Lesson 2

Selecting Data-Link Layer Protocols

1-13

The seven layers of the OSI model define functions that are implemented in various

networking protocols, such as Ethernet and TCP/IP. The functions corresponding to

the layers are as follows:

■

Physical The physical layer defines the nature of the network medium—the

actual fabric of the network that joins all the computers together—and the nature

of the signals transmitted over the medium. In most cases, the network medium is

a form of copper cable that uses electric currents for signaling, but fiber-optic and

wireless media are becoming increasingly prevalent.

■

Data-Link The data-link layer defines the interface between the network

medium and the software running on the computer. Among the data-link layer

functions are packet addressing (which allows computers to direct traffic to spe-

cific destinations on the local network); media access control (which allows mul-

tiple computers to share a single network medium without conflicting); and

formatting the frame used to encapsulate data for transmission over the network.

The data-link layer divides into two sublayers. The logical link control (LLC) sub-

layer controls elements such as error checking from node to node on the same

LAN, frame synchronization, and flow control. The media access control (MAC)

sublayer controls the movement of data packets to and from one network inter-

face card (NIC) to another across a shared channel.

■

Network The network layer defines the functions that provide end-to-end com-

munications between computers on different networks. Chief among these func-

tions is routing, which enables computers to relay traffic through intermediate

networks to a destination on a remote network any distance away. Other func-

tions include packet sequencing, end-to-end error detection from the sender to

the recipient, congestion control, and addressing. While the data-link layer is

responsible for local traffic on a single network, the network layer is responsible

for directing traffic to its ultimate destination.

■

Transport The transport layer provides functions that complement those of the

network layer, including guaranteed delivery (which uses packet acknowledg-

ments to ensure data is received), flow control (which regulates transmission

speed to avoid dropped packets), and end-to-end error detection (which enables

the receiving system to detect damaged packets).

■

Session The session layer provides many functions involved in the regulation of

the dialog between two computers communicating over the network. For exam-

ple, the session layer sets up, regulates, and terminates exchanges between the

applications at each end of the communication.

■

Presentation The presentation layer (sometimes referred to as the syntax layer)

is responsible for translating each computer’s native syntax into a common trans-

fer syntax readable by the other computers on the network. In some cases, the

transfer syntax can provide functions such as data compression and encryption.