Modelado CFD

The LLC provides a reliable link between the mobile device and the SGSN for both control and user data. It supports variable length information fields from 140 bytes up to a maximum of 1520 bytes for the payload and can transfer data and control messages, which may or may not be encrypted. The LLC protocol supports both acknowledged and unacknowledged modes of operation with the ability to also reorder frames that have been received out of sequence. This could occur, for example, in the case of retrans- mission of frames with errors. It is designed to be independent of the underlying radio protocols to enable different radio solutions to be adopted. In the control plane LLC car- ries GPRS mobility management (GMM) messages, such as attach, authentication, and session management (SM) information, such as PDP context activation, as well as also transporting SMS messages to the higher layers. In the user plane, LLC frames carry the

SNDCP packets which will contain the user data such as IP packets. Figure 4.9 shows how a single LLC is used for transporting signalling, SMS and different data connections concurrently. An LLC connection is identified by a datalink connection identifier (DLCI). This consists of the service access point identifier (SAPI) and the temporary logical link identifier (TLLI) of the mobile device.

As was seen previously, in GSM for security reasons, the transfer of the user IMSI is minimized, and thus a TMSI, assigned by the VLR, is used instead. In GPRS, the SGSN will assign a user the equivalent for the packet network, known as a P-TMSI. Like a TMSI, this is a 4-byte number, with local significance only. The mobile device will build a TLLI from the P-TMSI and use this TLLI to refer to itself in subsequent transactions with the network. This TLLI is a 32-bit number and uniquely defines the logical link between the mobile device and the SGSN. The mobile can build three different types of TLLI: local, foreign and random. A local TLLI is derived from the P-TMSI allocated by the SGSN in the current routing area (see later) and is therefore valid only within that area. A foreign TLLI is derived from a P-TMSI from a different routing area. A random TLLI is built if there is no P-TMSI available. Figure 4.14, shows how each is built.

The SAPI is carried within the LLC frame header and defines the particular SAP within the mobile device and SGSN with which it is associated. Since the TLLI is used to uniquely identify the mobile device, it is therefore required to be present. However, it can be seen from Figure 4.15 that there is no TLLI field within the LLC header. The underlying BSSGP (between the BSC and SGSN) and the RLC/MAC (between mobile device and BSC) during contention periods are required to transport the TLLI information that uniquely identifies the mobile device.

1 1 bits 29-0 of P-TMSI 31 30 29 0 1 0 bits 29-0 of P-TMSI 31 30 29 0 0 1 chosen randomly 31 30 26 0 local foreign random 1 1 1 29 28 27

Figure 4.14 Format of the TLLI

1 8

Control information

Information Field variable in length up to 1520 bytes CRC, 3 bytes long 0 A X N(S) 1 8 N(S) X N(R) S1 S2 N(R) 1 0 A N(R) S1 S2 N(R)X X 1 1 0 N(U) E PM N(U)X X 1 1 1 P/F M4 M3 M2 M1 S-frame I-frame UI-frame U-frame PD C/R X X SAPI

4.6.3.1 Unacknowledged mode

For this mode of operation an entity may initiate transmissions to a peer entity without having established a logical connection. The LLC does not guarantee in-order delivery of the frames and no error recovery procedures are defined. The LLC can however detect errors in a received frame, and depending on whether protected or unprotected mode has been used for the transmission will either discard or deliver the frame with errors. Recall that data has no tolerance of errors so an indication of errors being present is generally what is required, rather than a measure of how many errors are present. In protected mode, a frame check sequence (FCS) in the form of a CRC covers both the frame header and information field. In unprotected mode, on the other hand, the CRC check covers the header and only the first 4 bytes of the information field, corresponding to the maximum length of a SNDCP segment PDU header. The rest of the information is unprotected and permits applications which can tolerate errors to receive frames that may have errors. There is also no flow control for this transfer option. The frame format used is the numbered unconfirmed information (UI) frame. This mode of operation is referred to as asynchronous disconnected mode (ADM). Unacknowledged mode of operation is specified for all SAPIs that are not reserved, as indicated in Table 4.4. Both GPRS mobility management and SMS messages use this mode for transfer.

4.6.3.2 Acknowledged mode

In the acknowledged mode, each sending entity is responsible for the organization of its data flow and for error recovery procedures. To enable this, the link needs to be first established using the set asynchronous balanced mode (SABM) command. The frame format used is the numbered information (I) frame and the frames are acknowledged at the LLC layer. Procedures are specified for both retransmission of any frames that are

Table 4.4 SAPI identifiers

Value SAPI Service description SAP name Mode

0 0000 Reserved

1 0001 GPRS mobility management LLGMM Unack

2 0010 Tunnelling of messages 2 TOM2 Unack

3 0011 User data 3 LL3 Ack/Unack

4 0100 Reserved

5 0101 User data 5 LL5 Ack/Unack

6 0110 Reserved

7 0111 SMS LLSMS Unack

8 1000 Tunnelling of messages 8 TOM8 Unack

9 1001 User data 9 LL9 Ack/Unack

10 1010 Reserved

11 1011 User data 11 LL11 Ack/Unack

12 1100 Reserved

13 1101 Reserved

14 1110 Reserved

unacknowledged as well as for flow control. This mode of operation is referred to as asynchronous balanced mode (ABM) and provides a reliable in-order delivery service. This mode of operation is allowed for all SAPIs that are not reserved except for SAPIs 1, 2, 7 and 8 (see Table 4.4).

4.6.3.3 LLC frame formats and procedures

The LLC frames that are received will have different SAPIs to indicate what is being transported. For example, SAPI 1 is reserved for mobility management as shown in Table 4.4.

Figure 4.15 shows the format of the LLC header, and its fields are now described. Theprotocol discriminator (PD) bit in the frame address header should be set to a logic 0 to indicate that this is an LLC frame; if it is set to 1 the frame will be treated as an invalid frame.

Thecommand/response (C/R)bit indicates whether this is a command or a response to a command. The options are highlighted in Table 4.5.

The SAPI identifies the address of the higher layer services, to which this frame should be sent. There are 4 bits set aside for SAPI addresses and thus there can be a maximum of 16 SAPIs. A number of these are reserved and Table 4.4, correlates the addresses to the specific SAPI entities. The service access point (SAP) name identifies the actual service that is associated with the particular logical link frame.

It can be seen that there are four separate SAPs for user data. Each one of these can be assigned a different QoS. There are also two SAPs for tunnelling of messages. These give high and low priority to the messages being transferred. The tunnelling of messages is an optional procedure which uses the LLC unacknowledged mode of operation to tunnel non-GSM messages such as EIA/TIA-136 messages.

There are four types of control field:

• Supervisory (S frame)frames are used to perform LLC functions. They can acknowledge I frames using the sequence number of the received frame, N(R), and also temporarily suspend I frame transmission. The acknowledge request bit (A) is set to logic 1 by the sender if an acknowledgement is required, and to 0 if no acknowledgement is required. The S frame is sent if there is no information field data that needs to be transferred.

• Confirmed information transfer (I frame) where there is a sequence number for both the sent frames, N(S), and the received frames, N(R). Each I frame also contains a supervisory (S frame) and is sometimes referred to as an I+S frame.

Table 4.5 C/R values Type Direction C/R Command SGSN to UE 1 Command UE to SGSN 0 Response SGSN to UE 0 Response UE to SGSN 1

• Unconfirmed information transfer (UI frame) is used to transfer information to higher layers that does not need acknowledgements and in this respect no verification of the sequence numbers, N(U), is performed. This means that a frame could be lost with no notification given to the higher layer. The information may or may not be encrypted. This is indicated by the E bit. If it is set to logic 1 then the information is encrypted. The protected mode (PM) bit indicates whether the CRC checks across the header and payload or just the header. If it is set to logic 1 it indicates that the payload is also protected.

• Unnumbered (U frame) provides additional LLC functions. It does not contain any sequence numbers. The poll/final (P/F) bit is referred to as the poll bit if this is a command frame and the final bit if this is a response frame. The P bit is set to logic 1 to request a response frame from the receiver. The F bit is set to a logic 1 to indicate that this is a reply to a poll request command.

It can be seen from Figure 4.15 that the I and S frames have two further bits, S1 and S2, and also that the U frame has four M bits. These bits are used for control and response functions and Figure 4.16 highlights the values these fields can have. If a field has a value which is not defined, then the frame is rejected.

The function of the I frame is to transfer sequentially numbered frames containing information for higher layers across the logical link between the SGSN and the mobile station. Referring to Figure 4.16(a), it can be seen that there are four command/responses and these are briefly described below:

• The receive ready (RR) command/response is used by a receiver to indicate that it is ready to receive an I frame. It is also used to acknowledge received I frames.

• The acknowledgement (ACK) command/response is used to acknowledge a single or multiple I frames up to and including the last frame received, N(R)−1.

• The selective acknowledgement (SACK)command/response supervisory frame is also used to acknowledge I frames. In this case the information field contains a list of the frames that have been received successfully so that the sender needs to retransmit only the missing frames.

• The receive not ready (RNR) command/response is used to indicate that the entity is unable to accept incoming I frames at this time.

M4