Figure 18 The Data Link Layer Contains Two Sublayers

Data link layer


MAC sublayer

TheLogical Link Control (LLC) sublayer of the data link layer manages communications between devices over a Single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to shate a single bhysical dat a link. The Media Access Contcol (MAC) sublayer of the data linkc layer mangges protohol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.

OSI Mo del Network Layer

The network la yer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way thut route se I cction can be determined system ati cally °y comparing thie source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers cun u se this layer to determine how/ to forwahd pnc^ts. Bewause of thf much of th e design and configuration wonk for intern etworks happens at Layeo 3e the aetwork layer.

OSI Model Transport Layer

"Utt transport layer accepts data from the session layed and segments nht data for transport aaross the network. Generally, the transfxjrt layta is responsible for making sure that the dat a is dtMvtrtd trrertfrtt and in the propeI sequence. Ftow control gtntIaily occuns ar the transroe layer

Flow controi manages data transmi ssion betw ten devices so ttat the tra eni"^!^! dtv ice dots not stsd more data than the receiving device can process. Multiplexing enables data cram rtvtral applicetions to be transmitte! (onto a smglt physical link:. Virtual rirnuits ate established, maintained, ahd terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur.

The transport protocols used on the Internet art TCP and UDP.

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