DIAL-UP TELEPHONE CONNECTIONS
DEDICATED DIGITAL TELEPHONE LINES
CONNECTIONS OVER PACKET-SWITCHED NETWORKS
Are all WAN links

CONNECTION THROUGH THE TELEPHONE SYSTEM
Dial-up connection that uses the telephone line and it’s very popular in our daily life. It is analog and the computer uses modems to change the signals from analog to digital signals. DEDICATED LEASED LINES
It provides a full-time connection between two points. Lease lines are expensive but offer high speed data transmission. They are available as analog and digital lines. ISDN (INTEGRATED SERVICE DIGITAL NETWORK)
Offers voice, video, and data transmission where T1 = 23B + D (B: used for voice or data, D: used to handle control information, T1: 1.544 Mbps). Its speed can be up to 128 kbps. USING X.25
X.25 is the first wide speed implementation of packet switching. It is based on technology concepts that date back to the 1960’s. It was developed at 1992 and the speed of transmission increased up to 2Mbps and today it is used to allow communication between many different computer systems on a world wide basis. ADVANTAGES
It maybe the only way to set up international networks links to countries with unreliable telephone systems. It is reliable. It provides for any-device-to-any-device type connection for simultaneous users. DISADVANTAGES
Packets must travel through shared ports on routers. Not acceptable technology for most real time LAN to LAN applications. FRAME RELAY
It is a connection oriented service similar to X.25 which provides error-checking at each intermediate node in the network. Frame relay provides for error-checking only at the source and destination devices. ASYNCHRONOUS TRANSFER MODE (ATM)
It is a cell switching technique using time division Multiplexing. Information is transferred in packets of fixed size called cells. ATM cells posses fixed size 53bytes. IMPORTANT TERMS

The basic data communication configuration consists of a sending and receiving remote TERMINAL, a MODEM (modulator/demodulator) and a COMPUTER SYSTEM to receive the data and to process and prepare information for retransmission through the communication system. The telephone is still the main method of transferring data over a communication system. The telephone system is an analog system, but the terminals and computers produce digital pulses. The modem is a device that provides an interface for digital pulses to and from the terminals and computer, through the telephone circuits. The modem converts electrical signals to modulated signals that can be transmitted.

The HOST COMPUTER, also called the SERVER, is the main processing device in a network.