Physical Link

See Figure  on page  .

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Figure: The Physical Link Layer

In the physical link layer, communication is from physical network address to physical network address and is broadcasted over a logical single-wire, network, hooked together with hubs and bridges with the aid of the Address Request Protocol, ARP.

Examples of technology at this level include ethernet, token ring, and FDDI. A hub makes many physical wires look like one logical wire. A bridge is like a hub but with knowledge about the physical topology of the network that is used to optimize performance.

Token rings and FDDI are very similar. Both use a token to manage ownership of the physical wire. Once a computer on the wire has the token, the computer is allowed to send data or pass the token to the next computer on the wire. In token ring the token is not released until the data has moved around the entire ring. In FDDI the token is released as soon as the data is on the fiber. In theory, the FDDI is more efficient than a token ring but in practice, the token in a FDDI is susceptible to being lost, leading to wasted cycles.

If a computer on an ethernet wishes to place data on the wire, it first listens to see if the wire is calm and thus, available. If the wire is available then the data is placed on the wire. There is a race condition that might develop. Simultaneously, two computers might think that the wire is available and begin to send data. The data will collide during mid transmission. Each computer is responsible to monitor the transmission, detect if this occurs, and resend the data at a later time.

Every network interface card has a vendor-supplied physical address. This is also called a hardware address or a Media Access Control, a MAC, address.

At the very lowest level of the network every packet on the network is a frame packet sending information from one MAC to another MAC. The MACs reachable on a network are discovered with the help of the ARP protocol.

See Figure  on page  .

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Figure: The Frame or MAC Packet

Each hub maintains a MAC table to help it manage the local packets.

See Figure  on page  .

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Figure: A MAC Table