consists of the physical equipment that carries signals from one point of the network to another.

consists of instruction sets that make possible the services that we expect from a network.

sender site

receiver side

network support layers

user support layers

Other responsibilities of the data link layer include the following:

The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.

If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames.

It also uses a mechanism to recognize duplicate frames.

When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

If two systems are connected to the same link, there is usually no need for a network layer.

However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often no need for the network layer to accomplish source-to-destination delivery.

Other responsibilities of the transport layer include the following:

Source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other.

▪ A message is divided into transmittable segments, with each segment containing a sequence number.

▪ These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

▪ The transport layer can be either connectionless or connection-oriented.

▪ A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.

▪ A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets.

control at this layer is performed end to end rather than across a single link

The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication).

▪ The session layer allows two systems to enter into a dialog.

▪ It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two ways at a time) mode

The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

▪ The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods.

▪ The presentation layer at the sender changes the information from its sender-dependent format into a common format.

▪ The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

means that the sender transforms the original information to another form and sends the resulting message out over the network.

reverses the original process to transform the message back to its original form.

reduces the number of bits contained in the information.

particularly important in the transmission of multimedia such as text, audio, and video.

Specific services provided by the application layer include the following:

______ is a software version of a physical terminal, and it allows a user to log on to a remote host

This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

This application provides the basis for e-mail forwarding and storage.

This application provides distributed database sources and access for global information about various objects and services.

▪ It is used to associate a logical address with a physical address.

▪ On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC).

▪ is used to find the physical address of the node when its Internet address is known.

▪ It is used to associate a logical address with a physical address.

▪ allows a host to discover its Internet address when it knows only its physical address.

▪ It is used when a computer is connected to a network for the first time or when a diskless computer is booted.

▪ a mechanism used by hosts and gateways to send notification of datagram problems back to the sender.

▪ sends query and error reporting messages.

▪ It is used to facilitate the simultaneous transmission of a message to a group of recipients.

▪ It is the simpler of the two standard TCPIIP transport protocols.

▪ It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer.

It provides full transport-layer services to applications and is reliable stream transport protocol.

▪ This protocol provides support for newer applications such as voice over the Internet.

▪ It is a transport layer protocol that combines the best features of UDP and TCP.