1 Each layer provides a service to the layer above it (service provide).
Consumes services provided by the layer below it (service user)
1.1 The active elements in each layer are called entities
1.1.1 Entities in the same layer in different
machines are called peer entities
126.96.36.199 An entity can be either software or hardware or some combination
188.8.131.52.1 Layer 7 entities are often referred to as application entities,
Layer 6 entities as presentation entities, and so on.
2 Layers offer several classes of service
2.1 Fast (expensive)
2.2 Slow (Cheap)
2.3 reliable (expensive)
2.4 Unreliable (cheap)
3.1 Available at service access points
3.1.1 Each SAP has an address
184.108.40.206 There is a set of rules
governing the interface
3.2 Consists of a set of operations provided by 1 layer to the layer above
3.2.1 For 1 layer to send a PDU to its peer layer it must use the services of the layer below
220.127.116.11 PDU is packaged in an SDU (service data unit) and sent to the layer below
TCP/IP can be matched to this abstract description of services and layers in the following way. The protocol data unit in TCP is called a segment. When a TCP layer wants to send a segment to a peer TCP layer on another machine, it can’t transmit it directly, but has to rely on the transmission service of the IP layer below it. So the segment is packaged into an SDU that can be passed down to the transmission service of the IP layer. The IP layer receives the SDU and now wants to transmit the data in the SDU to its peer IP layer on another machine. It does this by packaging the data into one or more datagrams, as the datagram is the PDU at the IP layer. However, again the IP layer can’t send the PDU directly, but has to rely on the services of the network interface layer below. So the datagram is packaged into an SDU and passed down to the network interface layer, and so on, until a layer is reached where the protocol data unit can be directly transmitted to the peer layer on another machine.