WO2007077436A1

WO2007077436A1 - Method and communication system for carrying out ciphering and segmenting of data directed to a mobile device

Info

Publication number: WO2007077436A1
Application number: PCT/GB2006/050462
Authority: WO
Inventors: Ian Lasseter Phillips
Original assignee: Nokia Siemens Networks Gmbh & Co. Kg
Priority date: 2006-01-06
Filing date: 2006-12-18
Publication date: 2007-07-12
Also published as: GB0600208D0

Abstract

A method of communicating in a communication system comprising a mobile device (20), a base station (21) and a network access point (22) comprises, in downlink, carrying out ciphering (38) and segmenting (39) of data directed to the mobile device (20), for onward transmission to the base station (21), in the network access point (22). Data for the mobile device (20) is stored in a buffer (34) in the base station (21) and scheduling (32) of transmissions from the base station to the mobile device is controlled in the base station. Any incomplete transmissions according to the base station scheduling are retransmitted to the mobile device (20).

Description

METHOD AND COMMUNICATION SYSTEM FOR CARRYING OUT CIPHERING AND SEGMENTING OF DATA DIRECTED TO A MOBILE DEVICE

This invention relates to a method of communicating in a communication system, in particular for wireless mobile communication.

In the 3^rd generation partnership project (3GPP) release 6 (R6), there is a radio 5 link control (RLC) function which provides for segmentation and reassembly, retransmission of lost data in acknowledged mode, and ciphering. The RLC function is located at a radio network controller (RNC). For 3GPP long term evolution (LTE) there is a desire to move retransmission to the Node B, so as to be close to the air interface and hence to achieve latency and performance improvements. However, it is

10 also important to retain ciphering at a higher network node, known as the application gateway (aGW), to maintain user plane security. High speed packet access (HSPA) evolution aims to achieve similar gains, whilst retaining backwards compatibility with user equipments (UE) of 3GPP R6 and earlier.

Simply moving the RLC to the Node B might improve the latency and

15 performance, but at the cost of security. To address this aspect, global system for mobile communication (GSM) enhanced data rates for GSM evolution (EDGE) radio access network (GERAN) maintains two RLC-like layers. The first is a logical link control (LLC) layer, maintained between the UE and a core network node, known as a serving general packet radio service (GPRS) support node (SGSN). The LLC is based

20 on high level data link control (HDLC), which is a well known segmentation and reassembly mechanism with assured transmission. The LLC layer also includes a ciphering mechanism There is also a separate RLC layer, again with a segmentation and reassembly mechanism and assured transmission is maintained between the UE and the basestation, made up of a base transceiver station (BTS) and base station controller

25 (BSC).

This provides for assured transmission at the Node B equivalent, together with ciphering at a higher node. However, it does require two similar protocol layers, LLC & RLC, to be maintained in both the UE and base station (BTS+BSC), as illustrated in Fig. 1. Fig. 1 illustrates the method used in GERAN in which RLC layer 1, medium

30 access control (MAC) layer 2 and physical (PHY) layer 3, responsible for modulation, coding and the air interface, are provided in both the UE 10 and the BTS + BSC 11. RLC forms an additional layer between LLC 4 for the UE 10 and LLC 5 of the SGSN 12, with segmentation, reassembly, ciphering and retransmission Similarly in 3GPP LTE a separate ciphering mechanism is proposed at the u-plane between the UE 10 and the aGW 9, whilst segmentation and reassembly with assured transmission occurs between the UE 10 and the eNode B 8 Fig. 2 shows a similar scheme applied to 3GPP LTE with MAC(UE) 6 in the UE 10 and MAC (including scheduler ) 7 in an eNode B 8. For this example the LLC is replaced by PDCP including u-plane ciphering 13, 14 in the UE 10 and aGW 9 respectively. To adopt either of the se methods for HSPA Evolution would prevent backwards compatibility with older UEs.

In accordance with a first aspect of the present invention, a method of communicating in a communication system comprising a mobile device, a base station and a network access point comprises, in downlink, carrying out ciphering, and segmentation of data directed to the mobile device, for onward transmission to the base station, in the network access point; storing the data for the mobile device in a buffer in the base station; in the base station, controlling scheduling of transmissions from the base station to the mobile device; and retransmitting to the mobile device any incomplete transmissions according to the base station scheduling.

The present invention provides security enhanced radio link control for two node wireless architectures. In the present invention, the functions usually associated with radio link control in the base station have been redistributed, so that ciphering, segmentation and reassembly are in the network access point, so more secure; whereas buffering, retransmission and scheduling are supported in a modified base station to provide low latency for communication over the air interface with a mobile device. This has the advantage that the RLC at the UE is unchanged and so backwards compatibility is maintained, whilst avoiding duplication of RLC -like functions, as described above with GERAN. Preferably, the method further comprises, in uplink, carrying out deciphering and reassembling, in the network access point, of data directed from the mobile device to the base station for onward transmission to the wider network; storing the data from the mobile device in a buffer in the base station; providing control in the base station to enable transmissions of complete data from the base station to the wider network; and requesting lost data from the mobile device for any incomplete transmissions according to the base station analysis.

Preferably, the method further comprises policing user activity according to the base station scheduling. Preferably, a ciphered status message for use in retransmission is deciphered at the network access point and returned to the base station.

Under some circumstances, the UE may issue an RLC status protocol data unit (PDU) within an RLC Acknowledged Mode (AM) PDU. This message is of importance for retransmission and is a ciphered message. This message is deciphered at the gateway and fed back to the Node B.

In accordance with a second aspect of the present invention, a communication system comprises a mobile device, a base station and a network access point; wherein the base station comprises a first buffer, a retransmission function, a scheduler and means for communicating over the air interface to the mobile device; and wherein the network access point comprises segmentation and reassembly and a ciphering and deciphering function.

Preferably, the network access point further comprises a second buffer. Preferably, the first buffer contains a subset of the data held in the second buffer.

Generally, the first buffer only needs to hold immediate data being used for communication over the air interface and a copy of this is held with other data in the second buffer in the network access point.

In some situations, it is still desirable to provide some form of ciphering at the base station, so preferably, the base station is provided with a different localised ciphering function.

This avoids compromising the cipher keys for the protocols used in the network, but adds a further level of security to the communicatbns.

An example of a method and system according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a block diagram illustrating proposed arrangement of the RLC function in GERAN;

Figure 2 is a block diagram of illustrating proposed arrangement of the RLC function in 3GPP LTE; Figure 3 illustrates optional flow control between a base station and a gateway in an example of the present invention;

Figure 4 illustrates an RLC AM PDU; Figure 5 illustrates the division of RLC function for HSPA evolution in accordance with the method of the present invention;

Figure 6 is a block diagram showing handover in a typical radio access network; and, Figure 7 is a block diagram showing improved handover according to the present invention.

In a wireless radio access network comprising two nodes e.g. a base station plus a centralised access gateway, or network access point, forming a mobile anchor point to the terrestrial network, (e.g.: supporting Mobile IP (MIP)) and accounting, with ciphering and security keying required at the base station, or Node B, it is actually desirable from a security perspective to maintain ciphering and keying in the network and to cipher user data at the gateway, as this maintains a secure association across the radio access network. Radio access protocol stacks necessarily include a layer for segmentation & reassembly to match higher protocol packet sizes to smaller radio protocol packet sizes. This layer also typically supports control logic for assured transmission. It is convenient to co- locate ciphering with such a protocol layer as it provides appropriate blocks for ciphering and also provides sequence numbers suitable for enhancing block cipher algorithms. This is the situation for the 3^rd Generation Partnership Project (3GPP) Radio Link Control (RLC) layer in 3GPP Release 6.

However in a two node network it is also desirable to locate the retransmission at the base station to minimise latency on retransmission. This is in conflict with the earlier security requirement, since the RLC includes retransmission and ciphering which are conventionally provided together in the RLC layer. Furthermore, moving the RLC to the gateway would also put the buffering in the gateway, away from the base station, thus reintroducing real time aspects to the link between the base station and gateway. Another problem is that the system needs to operate with legacy mobiles which are set up to use the conventional configuration. The revised protocol architecture of the present invention addresses both requirements.

In current 3GPP architectures the RLC is located in the radio network controller (RNC) a centralised control node, which meets the security requirement. However 3GPP Release 6 suffers with high latency on retransmission. In IEEE 802.16AViMAX network architectures both ciphering and RLC type functions are co- located in a single medium access control (MAC) layer in the base station which allows efficient low latency retransmission. As ciphering occurs at the base station, ciphering keys must be known at the base station which is a security vulnerability identified in the ongoing 3GPP Long Term Evolution (LTE). For IEEE 802.16AViMAX this necessitates additional authentication algorithms which can update ciphering keys rapidly and additional security measures, such as IPSec, across the radio access network.

3GPP Release 6 avoids using authentication algorithms by maintaining the RLC wholly at the RNC, but this suffers from high latency. IEEE 802.16AViMAX either has to support context transfer, or suffer data losses during handover.

The present invention addresses the problems discussed, by having segmentation and ciphering functions co- located at the centralised gateway node, with buffering and retransmission control located at the base station. Scheduling, included in the MAC layer, is also located at the base station. Control links are established between the scheduler, retransmission logic and buffer management. The scheduler dynamically allocates shared air interface resources according to demand. It is desirable to have a fast response to a user request, especially in uplink. Retransmissions, which are usually given a higher priority than new user data, need their own resource allocations. In current systems, the allocation time interval may be 2ms and to take advantage of any reduction in traffic at a particular instant, the buffer needs to be co- located with retransmission, and any data there ready to send, rather than going back to the gateway for the data.

Optional flow control is provided between the buffer at the base station and the gateway to restrict user flows where limited transmission resources exist between the gateway and the base station. This is illustrated in Fig. 3. In a mobile device 20 segmentation and reassembly functions 24 and ciphering 25 are provided. An uplink buffer 26, retransmission 27 and scheduler 28 provides input via PHY 29 over the air 30 to corresponding PHY 31 in a base station 21. The base station similarly has a scheduler 32 and downlink retransmission 33 and buffer 34. The retransmission 33 sends retransmission requests 35 to the scheduler 32 and the buffer 34 is linked 36 to the scheduler 32 for discard or policing. The optional flow control 37 is provided from the downlink buffer 34 to a gateway 22 having ciphering 38 and segmentation and reassembly 39. The protocol is split and if there are any losses during transmissions from the base station 21 to the mobile 20, there is low latency because the buffering 34 is in the base station 21 allowing efficient retransmissions. Current latency times are of the order of 200ms, but sub 50ms is desired, so it is important to avoid changes which go against this trend.

The ciphering keys are vulnerable at base stations, although there is certain data that the base station needs to be sent in the clear, or else it must have its own ciphering as well. Current ciphers work on fixed block length and simply moving the ciphers alone to the gateway 22 requires use of different protocols. The loss of fixed block length due to moving the ciphering 38 is dealt with by also moving the segmentation 39. Conventionally, the segmentation is highly coupled with retransmission, so there has been no incentive to make this change. However, by putting segmentation and ciphering together in the gateway, small fixed block length and reuse of strong existing ciphering algorithms is enabled. This provides buffering 34 and retransmission 33 at the base station 21 with ciphering at the gateway 22 - meeting the requirements described earlier. The RLC Header (first octet for unacknowledged mode (UM) PDUs, first two octets for AM PDUs) are unciphered, although a potential difficulty arises in that piggybacked status PDUs in AM PDUs are ciphered, as shown in Fig. 4. However, this can be overcome e.g. by deciphering at the gateway and instructing the base station by either an enhanced Node B Application Protocol (NBAP), a protocol supporting logical operations and maintenance functions, or an enhanced user Frame Protocol, a protocol supporting user data transmission. This leads to the revised architecture in Fig.5, which shows an example of the present invention. Fig. 5 illustrates the architecture required whilst maintaining the 3GPP Release 6 air interface. Ciphering is maintained at the gateway avoiding the need for ciphering keys at the base station and hence improving security. This also meets 3GPP LTE security aspirations. MAC scheduling and RLC retransmission is maintained at the base station allowing low latency retransmission.

In an unmodified UE 40 is PDCP 41, RLC 42 with segmentation, reassembly, ciphering, buffer and retransmission functions, MAC-high speed (HS) (UE) 43 and

PHY 44. Communication with a modified Node B 45 has the buffer and retransmission part 47 of the RLC function, MAC-HS (including scheduler) 48 and PHY 49. A modified RNC gateway 50 has the PDCP function and the segmentation, reassembly and ciphering function part 52 of the RLC. Piggybacked status PDU' s can be sent 53 between the gateway RLC part 52 and Node B RLC part 47. The RLC AM PDU is made up of a header 54 and ciphered data 55 and optional ciphered status 56.

On handover from a serving base station to a target base station context information (buffer content and retransmission status) must be transferred. In a basic solution this requires two hops: one hop from serving base station to target gateway, and another hop from gateway to target base station. A further improvement reduces buffer context transfer to a single hop by incorporating the use of a majority buffer at the gateway and a minority buffer at the base station allowing efficient context transfers on handover.

Fig. 6 shows an example of handover, in which buffering is only present in the base station. A mobile 60 wishes to handover 61 from a serving base station 62 to a target base station 63. As in Fig. 3, each base station 62, 63 has a PHY 621, 631, a scheduler 622, 632, retransmission function 623, 633 and a buffer 624, 634. Context transfer 64 takes place between the buffers 624, 634. There is flow control 65 between each base station 62, 63 and a gateway 66 which has ciphering 67 and segmentation and reassembly 68. Discard or policing 36 is provided between the scheduler and buffer and retransmission requests 35 are sent between the retransmission and scheduler. With buffering 624, 634 exclusively at the base station then on handover from a serving base station 62 to a target base station 63, the context of the buffer 624 and the retransmission 623 must be transferred. In a typical radio access network, as illustrated in Fig. 6, this requires two hops, one hop from serving base station 62 to target gateway 66, and another hop from the gateway 66 to target base station 63. Fig. 7 illustrates an example in which a majority buffer 69 and minority buffer

70 are held in the gateway 66 and minority buffers 724 and 734 are provide in each base station.

The present invention provides an improvement by maintaining a majority buffer 69 at the gateway 66 which acts as a cache for a minority buffer 724, 734 at the base station. This still permits retransmission at the base station whilst maintaining context at the gateway. On handover from serving base station 62 to target base station 63, the gateway 66 can re-establish the minority buffer 70, containing immediate information, by a minority buffer update 72 from the base station buffer 724, directly to the target base station 63, requiring only one hop.

Where the gateway handles handover transfer, then faster handover is achieved through the use of a minority buffer 70 and majority buffer 69, due to fewer handover signalling hops. Traffic load due to handover is similarly reduced, which is a major advantage to limited capacity communications links between base station and gateway. Retransmission 35 works on segments due to the typically high error rate on the air interface.

Policing 36 is required to prevent other users losing quality when one user is exceeding the contracted rate. This is commonly achieved by separate policing algorithms in a hardware implementation. By the tightly coupled relationship between scheduler and buffer, then if one user is exceeding contract the scheduler can have feedback to discard data over the contracted rate and prevent other users losing quality, avoiding the need for separate policing implementation RLC is responsible for the buffering of user data and the present invention maintains the majority of buffered data at the gateway or modified RNC, whilst maintaining a minority of the data (i.e. data currently in transmission or awaiting acknowledgement from the UE) at the Node B. The benefits on Node B handover are that security is maintained at the gateway and so inter- Node B handover avoids the need to transfer security contexts between Node Bs and as the majority of data is maintained at the gateway, the need to transfer large user data contexts between Node Bs is also avoided. The functions normally associated with RLC at a single layer have been redistributed so as to improve latency and performance whilst maintaining security and backwards compatibility.

Claims

1. A method of communicating in a communication system comprising a mobile device, a base station and a network access point; the method comprising, in downlink, carrying out ciphering and segmentation of data directed to the mobile device, for onward transmission to the base station, in the network access point; storing the data for the mobile device in a buffer in the base station; in the base station, controlling scheduling of transmissions from the base station to the mobile device; and retransmitting to the mobile device any incomplete transmissions according to the base station scheduling.

2. The method according to claim 2, further comprising, in uplink, carrying out deciphering and reassembling, in the network access point, of data directed from the mobile device to the base station for onward transmission to the wider network; storing the data from the mobile device in a buffer in the base station; providing control in the base station to enable transmissions of complete data from the base station to the wider network; and requesting lost data from the mobile device for any incomplete transmissions according to the base station analysis.

3. A method according to claim 1 or claim 2, the method further comprising policing user activity according to the base station scheduling.

4. A method according to any preceding claim, wherein a ciphered status message for use in retransmission is deciphered at the network access point and returned to the base station.

5. A communication system comprising a mobile device, a base station and a network access point; wherein the base station comprises a first buffer, a retransmission function, a scheduler and means for communicating over the air interface to the mobile device; and wherein the network access point comprises segmentation and reassembly and a ciphering and deciphering function.

6. A communication system according to claim 5, wherein the network access point further comprises a second buffer.

7. A communication system according to claim 5 or claim 6, wherein the first buffer contains a subset of the data held in the second buffer.

8. A communication system according to any of claims 5 to 7, wherein the base station is provided with a different localised ciphering function.