Computing – Communications and Networks - Layering: logical communication

Layering: logical communication

•  E.g.: transport
• take data from app
•  add addressing, reliability check info to form ¡§datagram¡¨

• send datagram to peer

•  wait for peer to ack receipt

•  analogy: post office

Network Layer

 Set of functions required to transfer packets end-to-end (from host to host)

 hosts are not directly connected - need for intermediate systems

 examples: IP, Appletalk, IPX

 Intermediate systems

 routers: forward packets to the final destination

 interconnection devices

Modern networks have more than physical and data link. The network layer is the set of

mechanisms that can be used to send packets from one computer to another in the world. There

are two types of networks:

With Packet switching, data packets can be carried together on the same link. They are

differentiated by addressing information. Packet switching is the basis for all data networks

today, including the Internet, public data networks such as Frame Relay, X.25, or ATM.

Circuit Switching is the way telephone networks operate. A circuit emulates the physical signals

of a direct end-to-end cable. When computers are connected by a circuit switched network, they

establish a direct data link over the circuit. This is used today for modem access to a data

network.

Modern circuit switches are based on byte multiplexing and are thus similar to packet switches,

with the main difference that they perform non-statistical multiplexing (see later in this chapter).

A network has Intermediate systems (ISs): those are systems that send data to next ISs or to the

destination. Using interconnected ISs saves cable and bandwidth. ISs are known under various

terms depending on the context: routers (TCP/IP, AppleTalk,…), switches (X.25, Frame Relay,

ATM, telephone), communication controllers (SNA), network nodes (APPN).

The Internet's network layer has two principle components. It has a protocol that defines the

fields in the IP datagram as well as how the end systems and routers act on these fields. This

protocol is the celebrated IP protocol. There is only one IP protocol, and all Internet components

that have a network layer must run the IP protocol. The Internet's network layer also contains

routing protocols that determine the routes that datagrams take between sources and destinations.

The Internet has many routing protocols.