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Often called layer 2 protocols, data link protocols exist in the protocol layer just above the physical layer relative to the OSI protocol model. Data link protocols provide communication between two devices. Because there are many different ways to connect devices, there are many different data link protocols. The defining factors are

• Dedicated point-to-point links between two devices, such as modem, bridges, or routers


• Shared media links in which multiple devices share the same cable (i.e., Ethernet LAN)

The PPP (Point-to-Point Protocol) that people use to connect to the Internet via a dial-up modem is an example of a data link protocol. Because the link between two systems is point to point, the bits are always delivered from sender to receiver in order. Also, unlike shared-media LANs in which multiple stations attempt to use the network, there is no contention.

Data link protocols may provide any of the following services:

• Framing    Data is broken up into frames that are transmitted as independent units. If errors are detected in a frame, it is only necessary to retransmit that frame.


• Session setup and termination    For reliable services, session control messages are used by end systems to exchange status information about the session.


• Error detection    Determines whether a frame has been delivered accurately. A checksum is calculated on a frame by the sender, and the receiver must perform the same calculation and come up with the same checksum. If not, the frame is considered corrupted. If reliable services are being used, the frame is retransmitted. For nonreliable services, the frame is dropped and upper-layer protocols are relied on to handle the problem.


• Addressing on a multipoint medium such as a LAN    A computer's address is usually the hardwired address of the NIC (network interface card).


• Flow control    A technique that prevents the sender from sending more, overflowing the receiver with more data than it can handle.

Reliable services were essential in the days of dumb terminals that did not have the capabilities to perform error or frame checking. Today, end systems have their own processing power, so reliability services are not essential in the network itself. Instead, reliable services are executed on end systems. TCP is a reliable transport layer protocols that can replace network- level reliability services.