 9.6  CSMA/CD, with Ethernet frame format.

Carrier Sense Multiple Access/Collision Avoid (CSMA/CA) includes a priority scheme to guarantee the transmission privileges of high-priority stations. CSMA/CA requires a delay in network activity after each transmission is completed. That delay is proportionate to the priority level of each device, with high-priority nodes programmed for short delays and with low-priority nodes programmed for relatively long delays. As collisions may still occur, they are managed either through Collision Detect or through retransmission after receipt of a Negative Acknowledgment (NAK). CSMA/CA is more expensive to implement, as it requires that additional programmed logic be embedded in each device or NIC. CSMA/CA does, however, offer the advantage of improved access control, which serves to reduce collisions and, thereby, improve the overall performance of the network.

LAN Equipment

In addition to the attached devices (also referred to as nodes or stations), LANs may make use of other devices to control physical access to the shared medium, to extend the maximum reach of the LAN, and to switch traffic. Such hardware is in the form of NICs/NIUs, transceivers, MAUs, hubs, bridges, routers, and gateways. As is the case with much of the technology addressed in this book, the lines blur between these devices and their individual functions. Therefore, the focus will be on the classic definitions, expanding on those concepts and illustrating multifunctional devices as appropriate.

Network Interface Cards (NICs)

Also known as Network Interface Units (NIUs), NICs are printed circuit boards that provide physical access from the node to the LAN medium. The NIC can be fitted into the expansion slot of a PC, or it can exist as a separate box. A standalone, multiport NIC can serve a number of devices, thereby providing an additional level of contention control. A standard IEEE NIC contains a unique, hard-coded logical address. Transceivers are embedded in NICs/NIUs and MAUs. MAUs (Media Access Units, or Multistation Access Units) are standalone devices which contain NICs in support of one or more nodes.


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