Speech coding and channel coding

The speech coder is a regular pulse excited linear predictive coder (RPE-LPC) with long term prediction. This provides a net bit rate of 13kbit/s. It is a block based coder where the input samples are analysed in blocks with a 20ms duration. Work is also being carried out to specify a half rate speech coder which will effectively double the system capacity of GSM.

Before being assembled into the timeslots and frames, the digital speech and signalling data is encoded and interleaved. The speech coder output is divided up into three classes of bits and the most sensitive bits are encoded by adding parity check bits followed by a convolutional coder. Signalling data is encoded using a FIRE code. A process of interleaving is then used to spread the data blocks over a number of bursts.

For speech, an interleaving degree of 8 is used, i.e the speech block is spread over 8 bursts, whilst an interleaving degree of 4 is used for signalling. This overall process is shown in Figure 47.11, and the combined use of coding and interleaving provides good protection of channel data from the fading, dispersion and inter­ference effects on the radio path. With the addition of frequency hopping and diversity techniques, the GSM air interface is particu­larly robust.

One of the penalties to be paid for this is the overall transmission delay. The speech coder contributes about 25ms and the channel coding and interleaving a further 37ms. The rest of the transmission delay budget allows for analogue to digital conversions, 16kbit/s transmission and switching in various parts of the network. The overall one way transmission delay thus amounts to around 90ms. Such a delay means that echo control is necessary even on short national calls.

Table 47.4 GSM logical channels

Figure 47.11GSM channel coding and interleaving

GSM signalling

Figure 47.12 shows the overall signalling model. The Air Interface uses LAPDm Layer 2 signalling protocol and this is also used for the A-bis, BTS to BSC interface.

The layer 3 protocol consists of three sublayers, dealing with radio resource management (RR), mobility management (MM), and con­nection management (CM). Radio resource management is con­cerned with managing the logical channels, including paging, channel assignments, handover, measurement reporting, and other functions.

The mobility management layer contains functions necessary to support the mobility of the user which include authentication, loca­tion updating, attach and detach of IMSI (International Mobile Subscriber Identity), and registration. The connection management layer is concerned with call control, establishing and clearing cir­cuits, management of supplementary services and the short message service.

The BSC to MSC A-interface, and the various MSC to Register interfaces employ CCITT No.7 signalling using the Message Trans­fer Part (MTP), Signalling Connection Control Part (SCCP), Trans­action Capabilities Part (TCAP) and Mobile Application Part (MAP).

An example of the signalling messaging for establishing a mobile originated call is shown in Figure 47.13. The key events are:

1. Request and assignment of a channel, between MS and BSS.

2. A service request procedure which accesses the VLR.

3. An authentication and ciphering exchange which validates the mobile user and sets the encryption cipher.

4. Call set up which includes sending of dialled digits and estab­lishing the connection.

Location updating is shown in Figure 47.14 An update request is indicated by the mobile and passed to the VLR in the new location area. The new VLR requests the IMSI from the old VLR and then signals the new location to the HLR. The HLR provides the subscriber data to the new VLR and cancels the subscriber entry in the old VLR. Finally a confirmation message is set back to the mobile. There are, of course, many other signalling exchanges, dealing with mobile terminating calls, supplementary services, and short message service. There is not space in this chapter to deal with the detailed signalling for these cases; the examples above describe the general principle and illustrate the roles of the MS, BSS, MSC, VLR and HLR.

Figure 47.12GSM signalling model

Security features

The information on the air interface needs to be protected, to provide user data (including speech) confidentiality and to prevent fraudulent use of subscriber and mobile identities. The basic mech­anisms employed are user authentication and user data encryption. Each mobile user is provided with a Subscriber Identity Module (SIM) which contains the IMSI, the individual subscriber authenti­cation key (K_i) and the authentication algorithm (A3). After the mobile user has made an access and service request, the network checks the identity of the user by sending a random number (RAND) to the mobile. The mobile uses the RAND, K_i and A3 algorithm to produce a signed response SRES. This response is compared with a similar response calculated by the network, and access only continues if the two responses match.

The SIM also contains a cipher key generating algorithm (A8). The MS uses the RAND and A8 to calculate a ciphering key (Kc) which is used to encrypt and decrypt signalling and user data information.

The authentication centre (AUC) is responsible for all security aspects and its function is closely linked with the HLR. The AUC generates the Ki's and associates them with IMSIs, and provides the HLR with sets of RAND, SRES and Kc for each IMSI. The HLR then provides the appropriate VLR with these sets and it is the VLR which carries out the authentication check. Authentication of mobile users can be carried out on call set up, both mobile originated and mobile terminated, on location updating, and on activation of supplementary services. As the authentication sets are used up in the VLR, further sets are requested from the HLR.

An additional security feature of GSM is the equipment identity register (E1R). This enables monitoring ofthe mobile equipment 1MEI (International Mobile Equipment Identity) which is used to validate mobile equipments thus preventing non-approved, faulty or stolen equipment from using the system. This range ofsecurity features provide a high degree of protection to the user and the network operator.

Figure 47.13 Mobile originating call

Date: 2015-12-11; view: 1307