2.1.2 Х.25/х.75 protocol encapsulation

Frame Relay network can execute functions of transport medium for packet commutation networks based on X.25 protocol. In that case Frame Relay and X.25 protocols must be fully compatible, i.e. encapsulation and decapsulation procedure should save user information, as well as the structure of the protocol. Thus, we are talking about converting of X.25 protocol to the Frame Relay protocol, and vice versa. Since this procedure is significantly different from the procedures of LAN traffic encapsulation, it stands out in a separate class, called the interaction functions or IWF (Intel-working Functions). Schematically, the IWF procedure shown in Fig. 2.4

Fig. 2.4. Scheme of Frame Relay и Х.25 protocols interaction

X.25 protocols stack contains two levels: LAPB (Link Layer Protocol) and PLP (network layer protocol). On the network node, where there is support and X.25, and Frame Relay, X.25 encapsulation procedure is provided in the Frame Relay IWF (Figure 2.5). Herewith, network level information (PLP) is transmitted to a Frame Relay network transparently, without recognition and decoding, and recovery of the message structure is provided on the remote X.25 DTE, supporting the function of IWF. In the process of encapsulating the X.25 packet in the Frame Relay frame the placing of the address, control and information fields in the LAPB Frame Relay frame format is being provided. Then the new address of connection via DLCI network is being added and calculation of the new FCS value is being provided. The reverse process of decapsulation is performed on the remote DTE with the functions of IWF.

Fig. 2.5. Х.25 packets encapsulation to the Frame Relay frames

2.1.3 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.