Page 105 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes
Figure 7.1: Communication Channel between Two Hosts

Host (1) Sender Host (2) Receiver
Application Application
Presentation Presentation
Session Session
Transport Transport
Network Network
Data Link Data Link
Physical Physical

Briefly, the data-link layer provides transfer of a datagram across a communication channel
between two adjacent machines. Its main tasks, which will be described in this unit, are framing,

checksums, error detection and correction, acknowledgement, flow control, well-defined reliable

service interface to the network layer, encapsulating packets from network layer to frames, etc.

There are many different types of link-level technologies that can be used to connect two nodes
or machines. Examples of link-layer protocols are Ethernet, token ring, FDDI, and PPP.
7.1 Data Link Layer Design Issues

7.1.1 Services Provided to Network Layer

The data link layer provides well-defined interface to network layer, handles transmission
errors, regulate flow of data and keeps sender and receiver in harmony by offering the following
functionalities:
To provide a well-defined and reliable service interface to the layer 3 or network layer that will
also be dependent on the efficiency and error rate of the underlying physical layer. The data link
layer accomplishes these activities in the following manner:
(a) Unacknowledged connectionless service: This involves independent frames from source host
to the destination host without any acknowledgment mechanism. It does not include any
connection setup or release. It does not deal with frame recovery due to channel noise.

(b) Acknowledged connectionless service: Communication channel is more error prone. This
necessitates acknowledgement service for each frame sent between two hosts to ensure
that the frame has arrived correctly. However, the transport layer also sends a message for
acknowledgement.
(c) Acknowledged connection-oriented service: The data link layer provides this service to the
network layer by establishing a connection between the source and destination hosts
before any transfer of data takes place. The order of each frame is maintained and guaranteed
the receipt of frame by receiving hosts. Communication between source and destination
hosts completes in three phases. They are connection setup, actual frame transmission and
connection release.

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