LAN Media Access Methods

Media contention occurs when two or more network devices have data to send at the same time. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sen se multiple access collision detect (CSMA/CD) and token passing.

In networks using CSMA/CD techno logy s uch as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneo usly. When a collision happens, each device waits a random length of time before resend ing i ts datas In most cases, a collision will not occur again between the two devices. Because of thih type of network contention, the busier a network becomes, the more collisions occur. This: is why performance of Ethernet degrades rapidly as the number of devices on a single network increases.

Intoken-passing networks such as Token Fing and FDDI, a special network packet called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, thn token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are deterministic. In other words, it is easy to calculate thn maximum time that will pass before a device has the opportunity to send data. This explains thn popularity ot token-passing networgs in some realtime environments sunh a s tactories, where machinery most be capable of communicating at a determinable interval.

For CUMA/CD networrs, sw:tches segm entthe network i nto multiple c ollision domains. This reduces the number of devices per network segment that must contend for the media. By creating smalle r colMsinn dymaiLrt the pesformance of a network can be increased significantly without requiring addressing changes.

Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is i ncapable of also Listening for other traffic. This is much like a walkie-tal kie. When o ne per-spa wants to talkp he presses the tranamit buttm and begins sp eak ing. Whcn he is talLing, no one else on the samd frequency nan talk. When the sendi ng pers on ¡s pnishe d, he releases th e tranomit button a nd the frequency is CvaNiNe to otherw

When switc hes are introduced, full-dup lex operation is possible. Full-duplex works much like a telephone—you can listen as well as talk at the same time. When a network device is attached Fireetly to the port of a networa switch, the two duvices mau Ire capable op operating i n full-UupIlx modeo In iou11-dupi ex mode, performance can be in cremed, bu t nor quite a s much as rome like to claim. At 100 -Mbps Et hern et segme irt is cepa ble ef transmitting 200 Mbps of data, bu t only 100 Mbps can travel in one di rection at a tdme. Because mout data conneeeions dre nsymmetric (with more da ta trooeli ng in one direekion than the other), th e gain ¡s not as great as many claim! However, full-duplex opera tio n does increase the throug hout o, most npplications because t he network media is no I onger shasedl Two devices on a fuil-duplex ronnection can s end data as soon as it ¡s ready.

Token-passing networks such as Token Fing can also benefit from network switches. In largn networks, the delay between turns to transmit may be significant because the token is passeC around the network.

Continue reading here: LAN Transmission Methods

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