GB2386805A

GB2386805A - Differential compression of packet headers wherein the mask of changed bits is compared with previous bit mask and only transmitted if different

Info

Publication number: GB2386805A
Application number: GB0206742A
Authority: GB
Inventors: Robert Kutka; Stephen Mccann; Mark Alan West
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2002-03-22
Filing date: 2002-03-22
Publication date: 2003-09-24
Anticipated expiration: 2022-03-22
Also published as: GB2386805B; GB0206742D0

Abstract

In a communications network 1, a signalling of a communication packet is compressed by firstly determining a difference, (15,16) between a first and a second subsequent signalling portion/header. This difference is used to produce a bit mask (17) and this bit mask is transmitted with the data. If the bit mask for the next packet is the same (18/19), then an indication that this is the case is transmitted with the data. The bit mask is not transmitted. A receiver upon detecting the indication will use as the bit mask the earlier received one. Thus less information needs to be transmitted and the signalling portion is compressed in a first and a second stage of compression thus saving bandwidth.

Description

s Apparatus and Method for Compression of a Signalling Portion of a

Communications Packet This invention relates to an apparatus and method for compression of a signalling 10 portion of a communications packet in a communication network.

In many digital telecommunication networks data is sent in the form of discrete variable sized portions called packets. The communication packet comprises a signalling portion and a payload. The signalling portion comprises a number of 5 fields containing information such as addressing information important for the

routing of the packet to its destination or other signalling information such as timing. It is desirable to make best use of.communication resources and in particular to bandwidth which is often limited. This is particularly so in the field of radio

communications. One way to best utilise the available bandwidth is by the use of compression techniques. Packets have in the past been compressed in a number of ways. In one way, called "delta" encoding, the difference between a current signalling portion and a formerly transmitted signalling portion is transmitted rather 25 than the contents of the portion. By only sending the difference information less information is transmitted and bandwidth is saved.

The present invention provides an increase in the compression of a signalling portion of a communications packet in a communications network.

According to a first aspect of the invention there is provided apparatus for compressing a signalling portion in a communication packet comprising, a comparitor for comparing signalling portions of a first and a second packet to provide a comparison output, a bit mask generator responsive to the comparison as output to produce a current bit mask, a memory for storing the and at least a subsequent bit mask, a bit mask comparitor for comparing the bit masks to produce

. 2 s a bit mask comparison; and a packet packer responsive to the result of the bit mask comparison to load a bit mask into a communication packet.

According to a second aspect of the invention there is provided a method for compression of a signalling portion in a communications packet comprising; lo deriving from a first packet and a second packet a first signalling portion difference and a first bit mask, transmitting to a second node in the network the first packet, the first signalling portion difference and the first bit mask, determining from a third packet and the second packet a second signalling portion difference and a second bit mask difference, and in the event of there being a bit mask difference transmitting 5 the second signalling portion difference and the second bit mask difference and in the event of no bit mask difference transmitting an indicator that the bit mask is the same as earlier transmitted.

A specie c embodiment of the invention will now be described, by way of example 20 only, with reference to, and as illustrated by, the drawing in which: Figure 1 is a block diagram of a communication network in accordance with the invention; 25 Figure 2 is a block diagram of a transmitting apparatus in accordance with the invention and used in the network of claim 1; Figure 3 shows in more detail the apparatus shown in figure 2; 30 Figures 4 and 5 show a receiver used in the network shown in figure 1; and Figures 6 and 7 are explanatory block diagrams illustrating the method of operation of the network in accordance with the invention.

35 As is shown in figure 1, a communication network 1 comprises a first network node 2, a second network node 3 and the remaining elements of the network 4. The node

it: l 5 3 is a handheld mobile telephone hereinafter referred to as a "mobile" and node 2 is a base-station. The described embodiment is an example of one application of the invention. The invention may be utilised in base-station to base-station communication, base-station to mobile communication (and vice versa), mobile to mobile communication, terrestrial and satellite networks and landline telephony lo networks or indeed networks comprising all or a mixture of these elements, using radio, microwave, infra-red or other communication links.

The mobile and the base station are both provided with transmitter and receiver sections which are functionally identical. A transmitter section 5 is shown in figure 5 2 and it comprises a communications application 6 (program). The application 6 produces payload data 7 to a payload processor 8 and a signalling portion 9 to a signalling portion compressor 10. It should be appreciated that these blocks of functionality may be provided by a suitably programmed microprocessor or microprocessors. The payload processor 8 packages the data into the packet in a conventional manner to produce the payload portion 11.

The signalling portion compressor lo compresses the signalling information into a 25 compressed signalling portion 12 of the packet. The completed packet is then passed to a transmitter section 13. The transmitter section 13 then transmits the packet over the air interface 14 to the receiver of the other unit. It should be noted that in some embodiments two branches could be combined that is to say the payload processor and signalling portion compressor could be formed as one unit.

The signalling compressor is shown in greater detail in figure 3 and it comprises a comparitor 15 operably coupled to a memory 16. The memory 16 comprises a register that stores a current input signalling portion SC and the immediately former signalling portion SL. Thus, as a new SC arrives the currently stored SC shifts in as memory to overwrite SL and to become the current SL. The signalling portion comparitor 15 compares the current signalling portion SC with the last signalling

- s portion SL stored in the memory 16. In making the comparison, a bit mask BMC is generated by a bit mask generator 17 responsive to an output of the signalling portion comparitor 15. The bit mask will be described in greater detail later but in brief this is a mask of all the changed bytes in the current signalling portion as compared to the last signalling portion. The bit mask is written to a bit mask memory 17 by the bit mask generator 17 and in addition output to a bit mask comparitor 19. The bit mask comparitor 19 is operably coupled to the memory 18.

The memory 18 is used in a similar manner as the memory 16. It is provided with two divisions, a first which stores the current bit mask BMC and a second which stores the immediately former bit mask BML. Thus, as a new bit mask is input to 5 become the current mask BMC the former BMC shifts in memory to overwrite the former BML to become the current BML.

The bit mask generator also provides the bit mask to a packet packer 20. This is also operably coupled to an output of the bit mask comparitor 19. The output of 20 the bit mask comparitor provides a flag set to 1 or 0 (or indeed in alternative embodiments vice versa) depending upon a comparison of the current value of the bit mask with the stored value. In the event that BMC is the same as BML then the flag is set to 1 and in the event of there being a difference then it is set to 0.

Is The packet 21 has three fields. A first field being a data field 22, a second field

being a bit mask field 23 and the third field being a comparison result field 24. The

fields are packed in the following way.

In the event that the flag is set to O by the bit mask comparitor 19 detecting a 30 difference in the current bit mask and the last bit mask, then the packet packer 20 loads the field 24 with a zero and loads the bit mask field 23 with the current bit

mask BMC.

In the event that the flag is set to 1 by the bit mask comparitor not detecting a 3s difference in the current bit mask and the last bit mask that is the bit masks are the

s same, then the packer 2 leaves the field 23 empty. In essence the fields 24 and 22

are then immediately adjacent.

As is shown in figure 4, the transmitted packet is received by the receiver 25. The receiver outputs the packet to later receiver stages including a payload processor 26, lo a signalling portion expander 27 and a communications application 28 (program).

The payload processor 26 identifies the data field 22 in the packet and reads in the

data. The data is then placed in a form compatible with the communications application 28 and output to that application.

The signalling portion 27 identifies the comparison result field 24 and the bit mask

field 23 and reads in the contents. It performs an expanding operation on the

information stored in these fields to recreate the original noncompressed signalling

information. The signalling portion expander 27 is shown in more detail in figure S. The signalling portion expander 27 includes a packet reader 29 which identifies and reads the contents of the fields 24 and 23 of the packet. In the event of the

comparison flag held in the field 24 being 0 indicating that the bit mask has

changed since the last, the contents of the bit mask field 23 are written into a

25 memory 30 to become BMC. Any existing contents of this part of the memory will be shifted into a memory location which holds the last bit mask BML overwriting the contents held therein.

In the event of the flag being 1, an indication that the bit mask has not changed then 30 the reader has no need to amend the entry in the memory 30 and BMC remains the same ( as indeed does BML).

The memory 30 and the reader 29 are operably coupled to a bit mask reconstructor 31. When the reader completes its reading operation it sets a flag to 1 on the input 3s of the bit mask reconstructor 31 and the contents of the memory 30 both the BMC and the BML are used to reconstruct the bit mask. The reconstructed bit mask is

5 then output to a signalling portion reconstructor 32 which outputs to the communications application 28 the full signalling portion 33.

Let us assume that packet O has a signalling portion: lo portion O A4B23D6E 1 1 96FC3 1 This packet is the first and is therefore transmitted in full.

The next packet has a signalling portion: portion 1 A54E3D6E 1 2A60C3 For this example the field changes are:

Changed fields 011 1000001101

O 0 1 In this case the bit mask is: 25 Bit mask 7 0 6 9 in nibbles And the data is: Data 54E2A02 The packet then transmitted is 0 7 0 6 9 5 4 E 2 A 0 2 A further packet is to be transmitted and it has the signalling portion

: s Signalling portion 2 A 6 0 9 3 D 6 E 1 3 0 6 1 C 3 3

The changed fields are then:

Changed fields O 1 1 10 0 0 0 01 101

10 001

Which gives a bit mask of: Bit mask 7 0 6 9 in nibbles 15 That is to say the bit mask is the same as the previously transmitted one and by utilising the invention this field may be eliminated offering a second level of

compression. Accordingly with the data being: Data 6093013 The packet to be transmitted is 25 1 60930 1 3

The flag 1 indicating that the bit mask is the last transmitted bit mask.

As is shown in figure 6 in a first step the full signalling portion is transmitted by the 30 signalling portion 0 being packed into field 12 and data being packed into field 11.

Transmitter section 13 then transmits the assembled packet.

The portion O is stored in step 61 in memory 16 as SC. For subsequent signalling portions the incoming portion is first stored in memory 16 as SC with the current 3s SC being set as SL again this is step 61.

l 5 In the next step 62 the incoming portion SC is compared by the signalling portion comparitor 15 with SL held in memory 16. This comparison is shown as result 63.

The result is input to the bit mask generator 64 and the bit mask generated in step 64 to produce bit mask 65.

0 The generated bit mask 65 is then stored, as 13MC in memory 18, in step 66. A previously stored bit mask would then be stored as BML.

The current bit mask BMC and the last bit mask BML are compared in step 67 by the bit mask comparitor 19 to give result 68 which is a flag set to 1 or 0. The bit 5 mask packet packer 20 is responsive to the state of the flag in step 69. If the flag is set to one then the process follows the "yes" branch 73. If the flag is set O then the "no" branch is followed.

In this case there is no earlier bit mask stored and hence the result of the comparison 20 will be 0 and the "no" branch 70 is followed. The bit mask packet packer 20 then in step 71 sets the field 24 to 0 and in step 23 packs the current bitmask BMC into the

bit mask field 23.

The data is then loaded into field 22 in step 76 by the payload processor 8 and the

25 assembled packet transmitted in step 77 from the transmitter section 13.

With the next signalling portion, portion 2, the process is followed as before but this time the result of the bit mask comparison step, step 67 is that the flag is set to 1.

Accordingly at step 69, the "yes" branch 73 is followed. The packet packer 20 then 30 enters in the comparison result field 1 and leaves the field 23 empty in step 75.

The data is packaged into the data field 22 by the payload processor 8 in step 76 and

transmitted as before in step 77.

35 The way in which the receiver treats the transmitted packet will now be described with reference to figure 7. In a first step 78 the receiver receives the transmitted

s packet. The receiver locates the field 24 in the transmission and inspects the

contents of the field. Depending on the state of the flag contained therein, that is to

say, one or zero two decision branches 80 and 81 may be taken. Decision branch 80 is taken in the event that the flag in field 24 is zero and decision branch 81 is taken

in the event that the flag in field 24 is one. On taking decision branch 80, the next

0 step is to store the bit mask found in the field 23 by the reader 29 in the memory 30.

This causes the previously stored BMC to be written into the memory location for BML. The bit masks are then used in step 83 to derive the original signalling portion this being an expanding operation carried out by bit mask reconstructor 31 and the signalling reconstructor 32. The expanded signalling and payload are then 15 output to the communications applications in step 84.

In the event of the contents of the field 24 not being zero, that is to say the flag is

set to one, then branch 81 is followed to the signalling portion expansion step 83 and the process continues as before.

.

Claims

5 Claims

1. Apparatus for compressing a signalling portion of a communication packet . compnsmg: 10 a comparitor for comparing signalling portions of a first and a second packet to provide a comparison output; a bit mask generator responsive to the comparison output to produce a current bit mask; a memory for storing the and a subsequent bit mask; IS a bit mask comparitor for comparing the bit masks to produce a bit mask comparison; and a packet packer responsive to the result of the bit mask comparison to load a bit mask into a communication packet.

2. Apparatus as claimed in claim 1 wherein the compared bit masks are successive bit masks.

3. Apparatus as claimed in claim 2 or 3 wherein the packet packer is 25 responsive to the result of the bit mask comparison to load the bit mask into the communication packet when the compared bit masks differ.

4. Apparatus as claimed in claim 3 wherein the bit mask loaded into the communication packet is the current bit mask.

6. Apparatus for compressing a signalling portion of a communication packet substantially as hereinbefore described with reference to and or as illustrated by the drawings. 35 7. A communications network including apparatus as claimed in any one of claims 1 to 6.

f: -: 5 8. A method for compression of a signalling portion of a data packet in a communication network comprising, deriving from a first data packet and a second data packet a signalling portion difference and from the signalling portion difference a bit mask, transmitting the data of the first packet to a second node in the network with the bit mask, determining from a third packet a signalling portion lo difference and a bit mask difference and transmitting the bit mask in the event of there being a bit mask difference and in the event of there being no bit mask difference transmitting the packet with an indicator that the bit mask is the same as earlier transmitted.

S 9. A method for compression of a signalling portion in a data packet substantially as hereinbefore described with reference to and or as illustrated by the drawings. 10. A data packet produced by a method as claimed in any preceding claim.

11. A data packet comprising a comparison result field, a bit mask field and a

data field.

12. Apparatus for receiving a compressed communications packet comprising a 2s reader for reading a received packet, memory coupled to the reader to receive therefrom bit mask information, a bit mask reconstructor for determining from the stored bit mask and a former bit mask a signalling portion.

13 Apparatus for receiving a compressed communications packet substantially 30 as hereinbefore described with reference to, and as illustrated by, the drawings.