Local-Area Networks (LANs)

The topology of the LAN is determined by the technology. (For example, a ring topology is implemented by Token Ring or FDDI, and a star or tree topology is implemented by Ethernet.)

Token Ring

Token Ringis a technology developed by IBM and standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.5 committee for implementation in a LAN environment. Token Ring uses a special frame, called a token, to designate the authoritative speaker for that LAN segment. This technology can connect up to 255 nodes in a physical star or ring connection that can sustain 4 or 16 Mbps. Each node on a Token Ring LAN connects to a central wiring hub called the multistation access unit (MAU) using a twisted wire cable, such as UTP.

Token Ring is more deterministic than Ethernet, which means that it ensures that all users get regular turns at transmitting their data. With Ethernet, all users have to compete for network access to get on to the network. In a Token Ring network, a token is passed around the network from one workstation to the next, giving each workstation equal access to the network. Unlike an Ethernet workstation, which can send data if the line is idle, a Token Ring workstation cannot send data across the network unless it is in possession of the token.

FDDI

Fiber Distributed Data Interface, or FDDI (pronounced "fiddy"), is a LAN and metropolitan-area network (MAN) access method. It is a token-passing network, similar to Token Ring, and uses optical fiber cabling to transmit at 100 Mbps up to 10 kilometers. FDDI provides network services at the same OSI model layers as Ethernet and Token Ring (Layer 1 and Layer 2).

FDDI provides the option of a dual counter-rotating ring topology. This dual-ring topology is used for redundancy so that if one ring fails the other ring carries the traffic. Traffic on these rings travels in opposite directions: The traffic on one ring travels clockwise, whereas the traffic on the other ring travels counterclockwise.

Ethernet

Ethernet is the most widely deployed LAN access method, defined by the IEEE as the 802.3 standard. Ethernet has become popular such that a specification for a LAN connection or network card implies the use of Ethernet even if not explicitly stated. A 10/100 Ethernet port supports both 10BASE-T at 10 Mbps and 100BASE-T at 100 Mbps.

Ethernet is often considered to be a shared-media LAN, which means that all stations on the segment share the total bandwidth?10 Mbps (Ethernet), 100 Mbps (Fast Ethernet), or 1000 Mbps (Gigabit Ethernet). When Ethernet is deployed in a switched environment, it is no longer considered to be shared. Therefore, each sender and receiver pair has the full Ethernet bandwidth available for use.

Ethernet uses carrier sense multiple access collision detect (CSMA/CD) technology, broadcasting each frame onto the physical medium (wire, fiber, and so on). All stations attached to the Ethernet listen to the line for traffic, and the station with the matching destination MAC address accepts the frame and checks for errors before doing anything further with the frame. If the frame is error free, it is handed to the network layer (Layer 3) of the OSI model and ultimately the data is presented to the user, such as an e-mail. If the frame has errors, however, it is discarded.

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