Done by: Mardonov B

Purpose of each level- provide proper service for the overlying layer.

• Active elements of the levelcalled activities
• Activities can be software or hardware
• Activities of the same level on different machines are called equivalent activities.
• Level n+1 activities areusersservice created by level n activities, which are called suppliersservice

ACTIVITIES
The service is accessed through service access points - SAPs(service access points)
Each service access point hasunique address
SERVICE WITH AND WITHOUT CONNECTION
Connection serviceassumes that a connection is first established between the receiver and the sender, and only then the service is delivered. An example is the telephone network.
Service without connection acts like postal service. Every message has a recipient address. At the right points, it is routed along the right route. Regardless of other posts. With such a service, it is quite possible that a message sent later will arrive earlier.
Formally, a service is defined as a set of primitive operations (or primitives) by which a user or some activity accesses the service.
PROTOCOL AND INTERFACE
Protocol- this is a set of formalized rules that determine the sequence, format and purpose of messages (packets, frames) exchanged by equivalent activities (network components) that lie on the same level, but in different network nodes.
A hierarchically organized set of protocols sufficient for the interaction of devices on a network is calledstack of communication protocols.
Modules that implement the protocols of neighboring layers and are located in the same node also interact with each other in accordance with clear rules and using standardized message formats. These rules are calledinterface.
INTERFACE AND PROTOCOL
Pthe concepts "interface" and "protocol", in essence, mean the same thing, namely -formalized specified procedures for the interaction of componentsthat solve the problem of connecting computers in a network.
Pthe concept of "protocol" is more often used when describing the rules for the interaction of components of the same level located ondifferent network nodes
concept"interface" - when describing the rules for the interaction of components of neighboring levels located withinone node.