Circuit Switching, ATM, and Frame Relay: Key Concepts
Circuit Switching: Advantages and Operation
Circuit switching establishes a dedicated channel for the duration of a communication session. Once the session ends (e.g., a phone call), the channel is released for other users.
Key Features of Circuit Switching:
- Transparent service and constant speed.
- Delay due to call setup.
- Supports analog or digital data transmission.
Asynchronous Transfer Mode (ATM)
ATM uses small, fixed-size cells for data transmission. In 1988, the CCITT designated ATM as the transport mechanism for future broadband services. ATM is asynchronous because cells are transmitted without specific time alignment, unlike T1 frames. These cells are small (53 bytes) compared to variable-length LAN packets.
ATM Functions:
ATM allows different bandwidth allocation for various applications, optimizing resource utilization. This enables assigning different bandwidths based on the volume of information (e.g., email vs. multimedia applications).
Frame Relay
Frame Relay (Frame-Mode Bearer Service) is a communication technique using a frame relay virtual circuit network, introduced by ITU-T Recommendation I.122 in 1988. It’s a simplified form of packet switching that transmits variable-size frames, ideal for large data transfers.
Frame Relay is used for high-speed voice and data services, enabling interconnection of geographically separated local area networks at a lower cost.
Frame Relay in the OSI Model:
- Link Layer
- Physical Layer
Switched Virtual Circuit:
Exists only for the duration of the communication session.
Data Link Connection Identifier (DLCI):
The Data Link Connection Identifier (DLCI) identifies the channel in a Frame Relay circuit. It’s located in the frame and indicates the data’s path, i.e., the established virtual circuit.
The DLCI typically ranges from 0 to 1023 (10 bits), with values from 0 to 15 and 992 onwards reserved for special functions.
The DLCI has local significance; each end of the virtual circuit can have a different circuit ID to identify the same circuit.
ATM and Frame Relay: Advantages
ATM Advantages:
- High-speed communication
- Connection-oriented service, similar to traditional telephony
- Fast switching in hardware
- Single universal and interoperable transport network
- Single network connection for voice, video, and data
- Flexible and efficient bandwidth allocation
Frame Relay Advantages:
- Faster information transfer than X.25
- Reduced processor workload
- Minimized frame delays