1. Network Models

2. 2.1 what is the Protocol? A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow, to be able to communicate effectively. In the network telecommunication, we need to divide the communication task between different layers, in which case we need a protocol or more at each layer.

3. 2.3 Figure 2.16 Layers and protocols in the reference protocol model

4. Communication through an internet

5. Communication through an internet () bits frames packets Segments or datagrams messages Data units of each layer

7. 2.7 Figure 2.5 Physical layer The physical layer is responsible for movements of individual bits from one hop (node) to the next.

8. 2.8 Figure 2.6 Data link layer The data link layer is responsible for moving frames from one hop (node) to the next.

9. 2.9 Figure 2.7 Hop-to-hop delivery

10. 2.10 Figure 2.8 Network layer The network layer is responsible for the delivery of individual packets from the source host to the destination host.

11. 2.11 Figure 2.9 Source-to-destination delivery

12. 2.12 Figure 2.10 Transport layer The transport layer is responsible for the delivery of a message from one process to another.

13. 2.13 Figure 2.11 Reliable process-to-process delivery of a message

14. 2.14 Figure 2.14 Application layer

15. 2.15 The application layer is responsible for 1- providing services to the user. 2- dialog control and synchronization. 3- translation, compression, and encryption.

16. 2.16 Figure 2.17 Addresses in TCP/IP

17. 2.17 Figure 2.18 Relationship of layers and addresses in TCP/IP

18. 2.18 Figure 2.19 Physical addresses

19. 2.19 Figure 2.20 IP addresses

20. 2.20 Figure 2.21 Port addresses

21. THE DATA LINK LAYER

22. 3.1 DATA LINK LAYER functions 1. Providing a well-defined service interface to the network layer. 2. Framing: breaking up the bit stream into discrete frames 3. Error control 4. Flow control: Regulating the flow of data so that slow receivers are not swamped by fast senders.

23. 1-Providing a well-defined service interface to the network layer. the data link layer takes the packets it gets from the network layer and encapsulates them into frames for transmission. Each frame contains: – a frame header, – a payload field for holding the packet, – and a frame trailer,

24. Packets and Frames Relationship between packets and frames.

25. 3.1.2 Framing A good design of framing must make it easy for a receiver to find the start of new frames while using little of the channel bandwidth. We will look at four methods: – 1. Byte count. – 2. Flag bytes with byte stuffing. – 3. Flag bits with bit stuffing. – 4. Physical layer coding violations.

26. 1. Byte count. This method uses a field in the header to specify the number of bytes in the frame. When the data link layer at the destination sees the byte count, it knows how many bytes follow and hence where the end of the frame is. Trouble with this algorithm is that when the count is incorrectly received, the destination will get out of synchronization with transmission.

27. 2. Flag bytes with byte stuffing. This method resolves the problem of resynchronization after an error by having each frame start and end with special bytes (flag byte) is used as both the starting and ending delimiter. Two consecutive flag bytes indicate the end of one frame and the start of the next. Thus, if the receiver ever loses synchronization, it can just search for two flag bytes to find the end of the current frame and the start of the next frame.

28. 2. Flag bytes with byte stuffing. If the actual data contains a byte that is identical to the FLAG byte (e.g., picture, data stream, etc.) the convention that can be used is to have escape character inserted just before the “FLAG” character.

29. 2. Flag bytes with byte stuffing.

30. 3. Flag bits with bit stuffing. Each frame begins and ends with a special bit pattern, e.g 111111, five consecutive 1s. Whenever the sender’s data link layer encounters five consecutive 1s in the data it automatically stuffs a 0 bit into the outgoing bit stream. With both bit and byte stuffing, a problem is that the length of a frame now depends on the contents of the data it carries.

32. 4. Physical layer coding violations This method is to use a shortcut from the physical layer. We can use some reserved signals to indicate the start and end of frames. it is easy to find the start and end of frames and there is no need to stuff the data.

33. 3.1.3 Error Control 4 ways for error control: computing a checksum (a short token) for each frame, and include it in the frame. When a frame arrives at the destination, the checksum is recomputed. The usual way to ensure reliable delivery is to calls for the receiver to send back special control frames bearing positive or negative acknowledgements about the incoming frames.

34. 3.1.3 Error Control () When the sender transmits a frame, it generally also starts a timer. The timer is set to expire after an interval long enough for the frame to reach the destination; if either the frame or the acknowledgement is lost, the timer will go off, alerting the sender to a potential problem. frames may be transmitted multiple times; To prevent this from happening, it is generally necessary to assign sequence numbers to outgoing frames