Mini cells with variable payload size

(54) (56) (57)

INVENTOR INVENTOR

AGENT

NAME

Telefonaktiebolaget LM Ericsson, 126 25 Stockholm SE Göran Eneroth, Tyresö SE, Lars Göran Petersen, Tumba SE, Anders Näsman, Solna SE

Dr. Ludwig Brann Patentbyrå AB

Mini cells with variable for size of payload in a mobile phone network

CALLED PUBLICATIONS:

SE C2 503 317 (H04L 12/56), DE Al 4 326 377 (H04L 12/26),

JP A 58 181 392 (H04L 13/00)

SUMMARY:

One way to specify the length of minicells in a cellular network. Non-linear coding of a short fixed length field in the head of the mini-cell is described. Either an extension bit method or an extension code method is used to extend the length field so as to increase the number of values ​​that can be used to encode the minicells. The length of a minicell is specified in a single minicell or indirectly using a CID / length mapping table. Mini-cell sizes are changed during an ongoing connection and methods are described to make such length changes. Cell head readers that extract user information from individual mini cells are described. A cellular network using cell head readers is described.

The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

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TECHNICAL FIELD OF THE INVENTION

The present invention relates to telecommunication networks in general and the transport networks of a mobile telephone network in particular. ATM cells are used to transmit data. The payload of an ATM cell contains minicells.

RELATED TECHNOLOGY

EP-A1 528 085 describes the use of non-standard short cells, called minicells, for transmitting information. Mini cells are used to reduce the time it takes to fill an empty cell with information, the so-called packaging time. Reduced packaging time avoids the need for echo extinguisher for a connection between two STM switches via an intermediate ATM switch. Standard ATM cells consisting of 53 octets are used for connections between an STM selector and an ATM selector via an intermediate ATM selector.

PCT / SE95 / 00575 describes an ATM selector for emulating circuit-oriented traffic using small cells which reduce the delay through the ATM selector. Small cells are also used for the purpose of saving bandwidth for a physical path in the selector. A selector-internal interface defines the small cells. At the same ATM selector, mini cells of different sizes are used simultaneously. The size of a minicell is selected from the number of predetermined cell sizes. In the payload of a cell, and especially in the payload of an ATM cell, one or more mini cells are transported through the ATM selector. A central controller selects the cell size to be used for a single connection. The cell size is changed at a mapping unit located in the ATM selector. In the head of a small cell, a fixed length field, 4 bits long, is used to indicate the size of the cell. The cell head also contains a cell format indicator bit. If the cell format indicator is 0, the cell payload consists of three octets (time slots) and if the cell format indicator is 1, this indicates that

515 The 588 head is extended by an octet. In the extended head there is a fixed length field of 4 bits which is used to indicate the size of the cell payload. This patent also states that the size of the cell can be indirectly indicated by the cell identifier PRI for physical path and by means of the virtual path VP cell is assigned in the selector. The patent does not describe any method by which the cell size can be changed.

In ANSI T1S1.5 / 95-001 Revision 1, An AAL for Transporting Short Multiplexed Packets (SMAAL), Dec 95 encapsulates and transports the ATM adaptation layer AAL short user packets inside an ATM cell stream. A fixed length field is used to indicate the length of the minicell. The major disadvantage associated with using a fixed length field to indicate the size of a minicell is the poor transmission efficiency, especially when the size of user data in the cell is significantly small. For example, if the size of the payload is 17 octets, the size of the fixed length field must be 7 bits, which translates to bandwidth making up about 6% of the bandwidth.

Other disadvantages associated with using a fixed-length field in the head of a minicell are transmission delay. The transmission delay is due to inefficient bandwidth utilization when the mini-cells are subjected to statistical multiplexing.

SUMMARY

A main object of the invention is to save bandwidth on a link in a mobile telecommunication system.

Another object of the invention is to shorten the fixed-length field and use the bits thereby released either to save bandwidth or to extend the circuit identification field, called the CID field, in the head of a minicell.

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The main object of the present invention is to reduce and even completely eliminate the number of bits used in the head of a minicell to indicate the cell size.

Another object of the invention is to indicate the length of a minicell by using a short fixed length field in which non-linear coding is used to provide a wide range of many different cell sizes.

Yet another object of the invention is to provide an extension piece by means of which the fixed length field is extended. The extension bit is arranged in the short fixed length field in the head of a minicell.

It is yet another object of the invention to indicate the length of a minicell by using a length extension code provided in the short fixed length field at the head of the minicell.

Yet another object of the invention is to indicate the length of a minicell using a short fixed length field in combination with a length extension qualifying field.

Yet another object of the invention is to use the length extension qualifying field as an indicator of a minicell with an extended head format.

Another object of the invention is to indirectly indicate the length of a minicell by relating the circuit identifier CID of a single connection to a cell size selected from a group of predetermined cell sizes.

Yet another object of the invention is to indicate the cell size of a minicell indirectly by relating, on a system basis, a circuit identifier CID with a cell size. Each cell size is associated with each circuit identifier CID, which in turn is global in the transport network.

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Yet another object of the invention is to provide a method for dynamically modifying the size of a minicell during an ongoing connection.

In accordance with the invention, the size of the minicell is modified during an ongoing connection by means of a control message sent in a control channel that is separate from the user data channel in which the minicells are transported.

Yet another object of the invention is to provide a method of dynamically modifying the size of a minicell during an ongoing connection using cell size changing cells which are transported in the same user data channel as the one in which the minicells are transported.

In a mobile telephone system using ATM cells in the transport network, reduced bandwidth or improved use of the available bandwidth will make it possible to add more channels to the system.

A large number of connections, which require a larger CID field, will increase the bandwidth gain if statistical multiplexing is used.

DESCRIPTION

The invention will be explained in more detail below with reference to the accompanying drawings, in which

Fig. 1 shows the format of an ATM cell carrying mini cells,

Fig. 2 shows the head of a minicell transported in the ATM cell of Fig. 1,

Fig. 3 shows an octet of the cell head of Fig. 2, which comprises a fixed longitudinal field for indicating

515 588 the length of the minicell transported in an ATM cell,

Fig. 4 shows an octet in the head of a minicell transported in an ATM cell, which comprises a fixed length field linearly coded in accordance with the invention

Fig. 5 is a mapping table,

Fig. 6 shows the fixed longitudinal field and an extended fixed longitudinal field created by the extension bit method according to the invention,

Fig. 7 is a mapping table,

Fig. 8 shows a fixed length field and an extended fixed length field created by the extension code method according to the invention,

Fig. 9 shows the basic format of a minicell whose head is provided with a short fixed length field and a length extension qualifying field comprising different extension codes;

Fig. 10 is a table,

Fig. 11 shows the minicell of Fig. 9 in its extended format when predetermined extension codes are present in the length extension qualifying field;

Fig. 12 is a table,

Fig. 13 shows an operation and maintenance cell,

Fig. 14 is a block diagram showing a cell head analyzing circuit used to extract from the data channel

FIG.

FIG.

FIG.

FIG.

FIG.

FIG.

FIG.

this user data portion of a minicell in which the fixed length field contains the non-linear coding of the invention, shows a minicell's head and user data extracted from the user data channel, is a block diagram of a cell head analysis circuit used to extract the user data portion of a minicell from a user data channel according to the invention, is a block diagram of a modified cell header analysis circuit used to extract the user portion of a minicell from a user data channel using either the extension code method or the extension bit method, is a block diagram showing a cell header analyzer circuit used to extract the user portion of a user portion of a minicell the bit elongation method according to the invention, shows a minicell head in which the CID indicator is used to indirectly indicate the cell size, is a mapping table used in conjunction with the indirect method of indicating the cell size, shows various tables that together span an address space used on the links in the transport network of a mobile phone system ,

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<td>FIG.</td><td> 22</td><td>shows a minicell head provided with a synchronization bit which is used to synchronize a cell size change message,</td>

<td>FIG.</td><td> 23</td><td>shows the user data channel and entities involved in the method of changing cell size in accordance with the invention;</td>

<td>FIG.</td><td> 24</td><td>shows a system global specific minicell which is used to change the size of minicells belonging to a single connection;</td>

<td>FIG.</td><td> 25</td><td>shows a DOU minicell used for other size of minicells belonging to the connection to which the DOU cell is related,</td>

<td>FIG.</td><td> 26</td><td>shows a specific minicell used to change cell size, which specific minicell belongs to the connection whose minicells are to be resized;</td>

<td>FIG.</td><td> 27</td><td>shows a connection-related mini cell provided with an extension bit in its head and an extension field in its payload, which extension field contains the new cell size to be used for the mini cells in the connection, and</td>

<td>FIG.</td><td> 28</td><td>shows a mobile telephone system equipped with cell head analysis circuits.</td>

DETAILED DESCRIPTION OF EMBODIMENTS

In Fig. 1 an ATM cell 1 is shown comprising a head 2 and a payload 3. Conventionally, the payload consists of user data relating to a specific connection. In the above mentioned patent document PCT / SE95 / 00575 an ATM cell is shown which carries in its payload one or more mini cells. In the example shown in Fig. 1 there are three mini cells 4, 5 and 6 of different sizes. The ATM515 588 head 2 consists of 5 octets (1 octet = 8 bits = 1 byte) and its payload 3 consists of 48 octets.

Each minicell 4, 5, 6 consists of a header 7 and user data.

Fig. 2 shows an example of a minicell head 7 consisting of two octets 8, 9. Other sizes of the minicell head are conceivable depending on how the ATM system is constructed. The minicell's head may also consist of 3 or more octets. The minicell head 7 includes a circuit identifier CID, which identifies the established connection / circuit, a payload type selector PTS, which can be implemented as LEQ and EXQ fields, which identifies various types of payloads, eg DOU information or user data information, a length indicator LEN, sometimes EXQ and / or LENE is used to extend the length indicator, and a bit or field HIC that checks that the head is correct. The length indicator LEN defines the size of the payload of this minicell. The payload PSI indicator is not always needed as will be described in more detail below.

There is a need to be able to differentiate between different types of mini-cells. The following is required to indicate with the PTS field:

• Fixed length user information. The length indicator LEN is not necessary in the head but instead the length of the user information is configured into the system and the service. For GSM full bit rate, the length of user information is 35 octets, for PDC full bit rate it is 20 octets and D-AMPS full bit rate is 23 octets.

• User information of different sizes, ie user information with variable length. This is the preferred embodiment and will be described below. Using the PTS field to indicate the user of variable length formation is a future-proof solution.

• User information with different sizes with extended lengths.

• DOU information is circuit / connection.

• Sync information. This use of the PTS field for this purpose is optional.

In Fig. 3, the cell head 7 is shown to contain a fixed length field 10 which is used to indicate the size of user data in the minicell to which the head belongs. The size of the minicell is indicated in the field 10 using a linear code. For example, if the cell length is 5 octets, a binary 5 (000101) is written in this field. If the minicell is 6 octets, a binary 6 is written in this field. For short minicells, fixed field 10 will occupy large bandwidth, but all of the occupied bandwidth is not used for transmission of useful information as shown by the leading zeros in the given examples. A further disadvantage of a fixed length field 10 is that the range of cell sizes that can be expressed by linear coding is limited. If the fixed length field is 6 bits, cell sizes from 1 to 64 octets can be indicated. If larger cell sizes are to be indicated, the size of the fixed length field 10 must be extended, which in turn leads to even greater waste of bandwidth when small mini-cells are used.

In a mobile phone system, mini-cells are generated by speech encoders. Today's current IS 95 type encoder uses 2, 5, 10 or 22 octets. If a fixed length field 10 is used in accordance with the ANSI document then 7 bits would be required in the head of the minicell to indicate a cell size of 22 octets. With the non-linear coding according to Fig. 4, the fixed length field 11 has only 3 bits. This gives a bandwidth saving of 5% 588 for an IS 95 speech encoder operating at 2kbps (5 octets per 20 ms).

In Fig. 5, a mapping table 12 is used which is used in conjunction with a fixed length field 11 in accordance with the invention. As shown in the table, the code values ​​do not correspond to the size of the mini cells, but instead predetermined cell sizes are assigned different code values ​​of three bits. Examples of minicell sizes are given in the mapping list size column. The sizes range from 4 to 60 octets. Of course, the range can be expanded, but the maximum number of sizes is determined by the number of code bits used.

In order to extend the number of sizes that can be used in conjunction with the non-linear coding, it is possible to extend the fixed length field 11. If necessary, two methods for this will be described. Either an extension bit in the fixed length field 11 is used to extend the length field 11 and this method is called the extension bit method, or one of the length field codes is used for the same purpose, in which case the method is called the extension code method.

In Fig. 6, a bit 13 in a separate extension field 11A is reserved as an extension bit 13. When the extension bit 13 is set to 1, this indicates that the minicell head has an extended length field 14 of the same size as the fixed length field. When the extension bit is 0, the cell header contains only the fixed length field 11.

The extended longitudinal field 14 is represented in the example shown by 3 bits.

When the extension bit 13 is set, the number of bits that can be used for the mapping table 12 will increase from 3 to 6 bits, whereby the mapping table 15 shown in Fig. 7 is obtained. Since the extension bit 13 is reserved for this purpose, it cannot be used for coded size mapping.

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As a variant of the extension bit method, it is possible to add an extension field 11B to the extended length field 14. The added extension field is used to indicate if there is another extended length field in the minicell's head or not. If the added extension field contains a bit set to 1, this indicates that a second extension field 14A is to be added to the head. If the added extension field contains a bit set to 0, no such extension field is present.

Fig. 8 illustrates the extension code method. In accordance with this method, a code is reserved in the fixed length field 11 of Fig. 4 and is used as an extension code. Suppose, for example, the code 111 in the mapping table 12 is used as the extension code. When this code 111 occurs in the fixed length field 11, this means that an extended length field 14 should be included in the minicell head. Thus, an additional 3 bits are available for size mapping. This is shown in Fig. 8. This method reduces the number of sizes in the mapping table 12 by 1 and makes 7 additional sizes available, which can be mapped to the additional 8 code values ​​in the extended length field 14.

From the bandwidth utilization point of view, the extension code method is better than the extension bit method. If the value range is considered, the extension bit method is better than the extension code method.

In Fig. 9, the extension bit method has been combined with the extension code method in a manner that allows high utilization of the bits contained in the cell head while covering a wide range of cell sizes and utilizing the bandwidth effectively.

The basic format of a minicell using this combined coding method is shown in Fig. 9. The minicell comprises a head 21 consisting of 2 octets and a payload portion 22 which may contain from 1 to 48 octets. The four least significant bits of the length of a current minicell are indicated in a small fixed length field 23, designated LEN field, in the head. The LEN field 23 comprises 4 bits. The head also includes a CID field 24 which occupies 8 bits and which identifies the circuit to which the mini cell belongs. Also in the head is a length extension qualifying field 25, designated LEQ field, and a field 26, HIC field, indicating that the head is correct. The two latter fields are both 2 pieces long.

In accordance with the invention, the length extension qualifier LEQ is defined as a length extension of the payload and as an extension of the head. Binary LEQ values ​​of 00, 01 and 10, which means that the minicell has the basic format shown in Fig. 9, are bits to be added to the LEN field 23. Thus, the LEQ field will serve as an extension of the LEN field 23rd

Specifically, 2 is associated⁴ different values ​​in the LEN field 23 with the value 00 in LEQ 25. The value 01 in LEQ 25 is also associated with 2⁴different values ​​in the LEN field 23 and with the value 10 in LEQ 25 For this, 2 different values ​​are associated in ways

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<img file="SE515588C2_D0002.tif" />

totally

<img file="SE515588C2_D0003.tif" />

LEN 23 as shown

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longitudinal values ​​in Fig. 10.

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<img file="SE515588C2_D0006.tif" />

where m is the number of codes used to indicate the extended format of the minicell.

Thus, the minicell size can be selected from 48 length values. With the example shown, the length values ​​are coded as 1 to 48.

When LEQ 25 has the code value 11, this means that the basic cell format must be extended. When LEQ 25 has code 11, this has a dual meaning. The dual meaning of LEQ is:

(i) that it is used as the two most significant bits in

515 588 the length indication, eg LEQ x 2⁴ + LEN and (ii) it is used as an indication of an extended main format, ie, the LEN field 23 is interpreted as an extension qualifying field 27, EXQ field. EXQ field 27 consists of 4 bits.

Of the four bits in EXQ field 27, the binary values ​​0000 and 0001 are reserved for use with the additional length field 29, LENE field, in the manner shown in Fig. 12. Specifically, the least significant bit in EXQ field field 27 is added to the seven bits of the additional LENE field 29 as shown by the dashed rectangle 31 of Fig. 12. This is similar to that shown in Fig. 10. For the binary EXQ value 0 this will give additional 128 different length values ​​and for the binary EXQ value 1 another 128 different length values ​​are obtained.

The number of length values ​​that can be used with this method is given by the following expression:

_r the number of EXQ bits used, _r 2 the number of bits in LEN 29, [2] x [2]

In a preferred embodiment of the invention, the EXQ value 0 is used to indicate minicell lengths ranging from 1 to 128 octets and the EXQ value 1 is used to indicate minicell lengths whose sizes range from 129 to 256 octets.

It should be noted that the length of the minicells shown in Figures 9 and 11 is indicated using a linear coding.

An EXQ value of 2 (binary 0010) means that the minicell is an operating and maintenance cell, DOU cell, consisting of a head 32 and a DOU information field 33 as shown in Fig. 13. The head 32 is similar to the head 21 of Fig. 11. In LEQ field 25 is the binary code 11 and in EXQ field 27 is the binary code 0010.

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The EXQ code 3 (binary 0011) is used to indicate a fixed-length minicell, for example, for the DAMPS system standard. Other EXQ values ​​can be used for other system standards or services.

EXQ code values ​​lxxx are used as synchronization cells; where xxx is time information.

In the preferred embodiment, it is a major requirement that the head of the mini-cell has the maximum length of 2 octets. Given this restriction, the available bits are used effectively to cover all value ranges.

In Figs. 9-12, preferred sizes are shown below the different fields. The sizes given are examples only and many other sizes in the various fields can be considered. LEQ and EXQ codes other than those listed may be used as bits which are hung on LEN field 23 and LENE field 29.

In Fig. 14 a block diagram of a cell main reader is shown. The cell master reader includes a shift register 10, a first counter 20, a lock register 30, a ROM memory 40, a second counter 50, and a multiplexer 60. A bit stream containing user data in the mini cells is shifted into the shift register 10 at an input to the shift register. A clock signal controls the frequency with which the data bits are shifted into the shift register 10. The clock signals are counted by the first counter 20 which is used to extract the fixed length field 11 from a minicell and enter the data of this field in the register 30. The fixed length field, or more specifically the information contained therein, is used as the address of the ROM memory. 40, which is configured with the mapping table shown in Fig. 5. Thus, a single code, hereinafter referred to as the length code, will correspond to a specific length of user data. From the ROM 40 the size of user data (the length of the minicell minus the length of the head) is read and sent to the second counter 50 which controls the multiplexer 60 so that user data will appear on the output of the multiplexer 61. Suppose that the first counter 20 reads the binary

515 588 code 011 from the user data channel. This code is used as the address of the ROM and at this address the cell size 20 is stored. Thus, the length of user data is 20 octets. Then, the second counter 50 counts the following 20 octets bit by bit. By counting a corresponding number of clock pulses. The multiplexer 20 is shown to have an arm 62 movable between the two positions shown. Initially, the counter 50 places the arm 62 in the lower position shown in dotted line, and no output data will then appear at the output 61. When the second counter 50 receives the cell size from the ROM 40, it moves the arm 62 to its upper position. In the upper position of the arm, the arm is connected to a line 63 which in turn is connected to the input of the user data channel. When the second counter 50 has counted 20 octets, it moves back arm 62 to its initial position and the correct number of octets has now been produced at output 61.

In Fig. 15, the extraction of the fixed length field 11 from the user data channel is shown at the time t.₀. At time t₀ the counter 20 begins to count 20 octets bit by bit and at time t₁ the counter 20 has 20 octets. Thus, arm 62 will be in the upper position of Fig. 14 between times t₀ and tL ·

In the cell head reader shown in Fig. 14, there is a predetermined number of length codes and cell sizes stored in the ROM 40. In the cell head reader shown in Fig. 16, there is a RAM memory 70 to which length codes and cell sizes are written from a control system 80. it is possible to configure different specific mini-cell sizes for individual mobile phone systems.

The mini-cell sizes stored in the ROM 40 are global in that a single length code, e.g., 101, refers to all connections that utilize mini-cells with this length code.

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However, it is possible to have a specific minicell size for a specific connection or for a specific physical link using the control system 80 and RAM 70 in a manner that will be described in connection with Figs. 16-27.

Fig. 17 is a block diagram of a cell main reader for implementing the extension code method. Blocks which in Fig. 17 correspond to blocks in Figs. 14 and 16 have the same reference numerals. The circuit of Fig. 17 differs from those shown in Figs. 14 and 16 in that there is a comparator 90 used to detect the extension code. If matching occurs, the comparator triggers a subtractor 100 which counts down the first counter 20 by 3 count steps. When this is done, the extended length field, or more specifically the data contained in this field, is re-entered into the register 30. The different sizes associated with the extended length field must be added to RAM 70. This means that the number of cell sizes in The RAM memory will be doubled in practice, this means that a new memory bank is used in the RAM 70. The unit 110 is a D-lock circuit that locks the output value from comparator 90 and uses this to address the new memory bank in RAM 70.

The comparator 90 and the subtractor 100 are the units that handle the extended length field 14 so that the position of the head is moved when the extension code is detected. Three extra bits are added to the length field 11 and it is these three bits that are used to indicate the cell length. Thus, the fixed length field 11 is replaced with the extended length field 14 which is inserted into the data stream.

Compared to the operation of the circuits of Fig. 14 or 16, where one field is written into the memory, in Fig. 17 another field will be written into the memory 70.

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The cell main reader in Fig. 17 can also be used to implement the extension bit method. This is shown in Fig. 18. From register 30, which contains the fixed length field 11, the extension bit 13 is extracted and used to increase the address area. The extension bit counts down the first counter 20 by three bits, which subtractor 100 indicates. This means that three new bits will be entered in the register 30 and these new three bits plus the old three bits, i.e. six bits, are used to address the RAM 40 as symbolized by the six arrows. In this way, the number of cell sizes will be increased.

In the method described above, the size of a minicell is specified directly by the minicell itself. Instead of providing each cell with a fixed length field used to indicate the size of the mini cell, it is possible to use an implicit method to use any length field in the mini cell head. According to the implicit method of indicating cell size, the cell sizes are in the system's network. Instead of using a dedicated field to indicate the cell size, an existing field is used in the minicell's head. In the preferred embodiment of the invention, minicell strolls are mapped onto the identities of established connections. Thus, the sizes are not global but are connection oriented.

The identity of a connection is given by the CID field of a connection. In Fig. 19, the head 7 of a minicell is shown to comprise a CID field 71. The actual size of the CID field 71 depends on the system, but generally two octets should suffice. By using the same mapping method described in connection with Figures 6 and 7, a mapping table 72 is obtained.

Thus, the fixed length field 11 has been removed. This will increase bandwidth utilization. The CID value is used as the address of the RAM memory 70 in Fig. 16 and is provided by the control system 80. Instead of locking the length field 11 in the register 30, the CID value is locked in the register 30 and used as the address of the RAM515 588 memory 70. way, there will be a relationship between the identity of the established connection and the length of the minicells used in the connection. Thus, no additional memory locations are required to store the relationship between a CID value and the size of the minicell associated with this CID value.

When establishing a connection, control system 80 will receive a message requesting (a) that a connection should be set up between two identified endpoints and (b) that connection should use minicells having a size of X octets. X is assumed to be an integer selected from the available cell sizes. The control circuit then selects a free CID value from the logical addresses provided by the ATM network. For example, a CID value CID = 7 is selected. The control system 80 will now use 7 as an address to the RAM 70 and will enter at this address the mini cell striker X. The cell main reader in Fig. 16 will then operate in the same manner as described. It should be noted that the mapping takes place in connection with the connection setup.

Furthermore, it should be noted that the same CID value can refer to several different mini-cell sizes, due to the fact that cells having the same CID value can be transported on different virtual VC connections. This is illustrated in Fig. 21, where a typical address structure of an ATM network is shown. To each physical link, also referred to as the physical path, in the ATM network there is a physical link table 140 which has a number of inputs, for example the inputs 0-23. Each physical link includes VPI / VCI table 150 (virtual path / virtual circuit identifier). As an example, 256 virtual paths VP 0-255 are shown in each physical link. For example, in each VC connection identified with a VCI / VPI value, there are 256 mini-cell connections, each having a CID value each.

In some cases, it is necessary to change the size of a minicell during an ongoing connection. For example, the bit rate of speech must be changed from full bit rate to half bit rate or the service should be changed from number to data or a variable bit rate codec for the speech should be used. If the cell size is not to be changed too often, ie less than once per second, it is possible, in accordance with the present invention, to change the cell size with a control message sent over the access protocol between the base station and a controlling node, such as a mobile telephone exchange MSC. The controlling node handles and controls all equipment involved in establishing a mini-cell connection, in particular the control system 80 of Figs. 14, 17 and 18. The control message is sent over a channel that is different from the one in which the mini cells are transported. Thus, some form of synchronization will be required between the transmitting side of the mini-cells and the receiving side of the same cells. In accordance with the invention, synchronization is arranged in such a way that a flag is placed in a bit in the head of a minicell as shown in Fig. 22 where a flag bit is designated 82.

Fig. 23 shows an embodiment of a method for changing cell size. Hereinafter, the method is called control plane signaling. A cell head reader 83, identical to the one in Fig. 16, receives a user data stream 84 which is transmitted from a symbolically displayed transmitter 85. When the size of a minicell is to be changed, transmitter 85 sends a control message 86 which is transported in a control channel indicating that the connection having CID = N should change the size of the cells from the old length L1 to a new length L2, where L is the number of octets that form the mini cell.

The control message 86 is sent in a bit stream 87 in a control channel. The bit stream 87 is not synchronized with the bit stream 84. A signal processor 88 for the signal messages receives the control message and delivers it to the control system 80. The control system 80 will now write the new length L1 of the cell in the cell header reader 83 at the address of the identified connection CID = N.

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When enough time has passed for the control system 80 in the ATM network to process the control message, the transmitter 85 will change the cell size from L1 to L2 by setting a flag 83 in the first mini cell 89 using the new size L2. This signals to the recipient side that this cell and subsequent cells should have the new size L2.

Finally, when the first minicell 89 carrying the flag 82 is received by the cell master reader 83 and the CID value of the minicell 89 is received by the register 30, the new length L2 will be read from the mapping table associated with this CID value. Thus, the second counter 50 will control the multiplexer 60 in such a way that the new cell length will be applied to the mini cell 89 in the shift register 10 and to all subsequent cells in this connection. As a result, no information will be lost when the cell size is changed.

Track signaling can trigger cell size changes on a second basis. The case is so because the control system 80 has to process the control signals, which typically takes about-seconds. Thus, scheduling signaling is quite slow and requires synchronization.

It should be noted that Fig. 23 is somewhat simplified in order to clearly illustrate the synchronization procedure. In effect, the bit stream 87 is interleaved with the bit stream 84 on an irregular time base.

Some applications require that the cell size be changed on a millisecond basis. Other resizing methods that meet this requirement use cell resizing minicells, hereinafter referred to as "length change cells," which are transported in the user data channel. This will be described in connection with Figures 24-27. The length change methods used do not require machining in the control system and do not require any synchronization mechanism.

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Specifically, a particular minicell is used to indicate the new length in accordance with the methods described in association with Figs. 24-27. The new cell length is in payload 94. Four different types are used:

1) a specific EXQ value defines a size indicator cell shown in Fig. 26,

2) a definite EXQ value of 2, i.e. an OAM cell, is used, shown in Fig. 25,

3) the length change cell is indicated by a specific CID value, eg CID = 0, and the connection is identified by the CID field 93 in the payload shown in Fig. 24;

4) a type described under Method Four below.

The new cell length to be used for the subsequent mini-cells in a connection is specified in the length field 94. All mini-cells that follow the mini-cells 91, 95, 97 in the data stream and which have the same CID will have the new cell size and will have their size indicator set to zero, thus indicating that the cell is being used for user data.

A fourth method of changing the size of the minicells is to use a cell size-changing minicell 170, called a length-changing cell of the type shown in Fig. 27, where an optional field 171 is used to indicate the new length to be used for the minicells following this cell. 170 and belonging to the same connection. The compound is indicated by CID in the head of the cell. Also in the head is an extension bit 13 which, when set, indicates that the cell contains the optional length field 171. If the extension bit is set to 0, no field 171 is present in cell 170.

Instead of using an extension bit 13, it is also possible to use a specific payload type selection code, called the PTS code, similar to the extension code method of Fig. 8 to indicate that the cells are used to modify subsequent minicells in the same connection.

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In Fig. 28, a mobile telephone system is shown comprising an ATM network 200 to which a transmitter 201 and a receiver 202 are connected via each of links 205 and 206 respectively. User data sources 203 are connected to the transmitter over separate connections represented by lines 209. User data sinks 204 are connected to the receiver 202 over separate connections 210. The connections 209 formed by mini-cells are multiplexed together in transmitter 201 by means of a multiplexer not shown. Similarly, in receiver 202 there is a demultiplexer which demultiplexes mini cells belonging to connections terminated by user data sinks 204. In transmitter 201 there is a cell head reader 207 of the kind shown in Fig. 11 and in the receiver there is a similar cell head reader 208 of that type. as shown in Fig. 11.

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