WO1996035277A1

WO1996035277A1 - A method and apparatus for transmitting a digital data message according to a second frame format

Info

Publication number: WO1996035277A1
Application number: PCT/GB1996/000948
Authority: WO
Inventors: Richard John Albrow; Simon Alexander Black; Leigh Carter; Rupert Leslie Alexander Goodings; Paul Maxwell Martin; Neil Philip Piercy
Original assignee: Ionica International Limited
Priority date: 1995-05-05
Filing date: 1996-04-19
Publication date: 1996-11-07
Also published as: AU5342296A; IL118018A0; ZA963390B; GB9509199D0

Abstract

In a method of transmitting a digital data message in predetermined time slots within fixed length time intervals, data in a frame according to a first format is processed for transmission in a frame according to a second format. The formats are in accordance with different data link control protocols. Upon reception, the frame according to the first format is retrieved.

Description

A METHOD AND APPARATUS FOR TRANSMITTING A DIGITAL DATA MESSAGE ACCORDING TO A SECOND FRAME FORMAT

The invention relates to digital data transmission in time slots within fixed length time frames, in particular, time division multiplexing\time division multiple access (TDM/TDMA) digital telephony.

Data link control protocols are well known in conventional TDM/TDMA systems, such as

GSM ("Global System for Mobile Communications"). Data link control protocols are in accordance with level 2 of the ISO-OSI reference models as discussed, for example, in Telecommunication Networks and Services, Jan van Duuran, Peter Kastelein and Firts C. Schroute, Addison-Wesley Publishers 1992.

The present invention is defined in the claims, to which reference should now be made. Preferred features are laid out in the subclaims.

The present invention preferably provides a method of transmitting a digital data message

using a TDM/TDMA transmission scheme, in which a frame according to a first data link control protocol is processed for transmission in a frame according to a second data link control protocol. Transmission is preferably by radio. The present invention also relates to a corresponding transmitter.

The processing for transmission can include selecting data from the frame according to the first data link control protocol and including said data with other data in the frame according to the second data link control protocol. The said data can be enveloped by, that is, preceded and followed by, said other data in the frame for transmission.

The frame according to a first data link control protocol is preferably an LAPD frame. The frame according to the second data link control protocol is preferably an LAPI frame.

LAPD (Link Access Protocol Data) is defined for example in CCITT Q.920/921

Recommendation. LAPI is a data link layer protocol specifically for use over a radio link. LAPI provides an acknowledged data transfer service similar to the acknowledged data transfer service provided by LAPD. LAPI is a protocol in which a frame includes an address field, a length field, a control field, an information field and an error control field.

The present invention preferably also relates to a method of receiving a digital data message sent using a TDM/TDMA transmission scheme, in which a frame according to a second data link control protocol is processed to provide a frame according to a first data link control protocol. The present invention also relates to a corresponding receiver. The processing at the receiver can be reverse processing to that at the sending unit so that the frame according to the first data link control protocol is restored.

Preferably the data according to the first data link control protocol is stripped of frame checksum data. This has the advantage of improved transmission efficiency. Upon reception, this frame checksum data can be recomputed and reapplied. Second data link control protocol frames including data according to the first data link control protocol can be transmitted with other second data link control protocol frames. These two types of second data link control protocol frames can preferably be distinguished by a flag in their address fields. If errors are detected upon reception, processing steps can be selected dependent on the type of second data link control protocol frame received. In particular,

errors in second data link control protocol frames including data according to a first data link

control protocol can result in those frames being discarded; and errors in the other type of second data link protocol frame can result in error recovery procedures being undertaken.

The present invention is preferably suitable for transmission of ISDN, in particular D-channel signalling.

A preferred embodiment of the invention will now be described, by way of example, with reference to the drawings in which:

Figure 1 is a schematic diagram illustrating the system including a base station (BTE -Base Terminating Equipment) and subscriber unit (NTE - Network Terminating Equipment),

Figure 2 is a diagram illustrating TDM/TDMA frame structure and timing for a duplex link,

Figure 3 is a diagram illustrating Data Link Control (DLC) frame structure,

Figure 4 is a diagram illustrating an envelope frame including data from a different DLC frame format (LAPD data), Figure 5 is a diagram illustrating the address field of the envelope frame shown in Figure 4, and

Figure 6 illustrates the length field of the envelope frame shown in Figure 4.

The Basic System

As shown in Figure 1, the preferred system is part of a telephone system in which the local wired loop from exchange to subscriber has been replaced by a full duplex radio link between a fixed base station (BTE) and fixed subscriber unit (NTE). The preferred system includes the duplex radio link (Air Interface), and transmitters and receivers for implementing the necessary protocol. There are similarities between the preferred system and digital cellular mobile telephone systems such as GSM which are known in the art. This system uses a protocol based on a layered model, in particular the following layers: PHY (Physical), MAC (Medium Access Control), DLC (DataLink Control), NWK (Network).

One difference compared with GSM is that, in the preferred system, subscriber units are at fixed locations and there is no need for hand-off arrangements or other features relating to mobility. This means, for example, in the preferred system directional antennae and mains electricity can be used.

Each base station in the preferred system provides six duplex radio links at twelve frequencies chosen from the overall frequency allocation, so as to minimize interference between base stations nearby. The frame structure and timing for a duplex link is illustrated in Figure 2. Each duplex radio link comprises an up-link from a subscriber unit to a base station and, at a frequency offset, a down-link from the base station to the subscriber unit. The down-links are TDM, and the up-links are TDMA. Modulation for all links is π/4 - DQPSK, and the basic frame structure for all links is ten slots per TDM TDMA frame of 2560 bits, i.e. 256

bits per slot. The bit rate is 512kbps. Down-links are continuously transmitted and

incorporate a distributed broadcast channel for essential system information. When there is no user information to be transmitted, the down-link transmissions continue to use the basic TDM frame and slot structure and contain a suitable fill pattern.

For both up-link and down-link transmissions, there are two types of slot: normal slots which are used after call set-up, and pilot slots used during call set-up.

Each down-link normal slot comprises 24 bits of synchronisation information followed by 24 bits designated S-field which includes an 8 bit header followed by 160 bits designated D-field. This is followed by 24 bits of Forward Error Correction and an 8 bit tail, followed by 12 bits of the broadcast channel. The broadcast channel consists of segments in each of the slots of a TDM frame which together form the down-link common signalling channel which is transmitted by the base station, and contains control messages containing link information such as slot lists, multi-frame and super-frame information, connectionless messages, and

other information basic to the operation of the system.

During the call set-up, each down-link pilot slot contains frequency correction data and a training sequence for receiver initialisation, with only a short S- field and no D- field information. Up-link slots basically contain two different types of data packet. The first type of packet, called a pilot packet, is used before a connection is set up, for example, for an ALOHA call request and to allow adaptive time alignment. The other type of data packet, called a normal packet, is used when a call has been established and is a larger data packet, due to the use

of adaptive time alignment.

Each up-link normal packet contains a data packet of 244 bits which is preceded and followed by a ramp of 4 bits duration. The ramps and the remaining bits left of the 256 bit slot provide a guard gap against interference from neighbouring slots due to timing errors. Each

subscriber unit adjusts the timing of its slot transmissions to compensate for the time it takes

signals to reach the base station. Each up-link normal data packet comprises 24 bits of

synchronisation data followed by an S-field and D-field of the same number of bits as in each

down-link normal slot.

Each up-link pilot slot contains a pilot data packet which is 192 bits long preceded and followed by 4 bit ramps defining an extended guard gap of 60 bits. This larger guard gap is necessary because there is no timing information available and without it the propagation delays would cause neighbouring slots to interfere. The pilot packet comprises 64 bits of sync followed by 104 bits of S-field which starts with an 8 bit header and finishes with a 16 bit Cyclic Redundancy Check, 2 reserved bits, 14 FEC bits, and 8 tail bits. There is no D- field.

The S-fields in the above mentioned data packets can be used for two types of signalling. The first type is MAC signalling (MS) and is used for signalling between the MAC layers of the base station and the MAC layer of a subscriber unit whereby timing is important. The second type is called associated signalling, which can be slow or fast and is used for signalling between the base station and subscriber units in the DLC or NWK layers.

The D-field is the largest data field, and in the case of normal telephony contains digitised

speech samples, but can also contain non-speech data.

Provision is made in the preferred system for subscriber unit authentication using a challenge response protocol. General encryption is provided by combining the speech or data with a non-predicable sequence of cipher bits produced by a key stream generator which is synchronised to the transmitted super-frame number.

In addition, the transmitted signal is scrambled.

Data Link Control Enveloping

As mentioned above, the communication protocol is based on a layered model, of which the

DLC (data link control) layer basically corresponds to the upper part of the data link layer of the OSI standard model.

The DLC layer operates using a frame based protocol (known as a Link Access Protocol) such that each DLC frame contains an address field, a length field, a control field, a data field and an error control field. The DLC frame structure of the particular link access protocol used, known as LAPI, is illustrated in Figure 3. Figure 3 shows a one octet address field followed by a one octet length field, an m-octet control and information field, an n octet fill field and two octets of cyclic redundancy check (CRC) data.

Significantly, the DLC layer supports an enveloping function allowing foreign frame types

to be transmitted across the radio link. In particular, the enveloping function allows foreign link access protocol frame formats, such as LAPD, to be sent.

The basic operation is to process the foreign DLC frame into a format suitable for transmission from the radio link transmitter and then to reconstruct the foreign frame at the

receiver from the received signals.

More specifically, and as will be explained below, the checksum and delimiting is removed from the foreign DLC frames to be transmitted, an envelope address, new length delimiting and a recalculated checksum are added and the resulting data is sent as associated signalling in the S-field (mentioned above) over the radio link.

Envelope frames and other LAPI frames are transmitted, each having an address field including a link protocol discriminator field (LPD) as discussed below. This functions as a flag to indicate whether or not the frame is an envelope frame. If errors occur in reception, the processing undertaken depends on the LPD data. If the LPD data indicates that the frame is an envelope frame, the frame is discarded, a repeat frame being subsequently resent. If the LPD data indicates a non-envelope frame then automatic error recovery procedures are undertaken. Procedure for Processing a Foreign Frame to be Transmitted

An example of a foreign DLC frame to be transmitted is one of the LAPD type. This could be received at the subscriber unit from an ISDN telephone handset, or provided to the base station via the network, in both cases for transmission over the radio link. LAPD frames are

processed as follows to provide a frame for transmission known as an envelope frame:

a) Flag sequences are removed, b) Zero bit extraction is performed as defined in CCITT Recommendation Q.921 clause 2.6, c) The frame check sequence (FCS) field of the frame is checked as defined in CCITT Recommendation Q.921 clause 2.7 and removed, d) The remaining fields of the LAPD frame (address, control and information fields) is taken as the information field of the envelope frame. e) The length of the envelope frame is calculated and the length field prefixed with appropriate parameters, f) The address is prefixed to the envelope frame. (The DLC address parameter indicates the ISDN user port with which this frame is associated), g) The envelope frame is passed to the lower part of the DLC layer for

transmission.

Only valid LAPD frames are processed to provide envelope frames. Invalid LAPD frames are discarded without notification to the sender. An invalid LAPD frame is a frame which: a) Is greater than 266 octets in length following zero bit extraction, or b) Has fewer than 5 octets following zero bit extraction, or c) Does not consist of an integral number of octets following zero bit extraction, or d) Contains a frame check sequence (FCS) error.

An example envelope frame is shown in Figure 4. The envelope frame consists of 1 octet of address information, 2 octets of length information, the LAPD core data, fill data (if

necessary) and 2 octets of cyclic redundancy check. The maximum envelope frame length

is 280 octets which limits the maximum information field length to 275 octets.

Taking these fields in turn:-

Address Field

The address field is shown in more detail in Figure 5 and includes:

Address field extension bit (EA)

The address field range is extended by reserving the first bit of the address field octet to indicate the final octet of the address field. The presence of a digit 1 in the first bit of an address field octet indicates that it is the final octet of the address field. The Envelope frame always has the EA bit set to 1 to indicate a single octet of address information.

Reserved bit (RES)

The reserved bit is undefined for this frame structure and is always set to 0. Link Protocol discriminator field (LPD)

The link protocol discriminator field indicates that the frame is an envelope frame.

Data Link control address field (DLC Address)

The DLC address identifies the Layer 2 protocol which is using the enveloping function. For

ISDN, each DLC address refers to a complete CCITT Recommendation Q.921 data link layer

(which may consist of multiple instances of LAPD).

Length Field

The length field, which is two octets in length, is shown in more detail in Figure 6. The length field parameters are as follows:

Extended Length (EL) bit

The extended length bit is used to allow the length field to span more than one octet. It has the value 0 for the first octet and the value 1 for the second octet.

Reserved bit (RES) bit

The reserved bit is undefined for this frame structure and is set to 0.

Length parameter

The length parameter indicates the length of the information field in octets. Allowable values shall be 0 to 275. Information field

The information field contains the frame to be enveloped.

Fill Field

The fill field is added by the lower DLC layer when required.

Cyclic Redundancy Code (CRC) field

The CRC field is added by the lower DLC and is a 16 bit sequence. It contains an 8 bit colour code and an underlying sixteen bit cyclic redundancy code (CRC).

Processing a Received Envelope Frame to Restore the Foreign Frame

Envelope frames received from the lower DLC layer destined for an ISDN user port at the subscriber unit (NTE), or the access network at the base station (BTE), are processed according to the following procedure:

a) The ISDN user port with which the envelope frame is associated is determined from the DLC address within the address field. The address field is then discarded, b) The length field is discarded, c) The frame check sequence (FCS) field is calculated and appended to the frame as defined in CCITT Recommendation A.921 clause 2.7. d) Zero bit insertion is performed as defined in CCITT Recommendation A.921 clause 2.6, e) Flag sequences are added to the frame as defined in CCITT Recommendation Q.921 clause 2.2, f) The frame is routed according to the DLC address determined in (a).

Claims

1. A method of transmitting a digital data message in predetermined time slots within fixed length time intervals, the message comprising one or more frames of data, in which data in a frame according to a first frame format is processed for transmission in a frame according to a second frame format.

2. A method of transmitting a digital data message according to claim 1, in which transmission is by radio.

3. A method of transmitting a digital data message according to claim 1 or claim 2, in which the processing for transmission includes selecting data from the frame according to the first format and including said data with other data in the frame according to the second format.

4. A method of transmitting a digital data message according to claim 3, in which the said data is preceded and followed by said other data in the frame for transmission.

5. A method of transmitting a digital data message according to any preceding claim, in which the first format is that of a first data link control protocol and a second format is that of a second data link control protocol.

6. A method of transmitting a digital data message according to claim 5, in which the frame according to a first data link control protocol is an LAPD (Link Access Protocol Data) frame.

7. A method of transmitting a digital data message according to claim 6, in which the format of an LAPD frame is defined in CCITT Q.920/921 Recommendation.

8. A method of transmitting a digital data message according to any of claims 5 to 7, in which the frame according to the second data link control protocol is an LAPI frame.

9. A method of transmitting a digital data message according to claim 8, in which LAPI is a data link control protocol adapted for use over a radio link, and providing an acknowledged data transfer service.

10. A method of transmitting a digital data message according to claim 8 or claim 9, in which a LAPI frame includes an address field, a length field, a control field, an information field and an error control field.

11. A method of transmitting a digital data message according to any preceding claim in which the processing comprises stripping first frame checksum data from the data in the frame according to the first format.

12. A method of transmitting a digital data message according to claim 11, in which the processing further comprises computing second frame checksum data for transmission as part of a frame according to the second frame format.

13. A method of transmitting a digital data message according to any preceding claim, in which frames according to the second format including data according to the first format are transmitted with other frames according to the second format, these two types of frames according to the second format being distinguishable by flags in their address fields.

14. A method of transmitting a digital data message according to any preceding claim, for transmission of Integrated Services Digital Network (ISDN) signalling.

15. A method of receiving a digital data message sent in a predetermined time slot or slots

within fixed length time intervals, the message comprising one or more frames of data, in which a data in frame according to a second format is processed to provide a frame according to a first format.

16. A method of receiving a digital data message according to claim 15, in which first frame checksum data is recomputed and reapplied to reconstruct the data in the frame according to the first format.

17. A method of communicating a digital data message between a transmitter and receiver in predetermined time slots within fixed length time intervals, the message comprising one or more frames of data, in which at the transmitter data in a frame according to a first format is processed for transmission in a frame according to a second format, the frame being transmitted, and at the receiver the frame according to the second format is received and processed to retrieve the frame according to the first format.

18. A method of communicating a digital data message according to claim 17, in which frames according to the second format including data according to the first format are transmitted with other frames according to the second format, these two types of frames according to the second format being distinguished upon reception by flags in their address fields, and if errors are detected upon reception, processing steps are selected dependent upon

the type of frame determined.

19. A method of communicating a digital data message according to claim 18, in which determination of errors in frames according to the second format including data according to the first format result in those frames being discarded.

20. A method of communicating a digital data message according to claim 19, in which errors in the other type of frame results in error recovery procedures being undertaken.

21. A transmitter for transmitting a data message in predetermined time slots within fixed length time intervals, the message comprising one or more frames of data, in which the transmitter comprises processor means for processing data in a frame according to a first format for transmission in a frame according to a second format.

22. A receiver for receiving a data message sent in predetermined time slots within fixed length time intervals, the message comprising one or more frames of data, in which the receiver comprises processing means for processing data received in a frame according to a second format to retrieve data into a frame according to a first format.

23. Communications apparatus comprising a transmitter and a receiver, the transmitter being operative to transmit a data message comprising one or more frames of data in predetermined time slots within fixed length time intervals, the transmitter comprising first processor means operative to process data in a frame according to a first format for transmission in a frame according to a second format, the receiver being operative to receive the data message, the receiver comprising second processor means operative to process data in the frame according to the second format to retrieve data into a frame according to the first format.

24. A communications network comprising a base station and a plurality of subscriber units, the base station and subscriber units each comprising communications apparatus comprising a transmitter and receiver according to claim 23.

25. A communications network according to claim 24, in which the subscriber units are at fixed locations.