Data link layer of OSI model defines the ways of encapsulating data for transmission between two endpoints and the techniques of transferring the frames. Both Asynchronous Transfer Mode (ATM) and Frame relay are data link layer technologies and they have connection oriented protocols. Each technique has its own application dependent advantages and disadvantages.
Asynchronous Transfer Mode (ATM )
ATM is a network switching technology that uses a cell based methodology to quantize data. ATM data communication consists of fixed size cells of 53 bytes. An ATM cell contains a 5 byte header and 48 bytes of ATM payload. This smaller size, fixed-length cells are good for transmitting voice, image and video data as the delay is minimized.
ATM is a connection oriented protocol and therefore a virtual circuit should be established between sending and receiving points. It establishes a fixed route between two points when the data transfer starts.
Another important aspect of ATM is its asynchronous operation in time division multiplexing. Therefore cells are transmitted only when data is available to be sent unlike in conventional time division multiplexing where synchronization bytes are transferred if there data is not available to be sent.
ATM is designed to be convenient for hardware implementation and therefore processing and switching have become faster. Bit rates on ATM networks can go up to 10 Gbps. ATM is a core protocol used over the SONET/SDH backbone of the ISDN.
ATM provides a good quality of service in networks where different types of information such as data, voice, and are supported. With ATM, each of these information types can pass through a single network connection.
Frame relay is a packet switching technology for connecting network points in Wide Area Networks (WAN). It is a connection oriented data service and establishes a virtual circuit between two end points. Data transfer is done in packets of data known as frames. These frames are variable in packet size and more efficient due to flexible transfers. Frame Relay was originally introduced for ISDN interfaces though it is currently used over a variety of other network interfaces as well.
In frame relay, connections are called as ‘Ports’. All the points which need to connect to the frame relay network needs to have a port. Every port has a unique Address. A frame is made of two parts which can be called as ‘actual data’ and the ‘frame relay header’. Frame architecture is same as defined for LAP-D (Link Access Procedures on the D channel) which has a variable length for information field. These frames are sent over Virtual Connections.
Frame relay can create multiple redundant connections among various routers, without having multiple physical links. Since frame relay is not specific for media, and provides means to buffer speed variations, it has the possibility to create a good interconnect medium between different types of network points with different speeds.
Difference between ATM and Frame Relay
1. Although both techniques are based on end to end delivery of quantized data, there are many differences in terms of sizes of the data quanta, application network types, controlling techniques etc.
2. Although ATM uses fixed size packets (53 bytes) for data communication, frame relay uses variable packet sizes depending on the type of information to be sent. Both information blocks have a header in addition to data block and transfer is connection oriented.
3. Frame Relay is used to connect Local Area Networks (LAN) and it is not implemented within a single area network contrast to ATM where data transfers are within a single LAN.
4. ATM is designed to be convenient for hardware implementation and therefore, cost is higher compared to frame relay, which is software controlled. Therefore frame relay is less expensive and upgrading is easier.
5. Frame relay has a variable packet size. Therefore it gives low overhead within the packet which results it an efficient method for transmitting data. Although fixed packet size in ATM, can be useful for handling video and image traffic at high speeds, it leaves a lot of overhead within the packet, particularly in short transactions.