GSM Interfaces and Protocols (Global System for Mobile Communication (GSM)) Part 2

Abis interface

Abis is the interface between the BSC and the BTS. Figure 3-14 illustrates the protocol stack on the Abis interface. The following sections describe each layer in detail.

Physical layer. The physical layer, i.e., Layer 1, consists of a 2-Mb/s PCM30 link. This is based on ITU-T G.703 specifications. APCM30 link consists of 32 multiplexed 64-Kb/s timeslots. Thirty timeslots carry speech or user data, and the remaining two timeslots are used for synchronization and signaling purposes. Most of the vendors support further division of each 64b/s timeslot into four 16-Kb/s timeslots.

Protocol stack over the Abis interface.

Figure 3-14 Protocol stack over the Abis interface.

This has the obvious advantage of better link utilization. It also enables mapping of traffic channels at the Um interface directly to Abis. The traffic channels at the Um interface have almost the same data rates. A transcoder rate adoption unit (TRAU) is required to convert 64-Kb/s speech into 13-Kb/s GSM speech. The TRAU can be located at the BTS or BSC or MSC side. Generally, one 2-Mb/s link covers more than one BTS. The exact configuration of Abis links depends on the traffic requirements, TRAU location, and equipment capabilities.


Layer 2 protocol. The Data Link Layer, i.e., Layer 2, is based on the ISDN link access procedure on the D-Channel (LAP-D) protocol, with a few changes. The LAPD protocol is defined in ITU-T Q.920 and Q.921 specifications. The ITU-T Q.920 standard defines the general parameters of ISDN Layer 2. The ITU-T Q.921 defines the specifics of Layer 2. The main task of Layer 2 is to control the logical signaling links between a BSC and its connected BTSs. It also ensures error-free transmission of information between communicating entities.

Each BTS is connected on the Physical Link with the controlling BSC. However, the BTS have several logical LAPD data links over a physical link. The logical links are provided for Layer 3 information transfer and O&M of the BTS equipment and the links themselves. Each logical link is uniquely identified with a service access identifier (SAPI) and terminal equipment identifier (TEI) combination. The SAPI and TEI are parts of the address field in a LAP-D frame. The LAP-D frame is shown in Figure 3-16 and will be explained later in this section.

The SAPI identifies the Layer 3 protocol. The SAPI is 6 bits long and can have a value from 0 to 63. However, in GSM only three values, as given in Table 3-7, are used.

The TEI identifies one transceiver (TRX). The TEI is 7 bits long and hence can have a value from 0 to 127. GSM uses TEI values 0 to 63 for fixed TRX addresses. The values from 64 to 126 are used for additional TRX addresses in cases where TRX needs more than one signaling link.

TABLE 3-7 SAPI Values Used in GSM

SAPI (decimal)

Description

0 Radio signaling link (RSL)

This link is used to transfer Abis Layer 3 messages between BTSs and BSC. In addition, it serves the traffic management procedures of Layer 2.

62 Operation & maintenance link (OML)

This link is used to transfer BTS O&M messages.

63 Layer 2 management link (L2ML)

It is used for management of logical data link sharing of a physical connection.

Figure 3-15 shows the concept of uniquely identifying a logical data link using SAPI and TEI. One signaling link between the BTS and the BSC consists of three logical channels, RSL, OML, and L2ML, each of which is uniquely identified with a combination of SAPI and TEI.

LAP-D frame structure. Figure 3-16 shows a LAP-D frame structure. The flags indicate the beginning and the end of a frame. For consecutive frames, one frame is used to indicate the end of a first frame and the beginning of the next frame. A flag is 0111 1110 (hex 7E). In order to avoid repetition of this pattern within the information field, a zero is inserted after every five consecutive ones. This is called bit stuffing.

The two-octet address field, also known as the data link control identifier (DLCI), includes SAPI and TEI. The function of SAPI and TEI, as described in the previous section, is to identify logical data links. Each octet in the address field has one address extension (EA) bit. In the first octet, it is set to zero, indicating that one more address octet is to follow. The EAbit of the second octet is set to 1, indicating that it is the last octet of the address field. The command/response (C/R) bit is used to differentiate the commands from the responses. The BTS (user side) sets the C/R bit to one for responses and resets it to zero for commands. The BSC (network side) does the opposite, i.e., it resets the C/R bit when it sends a response and sets it when it sends a command.

There are three different formats of the control field.

Information transfer format (I frames). I frames control the transfer of the LAPD frame’s information field to Layer 3. I frames use N(S) (send sequence number), N(R) (receive sequence number), and P/F (poll/final bits) to number the frames and acknowledge correct receipt of frames.

Logical data links over the Abis interface.

Figure 3-15 Logical data links over the Abis interface.

LAP-D frame structure.

Figure 3-16 LAP-D frame structure.

Supervisory format (S frames). S frames handle Layer 2 flow control management, such as acknowledging the I frames and requesting retransmission and temporary suspension of I frames. N(R) and P/F bits are used in these frames.

Unnumbered information and control format (U frames). U frames provide additional transfer capabilities during unacknowledged transfer service or additional unacknowledged transfer service. N(S) and N(R) bits are not used. Only P/F bits are used.

Figure 3-16 shows the two different formats of the control field. The length (8 or 16 bits) of control field depends on the frame type and also on the sequence numbering used, i.e., modulo 8 or modulo 128.

Table 3-8 describes the different format and frame types. Table 3-9 describes the functions of different frame types.

The information filed is of variable length and carries Layer 3 information. A maximum of 260 octets can be sent over LAPD. The information field is present in all I frames and U frames that transfer information, i.e., UI frames. It is not present in S and U frames with only one exception, i.e., FRMR.

The frame check sequence (FCS) is used to detect errors in a frame. It is a 16-bit cyclical redundancy checksum (CRC) defined by ITU-T.

Layer 3 protocol. At Layer 3, between BSC and BTS, two different types of message flow take place, i.e., transparent and nontransparent messages.

TABLE 3-8 LAP-D Frame Formats

Control field format

Name of frame

Type of frame

Control field length (octets)

I frame

Information (I)

Command

2

S frames

Receiver ready (RR)

Command response

2

Receiver not ready (RNR)

Command response

2

Reject (REJ)

Command response

2

U frames

Set asynchronous balanced

Command

1

mode extended (SABME)

Disconnect mode (DM)

Response

1

Unnumbered information (UI)

Command

1

Disconnect (DISC)

Command

1

Unnumbered

Response

1

acknowledgement (UA)

Frame reject (FRMR)

Response

1

Exchange identification (XID)

Command

1

TABLE 3-9 LAP-D Frame Functions

Frame

Functions

Information (I)

I frame carries Layer 3 information across a data link connection during acknowledged transfer service.

Receiver ready (RR)

RR frames are used to indicate:

■ Layer 2 entity is ready to receive

■ Acknowledgment of previously received I frame

Receiver not ready (RNR)

It is used to indicate that a data link layer entity is busy and no more I frames can be accepted.

Reject (REJ)

A reject command frame requests retransmission of I frames starting with a frame numbered N(R). As a response, an REJ frame indicates the clearance of a busy condition.

Set asynchronous balanced mode extended (SABME)

SABME frame begins a data link connection for acknowledged information transfer service.

Disconnect mode (DM)

The transmitting side uses the DM frame to indicate that it can no longer maintain the Layer 2 connection.

Unnumbered information (UI)

UI frame carries Layer 3 information across a data link connection during unacknowledged transfer service.

Disconnect (DISC)

The transmitting side indicates its intention to tear down the Layer 2 connection by sending a DISC frame.

Unnumbered acknowledgement (UA)

A UA frame is used as a response to a SABME or DISC frame.

Frame reject (FRMR)

Unlike a reject frame, the FRMR is used to report an error condition that cannot be recovered by retransmission of frame. For example, a protocol error detected in a Layer 3 message cannot be set right simply by retransmission of a frame.

Exchange identification (XID)

Not used in GSM.

Transparent messages pass through the BTS without any decoding and action. These messages are from the MS and intended to be for the BSC/MSC or the other way around. The CM and the MM messages are examples of the transparent messages. The BTS does not process these messages. However, the RR layer contains messages of both types. The nontransparent messages, in this case, are those related to radio equipment and need to be handled by the BTS. The BTS management layer at the BTS interprets these messages and performs actions. An example of nontransparent RR messages is the ciphering message, where the ciphering key is sent to the BTS only, not to the MS.

As shown in Figure 3-15, the signaling channel between the BTS and the BSC carries three logical channels: RSL, OML, and L2ML. Table 3-7 lists the SAPI assigned to each logical channel. The RSL, which is assigned SAPI0, carries user signaling, i.e., all messages related to connection setup, release, SMS, and supplementary services (SS). The messages sent over RSL are divided into four groups.

Radio link layer management (RLM). The RLM contains the messages related to status and control of Layer 2 connection between the BTS and the BSC.

Common channel management (CCM). The CCM contains the messages that carry common control channel (CCCH) signaling data to and from the air interface.

TRX management (TRXM). The TRXM contains the messages that are related to TRX management.

Dedicated channel management (DCM). The DCM contains messages related to status and control of Layer 1 of the air interface.

Figure 3-17 shows the Layer 3 message structure for a transparent message. In this example, the LAP-D frame is carrying a Layer 3 CM service request transparently. The message discriminator is used to distinguish between RRM, TRXM, CCM, and DC messages. In the example shown, the message discriminator is set to 1, indicating an RRM message. The bit T is set to 1, indicating a transparent message. The protocol discriminator is used to discriminate between RR, MM, and CM (CC and SMS).

Table 3-10 lists RLM, CCM, TRXM, and DCM messages. The uppercase letters in the message name are mnemonics used in context and protocol presentations.

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