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7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

COMPUTER NETWORKS

OSI MODEL:

Physical Layer

Data Link

Network

Varna Free University

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

Source

1. Computer Networks, Andrew S. Tanenbaum
2. www.cisco.com
3. www.novell.com
4. www.rad.com
5. www.3com.com

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

NETWORK GOALS

The two main benefits of networking computers are… Communications Information can be distributed very quickly, such as email and video conferencing. Saving Money Resources such as information, software, and hardware can be shared. CPUs and hard disks can be pooled together to create a more powerful machine.

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

APPLICATIONS

A lot of things we take for granted are the result of computer networks.

• (^) Email
• (^) Chat
• (^) Web sites
• (^) Sharing of documents and pictures
• (^) Accessing a centralized database of information
• (^) Mobile workers

7 Application 6 Presentation 5 Session 4 Transport 1 Physical 2 Data Link 3 Network

NETWORK ARCHITECTURES

A set of layers and protocols is called the network architecture.

1. Protocol Hierarchies Networks are organized as layers to reduce design complexity. Each layer offers services to the higher layers. Between adjacent layers is an interface. Services – connection oriented and connectionless. Interface – defines which primitives and services the lower layer will offer to the upper layer. Primitives – operations such as request, indicate, response, confirm.

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

NETWORK ARCHITECTURES

2. Design Issues for the Layers - (^) Mechanism for connection establishment - (^) Rules for data transfer - (^) Error control - (^) Fast sender swamping a slow receiver - (^) Inability of processes to accept long messages - (^) Routing in the case of multiple paths

7 Application 6 Presentation 5 Session 4 Transport 1 Physical 2 Data Link 3 Network

OSI REFERENCE MODEL

1. Physical Layer a) Convert the logical 1’s and 0’s coming from layer 2 into electrical signals. b) Transmission of the electrical signals over a communication channel. Main topics: - (^) Transmission mediums - (^) Encoding - (^) Modulation - (^) RS232 and RS422 standards - (^) Repeaters - (^) Hubs (multi-port repeater)

7 Application 6 Presentation 5 Session 4 Transport 1 Physical 2 Data Link 3 Network

OSI REFERENCE MODEL

2. Data Link Layer a) Error control to compensate for the imperfections of the physical layer. b) Flow control to keep a fast sender from swamping a slow receiver. Main topics: - (^) Framing methods - (^) Error detection and correction methods - (^) Flow control - (^) Frame format - (^) IEEE LAN standards - (^) Bridges - (^) Switches (multi-port bridges)

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

OSI REFERENCE MODEL

4. Transport Layer a) Provides additional Quality of Service. b) Heart of the OSI model. Main topics: - (^) Connection-oriented and connectionless services - (^) Transmission Control Protocol (TCP) - (^) User Datagram Protocol (UDP)

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

OSI REFERENCE MODEL

5. Session Layer a) Allows users on different machines to establish sessions between them. b) One of the services is managing dialogue control. c) Token management. d) Synchronization.

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

OSI REFERENCE MODEL

7. Application Layer a) Provides protocols that are commonly needed. Main topics: - (^) File Transfer Protocol (FTP) - (^) HyperText Transfer Protocol (HTTP) - (^) Simple Mail Transfer Protocol (SMTP) - (^) Simple Network Management Protocol (SNMP) - (^) Network File System (NFS) - (^) Telnet

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

SERVICES

Each layer provides services to the layer above it.

1. Terminologies Entities – active elements in each layer (e.g. process, intelligent I/O chip). Peer Entities – entities in the same layer on different machines. Service Provider – Layer N. Service User – Layer N + 1. Service Access Points – places where layer N + 1 can access services offered by layer N.

7 Application 6 Presentation 5 Session 4 Transport 1 Physical 2 Data Link 3 Network

SERVICES

3. Service Primitives Request – entity wants the service to do some work Indicate – entity is to be informed about an event Response – entity responds to an event Confirm – entity is to be informed about its request Sending Computer Receiving Computer 3 Network

1. request 3 Network
2. confirm 2. indicate 3. response 4 Transport 4 Transport

7 Application 6 Presentation 5 Session 4 Transport 2 Data Link 3 Network

BANDWIDTH

The capacity of the medium to transmit data. Analog Bandwidth

• (^) Measurement is in Hertz (Hz) or cycles/sec. Digital Bandwidth
• (^) Measurement is in bits per second (bps). Q: Is 100MHz = 100Mbps? Q: Is 100Mbps = 100MBps?