And Data Link Layers

Service u&ei network layer protocol t

Service user newgfhi la/er protocol

Service provider {data link layer protocol)

Network layer

Data link layer

OSI Model Layers and Information Exchange

The seven OSI layers use various forms oi control information to communicate with their peer layers in other computer systems. This control information consists oi specific requests and instructions that are excha nged betwee n peer OSI ^yess.

Contra! information tynically takes nne of two So rms: headers and traile"sl ¡Headers are prepended to data that has been passed down from upper layers. Trailers are appended to data that has been pas sed down •rom upped layers. An Oni laym is not re quired to attach a header or a trailer to data from upper layers.

Headers , traMers, and data are re lat ivn concenrs , adponding on the lamer that analyzes the iL•ormation unit. At the network layer, tor example, an innormation unit consist of a Layer 3 header and data. At the data link layet, however, all the information aassed down by roe network layer (tie Layer p hender and the data) is treated as data.

nn other wo rds, the data portion of an information unit at a given OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is known as encapsulation. Figure s-6 shows h ow the header asd data from on e layer arn encapsulated into the h eader of the next lowest cayer.

0 0

Post a comment