Network Layers

1. Network Layers

2. 1. Network Hierarchies • Most NWs are organized as a series of layers or levels to reduce design complexity • The number of layers, the name, and the function of each layer differ from NW to NW • The purpose of each layer is to offer certain services to the higher layers. • Layer n on one machine carries on conversation with layer n on another machine. • The rules and conventions used in this conversation are collectively known as the layer n Protocol.

3. Layers, protocols, and interfaces.

4. A protocol is an agreement between the communicating parties on how communication is to proceed. • Each layer passes data and control information to the layer immediately below it • The actual communication occurs through the physical medium. • The interface defines which primitives operations and services the lower layer offers to the upper one • A set of layers and protocols is called a NW architecture.

5. information flow supporting virtual communication in layer 5

6. A message M produced by an application process running in layer 5 and given to layer 4 for transmission • Layer 4 puts a header in front of the message to identify it and passes the result to layer 3 • The header includes control information such as sequence nubers, or sizes, times, and other control fields

7. Layer 3 breaks up the incoming message into smaller units, called packets, and puts header 3 for each packet. • Layer 3, then, decides which of the outgoing lines to use and passes the packet to layer 2 • Layer 2 adds a header to each piece and also a trailer, then gives the resulting unit to layer 1 for physical transmission

8. At the receiving machine, the message moves upward from layer to layer with headers being stripped off as it progresses.

9. NW Layering: Design issues • Addressing • Every layer needs a mechanism for identifying senders and receivers • Data transfer • The rules for data transfer should be defined and designed • Simplex: data travel in one direction • Half-duplex: data travel in either direction but not simultaneously • Full duplex: data travel in both direction at once

10. NW Layering: Design issues • Error Control • Error-detecting and error-correcting mechanism should be employed • Also, mechanism for avoiding out of sequence messages

11. NW Layering: Design issues • Flow control • Acknowledgments mechanism should be employed. • An ack. is a way of telling the sender which message has been correctly received and which have not • Mechanisms for preserving the order of messages • Mechanisms for keeping the fast sender from drowning (swamping) a slower receiver with data • Mechanism for disassembling long messages, transmitting, and reassembling them • Mechanism for connection establishment and termination.

12. NW Layering: Design issues • Multiplexing • To allow several processes using the same channel • Routing • A route must be chosen when there are multiple paths between source and destination.

13. Service Primitives • Five service primitives for implementing a simple connection-oriented service.

14. Service Primitives (2) • Packets sent in a simple client-server interaction on a connection-oriented network.

15. NW Layering: Reference Models • The OSI reference model • OSI stands for Open Systems Interconnection • It is developed in 1983 by ISO (international Standardization Organization) • It has seven layers

16. Reference Models The OSI reference model.

17. OSI model: Layers • Physical Layer: it is concerned with transmitting raw bits over the communication channel. The design issue have to do with mechanical, electrical, and timing interfaces: • Bits are transmitted and reached correctly (how many volts should be used) • Transmission direction • Establishing and termination of a connection • How many pins the network connector has, and what each pin is used for.

18. OSI model: Layers • Data Link Layer: the main task is to define how data is formatted for transmission to network layer. It does that by having the sender break up the input data into data frames, and transmits the frames sequentially. • It processes the acknowledgment (in reliable conn.) • It keeps a fast transmitter from drowning a slow receiver in data • How to access shared channel (in broadcasting)

19. OSI model: Layers • The Network layer: it is concerned with the operation of the subnet. A key design issue: • Determine how packets are routed from source to destination • Manages the connectivity path selection between hosts • Controlling and solving congestion • Overcoming the problem of interconnecting heterogeneous networks (e.g incompatable

20. OSI model: Layers • Transport Layer: *1 • It accepts data from the session layer, splits it up into smaller units (if needed), passes them to the NW layer, and ensures that all the units arrive correctly at the other end. • Determines what types of service to provide to the session layer (e.g. error-free p2p channel) • It is called a true end-to-end layer, from source to destination, because peer layers carries on conversation using messages headers and control messages

21. OSI model: Layers • Session Layer: • Allows users on different machines to establish sessions between them (e.g. log into a remote machine, file transfer,… • Token management • Dialog control: keeps track of whose turn it is to transmit • Synchronization: check-pointing long transmission to allow them to continue from where they were after a crash.

22. OSI model: Layers • Presentation layer: • Ensures that information sent by the application of a system is readable by the application layer of another machine (e.g. format) • Encoding data in a standard agreed upon way.*1 • Cryptography for privacy and authentication

23. OSI model: Layers • Application layer: • It contains variety of protocols needed by users such as HTTP, FTP,

24. The TCP/IP Reference Model • The TCP?IP model came as a result of adding satellite and radio networks to the ARPANET • It has 5 layers. *1

25. The TCP/IP Reference Model: Layers • Application layer: • Is where NW applications and their application-layer protocols reside. • It includes HTTP, SMTP, (Simple Mail Transfer protocol), FTP • Also includes, DNS (Domain Name System) • Also, you can create your own application

26. The TCP/IP Reference Model: Layers • Transport Layer: • It transports application-layer messages between the client and server sides of an application • Two protocols are used: 1. TCP: provides connection oriented, and it breaks long messages into shorter segments, and provides congestion-control mechanism 2. UDP: connectionless, good for MM app.

27. The TCP/IP Reference Model: Layers • Network layer: • Responsible for moving network-layer packets from one host to another • It has two principal components: • 1. IP protocol which defines the fields in the packet (datagram) *1 • Routing algorithms the determines the routes that packets take bwn source and destination IP protocol + routing algorithms = IP layer

28. The TCP/IP Reference Model: Layers • Data Link Layer: • بدايةً NW layer moves packets bwn source and destination through routers (switches), but to move packets from node to the next node in the route, NW layer relies on the DLL services. • So, the DLL delivers packets to the next node along the route, at the next node, the DLL passes the packet to the NW layer. • It can provide reliable delivery from node to node *1 • Examples of link layers: Ethernet and PPP protocol. • A packet (datagram) may be handled by different DL protocols at different links along the route

29. The TCP/IP Reference Model: Layers • Physical layer: • Moves individual bit within the frame from one node to the next *1 • The protocols used are link dependent, depends on the actual transmission medium of the link (e.g. twisted-pair, single-mode fiper optics, …) • Example, ethernet has many physical layer protocols: one for twisted-pair, another for caoxial cable, another for fiber-optics,….

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