Layer 2 protocol for the random access channel and the access response channel

SWEDEN (12) PATENT WRITING (η) C2 (11) 516 740

<img file="SE516740C2_D0001.tif" />

(19) SE (51)

International class ⁷

H04J 3/12, 3/24, H04Q 7/38

PATENT AND REGISTRATION ₍₈₆₎

Patent issued 2002-02-26

Application generally available 1995-02-20 The patent application was submitted on 12/12/1994

Running day 1994-04-19

Tribal application number

International filing date 1994-04-19 Filing date for European patent application (83) Microorganism deposit (21) Patent application number 9404256 '!

Application received as:

Swedish patent application completed international patent application with number PCT / US94 / 04290 converted European patent application with number (30)

1993-04-19 US 047452 (73) (72) (74) (54) (56) (57)

Assignee

INVENTOR

AGENT

NAME

Ericsson GE Mobile Communications Inc,

Research Triangle Park NC US

John Diachina, Research Triangle Park NC US

Dr. Ludwig Brann Patentbyrå AB

Protocol on layer 2 for the random access channel and the access response channel

CALLED PUBLICATIONS:

US A 5,012,469 (370-95.3)

SUMMARY:

A method for transmitting messages between mobile stations and a cellular switching system is described, which method improves the channel burst capacity by allowing information from two distinct layer 3 messages (32, 34) to be transmitted in a unique layer 2 frame (50) transmitted in a channel burst. The end of the first layer 3 message (32) and the beginning of a second layer 3 message (34) are identified in a message end field (28) which is used to define the layer 3 message information contained in a subsequent data field.

<td> 7°,</td><td>32 ί</td><td>/ 't, / c</td><td>I3 (</td>

<td>5L'JI »A</td><td>j DATA FIELD</td><td>DATA FALL T</td><td>FIELD OF CYCLE</td>

<td>U '(</td><td>(MESSAGE 1)</td><td>(- •• LDCLL -'- '. R'F. 2;</td><td>RLDUNDANSKCNTROLl.</td>

1 ... 38 38 ... 1 16 r____y___J

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516 740

technology Area

The present invention relates to a method for transmitting messages between mobile stations and a central exchange, and in particular relates to a method for transmitting these messages using a more efficient communication link protocol over the air interface of a cellular telephone system.

Background of the invention

In typical land systems, remote stations and control centers are connected through copper or fiber optic circuits, which have a data throughput capacity and performance integrity that are generally substantially better than the data throughput capacity and performance integrity provided by the air interface of a cellular telephone system. As a result, the compactness of the administration required to handle a selected communication link protocol for the land systems is of secondary importance.

In cellular telephone systems, an air interface communication link protocol is required to enable the mobile station to communicate with a cellular switching system (CSS = Cellular Switching System). The communication link protocol is used to initiate and receive cellular telephone calls. The electromagnetic spectrum available to cellular telephone systems is limited and divided into units called channels. Individual channels are used as communication links either on a shared or dedicated basis. When the individual channels are used as communication links on a shared basis, several mobile stations can either listen to or compete for the same channels. In the competitive situation, each shared channel can be used by a plurality of mobile stations which compete to obtain exclusive use of the channel for a limited period of time. When individual channels are used as communication links on a dedicated basis

516 740, on the other hand, is assigned to a single mobile station the exclusive use of the channel as long as it needs it.

Given the generally reduced data throughput capacity and performance integrity offered by individual channels in a channel sharing situation in a cellular telephone environment, the choice of an effective air interface protocol such as basic seed communication link becomes paramount.

The communication link protocol is usually referred to as a layer 2 protocol in the communications industry, and its functionality includes delimiting or framing messages on higher layers. Traditional Layer 2 protocol bitmapping and flagging framing mechanisms are often used today in land networks for framing messages from higher layers, which are referred to as Layer 3 messages. These layer 3 messages can be transmitted between equal entities within the mobile station and cellular switching systems.

Communication between mobile stations and cellular switching systems will generally be described with reference to Figures 1 and 2. Figure 1 illustrates a plurality of layer 3 messages 11, layer frames 13 and channel bursts 14. A channel burst 15 is the shortest or smallest transmission event that can occur. between the mobile station and the cellular switching system. In Fig. 1, the channel bursts are separated in time, but they do not need to be separated. In other words, as soon as one channel burst ends, the next channel burst can begin. A channel burst 15 contains a complete layer 2 frame and other information, e.g. felkorrektionsinformation. Each layer 2 frame is divided into a number of different fields. One of the fields, namely the data field having a restricted frame, contains at least part of a layer 3 message 11. Since layer 3 messages have variable length depending on the amount of information contained in the layer 3 message, a plurality of layer 2 frames may be needed for transmitting a single layer message. Accordingly, a plurality of channel bursts may also be needed to transmit the entire layer 3 message, since

516 740 there is one-to-one correspondence between channel bursts and layer 2 frames. If multiple channel bursts are required to send a layer 3 message, these bursts are usually not successive bursts. Since it takes time to receive, process and respond to a received burst, the bursts required for transmitting a layer 3 message are sent in a zigzag format as illustrated in FIG. 2. In FIG. 2 illustrates a path A communication between a mobile station A and the cellular switching system. In this example, the mobile station A uses every six channel burst uplink (from mobile station to the cellular switching system) to transmit a layer 3 message to the cellular switching system CSS. Accordingly, other mobile stations, e.g. mobile station B, also send a layer 3 message to the CSS while mobile station A still transmits a layer 3 message using another set of associated uplink bursts.

In cellular telephone systems based on time-shared multiple access (TDMA), the built-in channel coding format has been proposed as a means of providing the basic layer 2 frame delimitation function, and this is an effective way of eliminating the aforementioned traditional layer 2 framing and flagging mechanism framing mechanisms. The TDMA channels can be used either dedicated or shared. These TDMA channels are essentially a sequence of transmission units or bursts, in which each burst has some fixed information-carrying capacity. Accordingly, each burst typically carries only a portion of a layer 3 message. When a TDMA channel is used in a shared way it can be referred to as a digital control channel in which several mobile stations either compete for its use in transmitting layer 3 messages to the cellular switching system (on the uplink) or collectively listen to the cellular switching system for potential layers 3 messages intended to receive them (on the downlink). In the uplink direction, several mobile stations try to communicate with the cellular switching system on a competitive basis, while several mobile stations listen for layer 3 messages sent from the downstream cellular switching system. Additional layers 2

516 However, 740 throughput performance improvements are still desirable, as both the uplink or random access channel (RACH = Random Access Channel) and the downlink or access response channel (ARCH = Access Response Channel) may have several different pending layer 3 messages at any given time.

In known systems, any given layer 3 message must be transmitted using as many TDMA channel bursts as are required to transmit the entire layer 3 message. The last TDMA channel burst used in transmitting a particular layer 3 message may not be fully utilized, since the last part of the layer 3 message does not need to fill the entire data field completely. In these systems, no measures have been taken to allow any pending layer 3 messages to be initiated within the remaining capacity of the data field. Accordingly, the existing systems with available TDMA channel burst capacity wastes when an additional separate and distinct layer 3 message is available and ready for transmission while a previously started layer 3 message transmission can be completed within a given TDMA channel burst.

Summary of the description

Accordingly, there is a need for a method for transmitting messages between mobile stations and a cellular switching system, which method improves channel tear capacity. It is therefore an object of the invention to provide a method for transmitting messages between mobile stations and a cellular switching system, in which a given channel burst method can contain information belonging to separate and distinct layer 3 messages.

Therefore, one aspect of the present invention is the provision of a method for transmitting messages between mobile stations and a cellular switching system using the concept of a layer 2 protocol frame transmitted within a channel burst, the frame first being divided into a plurality of fields. The frame type is then identified in a frame type field. End of a first layer 3 message

516 740 and the beginning of a second layer 3 message are identified in a message end field used to delineate the portions of layer 3 messages contained in a subsequent data field. Finally, layer 2 is transmitted the frame in a channel burst either uplink or downlink.

Brief description of the drawings

The present invention will now be described in more detail with reference to preferred embodiments of the invention, which are given by way of example only and illustrated in the accompanying drawings, in which:

Fig. 1 illustrates a plurality of layer 3 messages, layer 2 frames and channel bursts in a communication system;

Fig. 2 illustrates the communication path between a mobile station and a cellular switching system in the form of channel bursts;

Fig. 3 illustrates an initial frame in one embodiment of the present invention;

Fig. 4 illustrates an intermediate frame in one embodiment of the present invention;

Fig. 5 illustrates a final frame in one embodiment of the present invention;

Fig. 6 illustrates a dedicated layer 2 frame in one embodiment of the present invention; and

Fig. 7 illustrates a split layer 2 frame in one embodiment of the present invention.

Detailed description

Although the following description is made in connection with cellular communication systems of the type of time-shared multiple access, realize

516 It will be appreciated by those skilled in the art that the present invention may be applied to other digital communication applications, such as code division multiple access (CDMA)

In the present embodiment, layer 2 the protocol of the present invention is based on establishing four different types of layer 2 frames, i.e. an initial frame, an intermediate frame, an end frame, and an extension frame. These different types of layer 2 frames are suitable for transporting layer 3 messages in a cellular telephone system. For purposes of explanation, the following description assumes a TDMA channel burst structure for fast associated control channels (FACCHs) of EIA / TIA IS-54B, but the present invention is not limited thereto. The fast associated control channel is a signaling channel for transmitting control and monitoring messages between the cellular switching system and the mobile stations. In this embodiment, the fast associated control channel is structured in such a way that 65 information-carrying or utility bits are available in each TDMA channel burst.

In a cellular system, a plurality of mobile stations may at the same time be ready to communicate with the cellular switching system. However, only one mobile station can communicate with the cellular switching system in each channel burst uplink. Accordingly, the station stations that are ready to broadcast must compete for available uplink channel bursts (a contention event), with only one mobile station being granted access (winning a collision event) to a sequence of available uplink channels. System throughput performance will be degraded by an increased throughput delay if a mobile station is forced to transmit on a competitive basis, with each layer 3 message it may have waiting to attempt to access the cellular switching system. However, the present invention provides a means for avoiding multiple collisions by indicating that separate and yet distinct layer 3 messages from a mobile station are logically associated or composed. In the present invention, a mobile station may, since it has won a collision event, i.e. obtained access to the cellular switching system, sending all the separate and distinct layers 3 messages ready for transmission using this concatenation mechanism. This allows the system to exhibit better throughput performance by reducing competitive access of the cellular switching system. The present invention also helps to increase system throughput by allowing fewer channel bursts to be used when transmitting multiple layer 3 messages, since each given channel burst may contain information from two distinct layer 3 messages.

Each layer 2 frame is divided into a plurality of fields containing an additional burst indicator field, a data field, a message end field, a frame type field, a frame type modifier field, an initial / repeat field, and a reserved field. The various fields will now be described in connection with the figures.

Fig. 3 illustrates an initial frame in one embodiment of the present invention. Frame 10 is divided into four fields. A frame type field 12; an initial / repeat field 14; a data field 16; and a CRC field 18. Since layer 3 messages may be longer than a single layer 2 frame, multiple layer 2 frames may be needed to send a single layer 3 message. Accordingly, frame type field 12 identifies the type of frame such as either an initial frame, intermediate frame, end frame, or extension frame. In this embodiment, the frame type field 12 occupies two bits of the channel information utility information. The frame type field 12 is the first field in the initial frame 10, so that the first two bits in each channel burst identify the frame type, where 00 indicates an initial frame, 01 indicates an intermediate frame, 10 indicates an end frame, and 11 indicates an extension frame. The initial frame type is used in the initial burst of a transmission of a layer 3 message consisting of several bursts.

The initial frame 10 then has an initial / repeat field 14 at one bit. Transmission bursts are sometimes incorrectly received. Consequently

516 740, a transmission burst may need to be retransmitted. However, for the cellular switching system to function properly, it must be able to determine whether the received cut is an initial transmission burst or a repeated transmission burst. Accordingly, the initial / repeat field 14 indicates whether a channel burst is being transmitted for the first time or re-broadcast. For example, 0 in the initial / repeat field 14 indicates that the channel burst is transmitted for the first time, while 1 in the initial / repeat field 14 indicates that the channel burst is being retransmitted.

The data field 16 occupies most of the initial frame 10. The data field 16 contains a portion of layer 3 the message to be transmitted. Finally, the initial frame 10 contains a field 18 for cyclic redundancy control used for error detection. The cellular switching system uses the cyclic redundancy checkfield 18 to determine whether a given uplink channel burst is received correctly. The procedure for error detection using cyclic redundancy control (CRC) is well known in the art and is therefore not explained.

Fig. 4 illustrates an intermediate frame 20 in one embodiment of the present invention. The intermediate frame is used for intermediate channel bursts in a transmission of a layer 3 message containing multiple bursts. The intermediate frame 20 contains a frame type field 12, an initial / repeat field 14, a data field 16 and a CRC field 18, described above in connection with Fig. 3. A reserved field 22 is added to the intermediate frame to provide the header alignment required to support a clean and consistent transition point for transitioning from intermediate frames to final frames when the remaining layer 3 message information is more than 45 bits. The reserved field is added to the intermediate frame to avoid an unwanted transmission situation that could occur if 46 bits of a layer 3 message remain for transmission. Without the reserved field, all the remaining 46 bits of data would be transmitted in an intermediate frame which would be followed by a final frame which would

516 740 ί * contain a blank data field. This adverse situation is avoided by adding the reserved field in the intermediate frame, so that 45 bits of the remaining 46 bits are transmitted in the intermediate frame and the last bit is transmitted in a final frame.

Fig. 5 illustrates an end frame 30 in one embodiment of the present invention. The final frame 30 is used for the last cut of a layer 3 message transmission containing multiple bursts. The final frame 30 contains a frame type field 12, an initial / repeat field 14, a data field 16 and a CRC field 18, which are described above in connection with Fig. 3. However, the final frame also contains an additional burst indicator field 24 which provides an explicit indication of message composition or linking, since the additional burst indicator indicates whether an additional burst belonging to a separate and distinct layer 3 message sharing the same uplink access event is waiting. The additional burst indicator field 24 also provides an explicit indication of message composition when used on the downlink channel. If the additional burst indicator field 24 indicates that an additional uplink channel burst is waiting, the cellular switching system will continue to operate as if the initial layer 3 message is still being sent. This ensures that the remaining composite layer 3 messages share the same initial uplink access event and therefore are not subjected to further collision (contention).

The fourth type of frame, the extension frame, is illustrated in Figs. 6 and 7. In Fig. 6, a dedicated layer 2 illustrates frame 40 in one embodiment of the present invention. The dedicated layer frame 40 is used when a layer 3 message can be transmitted in its entirety within a single channel burst. In other words, the entire layer 3 message can be transmitted within the 46 bits provided for the data field in this embodiment. The dedicated layer 2 frame 40 contains a frame type field 12, a data field 16 and a CRC field 18, which are described above in connection with Fig. 3. However, the layer 2 frame also contains a frame type modifier field 26, in

516 740 which 0 indicates a dedicated layer 2 frame and 1 indicates a split layer 2 frame.

A split layer 2 frame is illustrated in Figure 7. The split layer 2 frame 50 is used when a first layer 3 message ends and additional space is available in the data field and an additional separate and distinct layer 3 message is available for transmission. The use of channel burst splitting essentially provides an implicit indication of layer 3 message composition or linking when the split layer 2 frame is used in the uplink direction. However, when the split layer 2 frame is used in the downlink direction, the use of the channel burst split does not provide an implicit indication of the layer 3 message composition. In other words, in the uplink direction, the two messages 32 and 34 illustrated in Fig. 7 must come from the same mobile station. However, in the downlink direction, the two messages from the cellular switching system may be intended for two different mobile stations. Consequently, unavailable channel burst capacity is wasted when a layer 3 message ends a bit into the data field. The split layer 2 frame 50 contains a frame type field, an initial / repeat field 14, a data field 16 and a CRC field 18, described above in connection with FIG. 3, and a frame type modifier field 26 described above in connection with FIG. 6. As illustrated in FIG. 7 an end of message field 28 is added to the shared layer 2 frame to provide boundary information for layer 3 messages. In other words, the end of the message field 28 indicates where the first layer 3 the message 32 ends and the second layer 3 the message 34 begins in the data field. Accordingly, the receiver (cellular switching system or mobile station or stations) can distinguish between the two layers 3 messages.

Since there may be many mobile stations competing for the same access possibilities on one channel, there must be a means of distinguishing the mobile stations from each other. In one embodiment of the present invention, the first TDMA channel burst used to transmit a layer 3 message.

516 740, either partially or in full, and transmitted from a mobile station to the cellular switching system on a competitive basis, information unique to the transmitting mobile station. The introduction of such unique information, called partial echo, allows the cellular switching system to provide, after properly receiving a first channel burst, partial echo feedback to the competing mobile stations which clearly indicates the mobile station whose first channel burst has been correctly received.

Although a particular embodiment of the present invention has been described and illustrated, it will be appreciated that the invention is not limited thereto, as modifications may be made by those skilled in the art. The present invention covers all modifications that fall within the spirit and scope of the underlying invention described herein and in the claims.

516 740 /2