Introduction to Frame Relay

Frame relay is a synchronous HDLC protocol based network. Frame Relay is a standardized wide area networking technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. Data is sent in HDLC packets, referred to as "frames".
Frame relay technology was developed to specifically address these needs :
- A higher performance packet technology.
- Simpler network management.
- More reliable networks.
- Lower network costs.
- Integration of traffic from both legacy and LAN applications over the same physical network.

Advantages of Frame Relay

Frame Relay offers an attractive alternative to both dedicated lines and X.25 networks for connecting LANs to bridges and routers. The success of the Frame Relay protocol is based on the following two underlying factors:
- Because virtual circuits consume bandwidth only when they transport data, many virtual circuits can exist simultaneously across a given transmission line. In addition, each device can use more of the bandwidth as necessary, and thus operate at higher speeds.
- The improved reliability of communication lines and increased error-handling sophistication at end stations allows the Frame Relay protocol to discard erroneous frames and thus eliminate time-consuming error-handling processing.

These two factors make Frame Relay a desirable choice for data transmission; however, they also necessitate testing to determine that the system works properly and that data is not lost.

Framing in Data Link Layer

The data link layer detects the number of bits transmitted by physical layer to be error free. The approach used is to break the bit stream up into discrete frames and compute the checksum for each frame. When a frame arrives at the destination, the checksum is recomputed. If this checksum is different from the one obtained in the frame, the data link layer knows that an error has occurred and takes steps to deal with.
There are three different types of framing, each of which provides a way for the sender to tell the receiver where the block of data begins and ends:
- Byte-oriented framing : Computer data is normally stored as alphanumeric characters that are encoded with a combination of 8 bits (1 byte). This type of framing differentiates one byte from another.
- Bit-oriented framing : This type of framing allows the sender to transmit a long string of bits at one time.
- Clock-based framing : In a clock-based system, a series of repetitive pulses are used to maintain a constant bit rate and keep the digital bits aligned in the data stream.

The following methods are commonly used for calculating the check sum :
- Character Count : It uses a field in the header to specify the number of characters in the frame. When data link layer at destination sees the character count, it knows how many characters follow, and hence where the end of frame is. The disadvantage with this method is that the count can be garbled by a transmission error.

- Character stuffing : This method gets around the problem of resynchronization after an error. Each frame starts with the ASCII character sequence DLE(Data Link Escape) STX(Start of Text) and end with the sequence DLE ETX(End of Text). If the destination loses track of frame boundaries, all it has to do is to look for DLE STX or DLE ETX. Problem occurs when binary data such as object programs or floating point numbers are transmitted. Solution to this problem is to have sender's data link layer insert an ASCII DLE character just before each accidental DLE character in the data. The data link layer on receiving end removes the DLE before the data is given to the network layer. This is called character stuffing. A disadvantage of using this framing method is that it is closely tied to 8-bit characters.
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Ethernet Hubs

A hub connects multiple devices together. Ethernet hubs, and are most commonly used in computers for networking purposes. Ethernet hubs are available in different types, depending on the speed of the network connection or broadband speed. The number of ports an Ethernet hub supports also varies. Older Ethernet hubs were relatively large in size and sometimes noisy as they contained built in fans for cooling the unit. Newer devices are much smaller, designed for mobility, and noiseless.

Working of an Ethernet Hub

The main purpose of the Ethernet hub is to transmit the large pockets or cluster of data it receives from one computer onto another through all the ports connected to it. Ethernet uses a protocol called CSMA/CD, which stands for Carrier Sense, Multiple Access with Collision Detection.

- Carrier Sense - When a device connected to an Ethernet network wants to send data it first checks to make sure it has a carrier on which to send its data.
- Multiple Access - This means that all machines on the network are free to use the network whenever they like so long as no one else is transmitting.
- Collision Detection - A means of ensuring that when two machines start to transmit data simultaneously, that the resultant corrupted data is discarded, and re-transmissions are generated at differing time intervals.