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Reference Models

Overview

The OSI Reference Model The TCP/IP Reference Model A comparison of the OSI and TCP/IPReference Models A hybrid model

The OSI Reference Model The OSI models is shown on Fig. 1. (minus the physicalmedium). This model is based on a proposal developed by theInternational Standards Organization (ISO) as a firststep toward international standardization of the protocolsused in the various layers. The model is called the

ISO OSI (Open Systems Interconnection)

Reference Model because it deals with connecting open systems – that is, systems that areopen for communication with other systems. We willusually just call it the OSI model for short.

Fig. 1. The OSI reference model

The OSI Reference Model Note that the OSI model itself is not a networkarchitecture, because it does not specify theexact services and protocols to be used in eachlayer. It just tells what each layer should do. However, ISO has also produced standards forall the layers, although these are not part of thereference model itself. Each one has beenpublished as a separate international standard. Next, we will discuss each layer of the model inturn, starting at the bottom layer.

OSI^ –

The Physical Layer The^ physical layer

is concerned with transmitting raw bits over a communication channel. The design issueshave to with making sure that when one side send a 1bit, it is received by the other side as a 1 bit, not as a 0bit. Typical questions here are:^
^ how many volts should be used to represent a 1 and how mayfor a 0^
^ how many microseconds a bit lasts^
^ whether transmission may proceed simultaneously in bothdirections^
^ how the initial connection is established and how it is torn downwhen both sides are finished^
^ And how many pins the network connector has and what eachpin is used for. The design issues here largely deal with

mechanical,

electrical

, and procedural interfaces, and the

physical

transmission medium

, which lies below the physical layer.

10

OSI^ –

The Data Link Layer A noise burst on the line can destroy a framecompletely. In this case, the data link layer software onthe source machine can retransmit the frame. However, multiple transmissions of the same frameintroduce the possibility of duplicate frames. (if theacknowledgment frame from the receiver back to thesender was lost) It is up to this layer to

solve the problems caused by damaged, lost and duplicate frames

Another issue that arises in the data link layer (and mosthigher layers as well) is how to keep a fast transmitterfrom drowning a slow receiver in data. Some

traffic

regulation mechanism

must be employed to let the transmitter know how much buffer space the receiverhas at the moment. Frequently, this flow regulation anderror handling are integrated. If the line can be used to transmit data in both directions,this introduces a new complication that the data linklayer software must deal with – acknowledgment framesfrom A -> B traffic compete for the use of the line with B - > A traffic.