Fragmentation

In some upper layer protocols informational fields of big length are used for transmitting of information, and therefore they cannot be transmitted in the format of one Frame Relay frame. In this case, for data transmission via the Frame Relay network a procedure of partitioning of the data message on frames is applied. Or simply fragmentation; it’s very similar to the procedure of the traffic encapsulation. As an example, let’s consider the fragmentation of IP datagrams using the procedure of SNAP (Fig. 2.6).

Рис. 2.6. Fragmentation of IP datagram to the Frame Relay frames

As can be seen from the figure, the SNAP frame format extension is used for fragmentation. In the frame format the fragmentation header is added for transmitting of information about fragments. The fragmentation header consists of four fields: the " sequence" (Sequence - Seq, 2 octets), which is assigned to each new fragmented message; reservation field (Res. 4 bits, all of them equal zero), А fragmentation end bit (in the frame, that contains the last fragment, it equals 1, all others - 0), and also an offset field (Offset - 11 bits) that specifies the offset of the transmitted fragments at division by 32.

3 Key questions

3.1 The essence of the main approaches of the formation of routes in data networks.

3.2 Describe the main procedures used to support protocols of higher levels by the Frame Relay protocol.

3.3 Specify the assignment of identifier and the fields included in the frame information field of the multiprotocol encapsulation.

3.4 Draw the structure scheme of the LAN / Ethernet frame encapsulation in the Frame Relay and explain assignment of identifiers and fields included in this structure.

3.5 Draw the structure of the IP datagrams encapsulation frame in the Frame Relay and explain the assignment of identifiers and fields included in this structure.

3.6 Explain the meaning of the Frame Relay and X.25 protocols interaction scheme.

3.7 What purpose is the fragmentation procedure during the data transmitting via Frame Relay network implemented for?