1. More on TCP/IP and Networking CS-328 Dick Steflik

2. Open Systems Interconnection Model 7 Layer model –7 Application –6 Presentation –5 Session –4 Transport –3 Network –2 Data Link –1 Physical Hardware Connection

3. 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)

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

5. 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

6. 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

7. Transport Layer provides end-to-end reliability double checks what what was done at the lower layers

8. Session Layer Remote Terminal Access –so important that it occupies its own layer

9. Presentation Layer services used by applications –text compression/decompression –graphic object serialization

10. Application Layer applications that use the network –e-mail - x.400 –file transfer

11. 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

12. 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

13. 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