Link Layer

Added 26 Jul 2008

TCP/ IP is a four- layer protocol. The lowest level, the link layer, is implemented within the network adapter and its device driver. Like all the TCP/ IP protocols, it is defined by standards. The standards for generic Ethernet- type networks are defined by the IEEE 802 Committee: for example, IEEE 802.3 for Ethernet networks, or IEEE 802.5 for Token Ring networks.

Other link layer protocols that could be used include Serial Line IP (SLIP) or Point- to- Point Protocol (PPP), which are used when connecting to a network over an asynchronous dial- up link.

Since Ethernet is the most common type of network, we will look at it in a bit more detail. The Ethernet protocol is designed for carrying blocks of data called frames. A frame consists of a header containing 48- bit hardware destination and source addresses (which identify specific network adapters), a 2- byte length field, and some control fields. There follows the data, and then a trailer which is simply a 32- bit cyclic redundancy check (CRC) field. The data portion of an Ethernet frame must be at least 38 bytes long, so filler bytes are inserted if necessary.

All this means that frames are at least 64 bytes long, even if they carry only one byte of user data: a significant overhead in some types of application.

Frames also have a maximum size. Less headers, the maximum size for an Ethernet frame is 1492 bytes, which is the maximum transmission unit (MTU) for Ethernet. All link layer protocols have an MTU. It is one hardware characteristic that the higher- level protocol needs to be aware of, because larger blocks of data must be fragmented into chunks that fit within the MTU and then reassembled on arrival at their destination.