More on TCP/IPand

Networking

CS-328

Dick Steflik

Open Systems Interconnection Model

• 7 Layer model– 7 Application– 6 Presentation– 5 Session– 4 Transport– 3 Network– 2 Data Link– 1 Physical Hardware Connection

X.25 and the ISO Model

• International Standard

• Fully compliant with the OSI 7 layer model

• X.25 is the set of protocols recommended by Telecommunications Section of the International Telecommunications Union (ITU-TS)

Physical Layer

• X.25 specifies a standard for physical interconnection– electrical specs (voltages, current)– connectors

Data Link Layer

• specifies how data travels between a host and the packet switch it is connected to– frame - unit of data– specs for framing of data

• what defines start of frame and end of frame

• frame checksuming and acknowledgement and receipt

– HDLC - High level data Link Control

Network Layer

• also called the network or communication subnet layer

• defines the basic Unit of Transfer– includes destination addressing and routing

• assembles a packet and uses layer to transfer it to the packet switch

Transport Layer

• provides end-to-end reliability

• double checks what what was done at the lower layers

Session Layer

• Remote Terminal Access– so important that it occupies its own layer

Presentation Layer

• services used by applications– text compression/decompression– graphic object serialization

Application Layer

• applications that use the network– e-mail - x.400– file transfer

X.25 vs TCP/IP

• Main difference is approach to reliability– X.25 - reliability is at link level and insurance

is provided at all layers• causes possible retries many times

– TCP/IP - reliability is an end-to-end problem • done almost exclusively at Transport layer

– retries only done at transport layer rather than every layer

Another Difference

• X.25 - idea of a network is a utility that provides transport service– network vendor provides billing, handling

routing problems, flow control– network is complex with simple hosts at the

end points

Another Difference (more)

• TCP/IP

• hosts actively participate in reliability, routing and network control (ICMP)

• the network is a simple packet delivery system with complex hosts at end points