McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Lecture 3 : Network Architectures 1

Protocols and Standards 2 Protocol  set of rules that govern data communication القواعد التي تحدد كيف يمكن لأجهزة الكمبيوتر أن تتفاهم مع بعضها البعض عبر الشبكة التي تتواجد عليها.  It represents an agreement between the communicating devices  Key elements  Syntax: defines the structure of information communicated, including the data format.  Semantics: defines the meaning of the signals, exchanged including control information for coordination and error handling  Timing: defines speed matching and sequuencing.

3 Standards  Required to allow for interoperability between equipment  Advantages  Ensures a large market for equipment and software  Allows products from different vendors to communicate  Disadvantages  Freeze technology  May be multiple standards for the same thing

Standards 4 Defined by public organizations. Open standards  Available to everyone (but not necessarily for free)  Everyone has the possibility to propose, criticize, and influence  Standards organizations  ISO: International Organization for Standardization  IETF: Internet Engineering Task Force  IEEE: Institute of Electrical and Electronics Engineers  ANSI: American National Standards Institute  ETSI: European Telecommunications Standard Institute  ITU: International Telecommunication Union  ITU-T: International Telecommunication Union—Telecommunication Standards  EIA: Electronic Industries Association

The task of designing a communication network is too complex to be single model An alternative, a structured approach. Divide the communication task into manageable subtask. Need to describe the communication functions in terms of an architecture. The architecture defines the relationship and interactions between network services and functions through common interfaces and protocols. Network Architectures 5

The task of designing a communication network is too complex to be single model An alternative, a structured approach. Divide the communication task into manageable subtask. Need to describe the communication functions in terms of an architecture. The architecture defines the relationship and interactions between network services and functions through common interfaces and protocols. Network Architectures 6

Figure 3.1 Tasks involved in sending a letter A network model is a layered architecture Task broken into subtasks Implemented separately in layers in stack Functions need in both systems Peer layers communicate  layers are independent  Modify one without affecting the other 2.7 Network architecture( model)

The architecture is divided into multiple layers.  Each layer performs a related subset of functions required for communication.  Layer N relies on services of layer N-1 to provide a service tolayer N+1  Service required from lower layer is independent of how that  service is implemented Information and complexity hiding Changes in layer N do not affect other layers Layered Architecture 8  interface between each pair of adjacent layers what information and services a layer must provide for the layer above it

2.9  The two important network architecture model is IOS and TCP\IP

2.10 ISO is the organization. OSI is the model. Note

THE OSI MODEL 2.11  Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards.  An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s

2.12 Figure 3.2 Seven layers of the OSI model

PEER – TO – PEER PROCESS 2.13  Communication must move down through the layers on the sending device, over to receiving device  Are the receiving device, communication must move up through the layers  Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it  At the receiving device, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it

LAYERS IN THE OSI MODEL LAYERS IN THE OSI MODEL we briefly describe the functions of each layer in the OSI model. Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Topics discussed in this section:

The physical layer 2.15  is responsible for movements of individual bits from one hop (node) to the next Transmission of raw bits (0/1) over physical wires  Physical characteristics of interfaces and media Multiplexing Guided and Unguided transmission media Data rates Transmission impairments Transmission mode Parallel vs. serial Synchronous vs. asynchronous Direction of transmission  Representation of bits Analog and Digital signals

2.16 Figure 3.3 Physical layer

A node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. Example 2.1

The data link  layer is responsible for moving frames from one hop (node) to the next  Function  Framing: break up the sending data into data frams  Physical addressing  Flow control : how to coordinate between fast sender and slow recaiver.  Error control

2.19 Figure 3.4 Data link layer

Network layer 2.20  Responsible from the delivery of packets from the original source to the final destination across multiple networks  If 2 systems are connected to the same link, there is no need for network layer  Functions of the layer  Logical addressing  Controls access to the network  Provides routing of packets within the network  Manages contention and bottlenecks within the network

Transport layer 2.21  Responsible for delivery of a message from one process to another (End-to-end data delivery)  Functions  Port addressing  Segmentation and reassembly  Connection control Connection oriented Transport layer makes a connection with its peer layer at the destination before sending packets After all data are transferred, the connection is terminated Connectionless oriented A connectionless Transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine  Flow control  Error control

2.22 Figure 2.11 Reliable process-to-process delivery of a message

Session layer  It is the network dialog controller  It establishes, maintains and synchronize the interaction between communicating system  Function Dialog control It allows the communication between two processes to take place in either half-duplex or full-duplex. Synchronization It allows a process to add checkpoints into a stream of data

Presentation layer 2.24  Concerned with the syntax and semantics of the information exchanged between two systems  Conversion of data to conform to a common format  Functions  Translation The running programs in two systems are usually exchanging information in the form of char strings, numbers,… Information must be changed to bit stream before being transmitted But different computers use different encoding methods The presentation layer is responsible for interoperability between these different encoding systems It changes the information at the sender from its sender-dependent format into its receiver-dependent format  Encryption  Compression

Application layer 2.25  Responsible for providing services to the user  Functions  Mail services  File transfer and access  Remote log-in  Accessing the World Wide Web www