Data Communication

Communications Media

• –twisted pairs 
• –coaxial cables 
• –line-of-sight transmission: lasers, infra-red, microwave, radio 
• –satellite links 
• –fiber optics 
• –Power line

 Types of Communication Networks

Local Area Networks (LANs) 

– < a few km
–high data transmission rate (at least several Mbps)
–ownership usually by a single organization
–e. g., Ethernet, IBM Token Ring, Token Bus, FDDI, Fast Ethernet, ATM, Gigabit Ethernet

Metropolitan Area Networks (MANs) 

–up to 50 km
–fibre optics is a popular technology for MANs
–may be private or public
–may involve a number of organizations
–e.g., cable TV networks (CATV), ATM networks

Wide Area Networks (WANs)
–a few km to thousands of km
–point-to-point networks (also called long-haul networks)
–lower data transmission rate than LANs
–fiber optics is a popular technology for MANs ownership usually by more than a single organization
–e.g., ARPANET, MILNET (US military), CA*NET, NSFNET, KREONET, BoraNet, KORNET, INET, Internet

Functions of the OSI Layers

1. Physical layer – responsible for the electro-mechanical interface to the communications media

2. Data link layer – responsible for transmission, framing and error control over a single communications link.

3. Network layer – responsible for data transfer across the network, independent of both the media comprising the underlying subnetworks and the topology of those subnetworks.

4. Transport layer – responsible for reliability and multiplexing of data transfer across the network (over and above that provided by the network layer) to the level required by the application.

5. Session layer – responsible for establishing,, and managing sessions between cooperating applications.

6. Presentation layer – responsible for providing independence to the application process from differences in data representation (syntax).

7. Application layer – ultimately responsible for managing the communications between applications.