Acknowledgment Operations in the RLC Layer of ... - Semantic Scholar

Acknowledgment Operations in the RLC Layer of GPRS

Wessam Ajib and Philippe Godlewski E.N.S.T., Dept: Inf-Res 46, Rue Barrault, 75634 Paris Cedex 13 FRANCE Email: (wajib&godlewski)@infres.enst.fr

Abstract- This paper considers the acknowledgment procedure used to assure the delivery of packets on the radio interface of General Packet Radio Service (GPRS), where GPRS system is a new GSM service. It is currently being standardized for GSM Phase 2+ and provides packet switched data services over GSM network resources. This paper gives a rapid description of the GPRS radio interface, and its sublayers, with a special attention to the Radio Link Control and Medium Access Control (RLC/MAC) layer procedures. Particularly, the acknowledgment parameters and operations are described and its performance is analyzed. The delay introduced by the acknowledgment procedure is evaluated analytically. In order to ameliorate the performance of the RLC acknowledgment mechanism, we propose and analyze a new additional hybrid FEC/ARQ mechanism, which can operate with the current one. The new mechanism permits to decrease the number of control blocks used for RLC acknowledgment and thus reduce the delay requested for a packet delivery. Keywords: mobile network architectures, performance, wireless data communications, communications system.

I.

protocol personal

Introduction

Data communication is supported in today's GSM network, although the main objective of GSM is to support mobile telephony services, by two services: circuit switched data bearer services with different data rates ranging up to 9.6 kb/s and the Short Message Service (SMS). A need for new data services for GSM is resulting from the increasing demand for mobile data communication and the recent developments of mobile data applications. Therefore, the European Telecommunications Standards Institute (ETSI) standardizes two additional data bearer services: High Speed Circuit Switched Data (HSCSD) and General Packet Radio Service (GPRS). The GPRS service provides a packet switched data transmission within GSM network and packet access to data networks. GPRS have to offer two types of services: Point-ToPoint (PTP) services, for either connectionless and connection-oriented network protocols, and Point-ToMultipoint (PTM) services. In order to offer requested services at low cost to the user and support packet data routing, the conventional GSM structure has been extended by two new logical network nodes [4]. Firstly,

Serving GPRS Support Node (SGSN), who is at the same hierarchical level as the MSC, is responsible for the delivery of packets to the mobile stations within its routing area. Secondly, Gateway GPRS Support Node (GGSN) acts as a logical interface between the external Packet Data Networks (PDNs) and the SGSNs. The physical channels (i.e., timeslots), available in the cell, are dynamically shared between the GSM service and the other GSM services. The ones associated with GPRS are called Packet Data Channels (PDCHs). New types of packet data logical channels are defined and mapped dynamically onto either a 52-multiframe or 51-multiframe physical channel [5]. The new logical channels are classified into packet broadcast control channels, packet common control channels and packet data traffic channels. The rest of this paper is organized as follows. The next section contains a rapid description of the radio interface, noted Um. The radio interface layers protocols, which present an important part of the transmission architecture plane of GPRS, are given. In the third section, we focus on the RLC/MAC layer functions and present simplified scenarios of uplink and downlink transmission. After which, we centralize our studies on the procedures, which define the acknowledgment mechanism. Starting from analysis done in [9], the acknowledgment messages number and the delay introduced by the acknowledgment mechanism are analyzed and evaluated for any packet size. So as to ameliorate this mechanism performance, a proposal modification is exposed in section IV. The modification is derived from Non Selective Repeat (NSR) protocol presented in [3], therefore the resulting mechanism can be presented like a new hybrid FEC/ARQ one. The simulation results are given and discussed in section V, showing the interest of the modification proposed. Finally, conclusions are drowning. II.

The GPRS radio interface

The transmission plane, defined in [4] and summarized in [1] and [2], consists of a layered protocol structure providing user information transfer and associated information transfer control procedures like flow control or error handling.

Netwok Layer

PDP = IP, X.25, … etc.

SNDCP

SNDCP

LLC

LLC

RLC

RLC

MAC

MAC

PLL

PLL

Physical RF

Physical RF

Mobile Station

GGSN

Network Radio interface Um

Fig. 1. GPRS radio interface (Transmission plane) Fig. 1 shows the part of the transmission plane, which contains radio interface layers. The SubNetwork Dependent Convergence Protocol (SNDCP) maps network-level characteristics onto the ones of the underlying network. A variety of network layers are supported e.g., IP, X.25. The network layer Packet Data Protocols (PDPs) share the same SNDCP, which then performs multiplexing of data coming from different sources to be sent across the underlying layers. Therefore, all functions related to transfer of Network layer Protocol Data Unit (N-PDU) shall be in a transparent way by the GPRS network entities. Another requirement for the SNDCP layer is to improve channel efficiency by means of compression techniques. The SNDCP layer is specified in [7]. The Logical Link Control (LLC) layer provides a highly reliable ciphered logical link. Two LLC modes of operation are available: unacknowledged and acknowledged. The LLC layer shall be independent of the underlying radio interface protocols, notably the mode of operation. The LLC layer is specified in [6]. The two lower layers of the radio interface are physical layer and RLC/MAC layer. The physical layer has been separated, as shown in Fig. 1, into: physical RF layer and Physical Link Layer (PLL). The physical RF layer specifies, among other things, the carrier frequencies and the modulation and demodulation requirements. The PLL layer provides services for information transfer over physical channels between the Mobile Station (MS) and the network, such as data unit framing, cell (re-) selection, timing advance, power control management and

particularly data channel coding. The Forward Error Correction (FEC) coding, including the Block Check Sequence (BCS), performed by four coding channels schemes and introduced in [4], allows the detection and correction of transmitted code words and the indication of uncorrectable code words. The RLC layer represents an interface between the LLC layer and MAC sublayer. It performs the segmentation and re-assembly of LLC-PDUs into RLC data blocks. It includes Backward Error Correction (BEC) procedures, which enables the selective re-transmission of uncorrectable data blocks. Two modes of operations are defined at RLC level: unacknowledged and acknowledged. The PTP, connectionless and connection-oriented, services utilize acknowledged transfer mode on the radio interface for reliable delivery. Therefore, only the acknowledgment mode of operation at RLC level is considered in the rest of this paper. The MAC layer defines the procedures that enable multiple MSs to share a common transmission medium, which may consists of several physical channels, PDCHs. It provides collision avoidance, detection and recovery procedures and priority handling. III.

Procedures of RLC/MAC layer

The RLC/MAC mechanism, defined in [5], is designed according the GSM/GPRS features, but the overall concept can be implemented on any TDMA system ([9]). It realizes packet access and can be seen as a possible and detailed implementation of the general Packet Reservation Multiple Access (PRMA) protocol, defined in [8]. A RLC/MAC data block, simply called data block, is the basic radio packet using a radio resource corresponding to four bursts (i.e. 456 encoded bits after channel coding), but the access packet uses smaller amount of resource (i.e. one burst). It uses a sequence of four timeslots on a PDCH, called block period, to be transmitted. A Temporary Block Flow (TBF) is a physical connection used by the network and the MS to support the unidirectional transfer of a number of data blocks carrying one or more LLC-PDUs on one or more PDCHs. Each TBF is assigned a Temporary Flow Identifier (TFI), which have to be unique among concurrent TBFs in each direction (uplink and downlink). The RLC/MAC mechanism, which consists in the establishment and transmission of a Temporary Block Flow (TBF), includes the following (simplified) steps. • The paging procedure is necessary in the case of downlink TBF in order to change the mobility management state from "standby" state to “Ready” state. This procedure is performed by GPRS mobility management entity.

NAck F

F

R1

R2

R1

R1

F

R1

R1

R2

F

Ack R1

R2

R1

R1

Downlink↓ USF↓ Uplink↑

Access burst from MSO Immediate assignment from the network to MS0 Data blocks transmitted from MS0 Ack/NAck message

USF is a flag used for resource reservation on uplink direction (e.g., USF = R1, reserved for MS0), Fig. 2. A TBF uplink transmission scenario • The MS sends an access request burst on a Packet Random Access Channel (PRACH). The cause value of the access request can be, among other things, a paging response for a downlink TBF establishment or an uplink TBF establishment. The PRACH position can be determined dynamically by the Uplink State Flag marked as "Free" (USF = "F") on the corresponding downlink block, where the USF is a flag sent within all the downlink blocks in order to determine the allocation state of the next block period on the uplink resources. • The network responds by a packet resource assignment sent on a Packet Access Grant Channel (PAGCH), indicating the PDCHs used for downlink transfer or reserving resources for uplink transfer. • The sender transmits a sequence of data blocks numbered by a Block Sequence Number (BSN). The data blocks are transmitted on Packet Date Traffic Channels (PDTCH). • The receiver sends an acknowledgment message, noted Ack/Nack and transmitted on a Packet Associated Control Channels (PACCH), indicating (if any) the erroneous blocks. • Eventually, the sender re-transmits the erroneous data blocks and waits for an Ack/NAck message, until the last positive Ack message. The data blocks transfer in RLC acknowledged mode is controlled by a Selective Repeat Automatic Repeat reQuest (SR-ARQ) mechanism which involves a modulo128 numbering of data blocks. The acknowledgment mechanism uses windows with maximum size W set to (W = 64). Each RLC transmitter entity, simply called Tx, sends data blocks within a window of W blocks. The RLC receiver entity, called Rx, periodically sends a temporary Ack/NAck. A Tx has a transmit window delimited by V(S), the BSN of the next block to be transmitted, and

V(A), the BSN of the oldest data block positively acknowledged. An Rx has a receive window delimited by V(R), the number of the next block expected to be received, and V(Q), the number of the oldest data block received. The value of V® is updated at the reception of every data block and V(Q) is updated at the sending of an Ack/Nack. When Tx detects a transmit window stall condition (i.e., [V(S) = V(A) +W] modulo 128), it indicates this condition to Rx and retransmits the oldest data block not yet acknowledged, then the next oldest one, etc. An Ack/NAck message corresponds to a control block and contains a bitmap indicating the acknowledgment status of, at most, 64 previous received blocks. On reception of an Ack/NAck, the Tx retransmits (if any) the erroneous data blocks and readjusts its sending window. A missing Ack/NAck is not critical and a new one can be issued whenever. The network manages the scheduling of Ack/NAck for both uplink and downlink traffic. Making reference to ETSI Technical Specifications ([5]), the network can send, or asks the MS to send, an Ack/NAck "when needed". Fig. 2 presents an example of a TBF uplink transmission scenario. It firstly comprises an access request burst sent from the mobile station to the network. Thereafter, an immediate resource assignment is sent to the MS, a transmission of a sequence of three data block, reception of a NACK message indicating the existence of one erroneous data block, retransmission of the erroneous block, reception of an Ack message and the TBF end. Notations and assumptions • Let DAckProcRx denotes the delay between the Ack decision and Ack transmission start by Rx (same for NAck [9]). It is estimated to 10 ms. The transmission delay of an Ack (or NAck), DAckTr , is estimated to 20 ms, corresponding to one block period. The delay, DNAckProcTx, required by Tx

to prepare a retransmission, is estimated to 10 ms ([9]). The intrinsic delay introduced by one Ack, DAck , (or NAck, DNAck,) message transmission and handling can be evaluated as follows: DAck = DAckProcRx + DAckTr and DNAck = DAck + DNAckProcTx. (1) • We consider that the transmission of a TBF is segmented into sub-flows called "batches": (1) Tx transmits (min {B, W}) blocks (corresponding to one "batch"), where B denotes the number of data blocks to be transmitted, and waits for an Ack/NAck. (2.a) Tx receives a NAck indicating k erroneous blocks: go to (1) with: B ← B – min{B, W} + k. (2.b) Tx receives an Ack: go to (3.a) or (3.b) (current batch end). (3.a) It still remains data blocks to send: go to (1) with: B ← B – W. (3.b) There is no more data block to send: the TBF transmission is finished. • The feedback channel is assumed noiseless [13], and this can be justified by considering proper transmission of Ack/NAck on downlink channel (e.g., high transmission power) to guarantee with a high probability reliable transmission in all downlink channel conditions. • We assume the using of an error detecting code able to detect all the error patterns. Let N –(B) denotes the number of NAck messages sent for one TBF, initially consisting of B data blocks, and ρ is the block error rate. To evaluate N –(B), two basic cases are considered: B ≤ W and B > W where W is the maximum window size. The first case is studied in [9]. For the second case, the probability to have m NAck is: Pr(N – (B ) = m) =

W

 W 

∑  k  ρ (1 − ρ)

W −k

k

k =1

(

+ (1 − ρ)

W

 Pr (N – ( B − W + k) = m − 1) . 

× Pr( N ( B − W ) = m) −

(2)

)

Assuming that Pr(N –(i)=m) = 0 if i < 0, this expression is compatible with the first case. +

Let N (B) denotes the number of Ack messages sent for one TBF of B data blocks. If B ≤ W, then N +(B) = 1. In the case of B > W, the probability to have m Ack is: W  W   k W −k  ρ (1− ρ )  Pr( N + ( B −W + k) = m) . (3) Pr(N + ( B) = m) = ∑     k   k =1

(

+ (1 − ρ) Pr ( N + ( B −W ) = m −1) W

)

The average delay, DTBFAck, introduced by Ack/NAck mess-ages for one TBF is: DTBFAck = DNAck × E [N –(B)] + DAck × E [N +(B)]. Using (1) and (4), DTBFAck can be given by:

(4)

DTBFAck (ms) =40 × E [N –(B)] + 30 × E [N +(B)],

(5)

where N –(B) and N+(B) are evaluated using (2) and (3). IV.

A proposal modification on acknowledgment mechanism

Basically, there are two fundamental techniques for maintaining reliable and efficient communication over noisy channels: FEC and ARQ mechanisms. Since in FEC mechanisms, the transmitted information rate (K/N related to coding rate) is constant regardless of the channel conditions, but the reliability falls as the channel degrades. On the other hand, transmitted information rate of ARQ mechanisms depends mainly on channel quality, but the reliability is almost independent of the channel error rate. The three classical ARQ schemes are Stop and Wait (SW), Go-Back-N (GBN) and Selective Repeat (SR); the current RLC-level GPRS acknowledgment mechanism uses the last one. In GSM and GPRS, the "FEC" and ARQ procedures are not at the same level. In this section, a new hybrid FEC/ARQ mechanism is proposed for RLC-level GPRS, uplink and downlink, transmission. The coding in the proposed mechanism has basic parameters [nMAX , k MAX , dMAX ] with dMAX = nMAX –k MAX +1, where dMAX is a defined minimum distance. Reed Solomon (RS) code can be used. The code is defined over an alphabet of symbols where a symbol presents the payload of a block. A code word involves information blocks and redundancy blocks (R-blocks). Assuming perfect error detection at the reception i.e. the only kind of errata blocks are the erased blocks, the code can correct e erased blocks if: e < d MAX . Specific versions of the RS code are obtained by shortening the code using some parameter K' and puncturing by "deleting" Λ' R-blocks. Starting from a [nMAX , k MAX , dMAX ] code and using shortening and puncturing, a new code with flexible parameters is resulted [N = nMAX –K'–Λ', K = k MAX –K', D = dMAX –Λ'] and the relation D = N–K+1 is still valid ([10]). The reference parameters of the coding scheme associated are fixed by a [nMAX , k MAX , d MAX ] code which may correspond to some shortened version of the RS code. The W blocks of an ARQ window are divided in groups of, at most, K blocks where K ≤ k MAX . The proposed scheme uses feedback signaling (i.e., OK, NOK and SNAck) indicating the reception status of a group of blocks. The OK signal indicates that Rx has received correctly the previous group but NOK signal means that sending Rblocks could recover missing blocks. A NOK signal could indicate the requested number of redundancy blocks. The SNAck signal indicates that missing blocks can not be recovered by FEC procedure and Ack/NAck procedure will be responsible of the recovery. The new mechanism considers a time window size corresponding to the channel memory for a mobile class.

time

1

2

..

16

17 18

..

31

32

..

...

R1 36

R1 R2

..

64

...................

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... X

OK

Rx

NOK1

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..............

...

.....

XX

XXXX

NOK2

...

SNAck NAck

= one erased block

One block transmission

A feedback signaling transmission

Fig. 3. Example of a TBF transmission using the proposed mechanism In order to explain the functionality of the additional mechanism; let's consider a simple TBF transmission scenario. The RLC transmitter entity, Tx, sends K (e.g., K=16) I-blocks. If Tx receives an OK signal; it discards the last group of blocks, transmitted before a time window size, from its buffer considering that they are correctly received. If Tx receives a NOK signal; it sends, at most, Λ R-blocks. The Tx stops to send R-blocks if it receives an Ack/NAck or an OK signal for the concerned group. If Tx receives a SNAck signal; it keeps the previous group in its buffer and switch to SR mechanism. The RLC receiver entity, Rx, responds with a feedback signaling every time it detects a group end. When Rx have to send the fourth (for K=16) SNAck, it sends a NAck message to acknowledge the four "noisy" corresponding groups. Both K and Λ could be not fixed at the beginning of the transmission and their values could be adapted to the channel condition. A data block will be • IC, (I-block Continue) an information data block which is not the last one of the TBF, • IE, (I-block End), last information data block of a TBF, or • R, R-block numbered and called: R1, R2, ... etc. Fig. 3 gives a TBF transmission example, with the proposed mechanism, considering that K=16 and Λ=2 and NOK signals tell Tx to transmit 1 or 2 R-blocks. The first group, BSN ∈ {1, 16}, is received correctly. The second has one error; therefore Tx receives a NOK1 and transmits one R-block. The third group has two errors, so the Rx

sends a NOK2 and Tx responds with two R-blocks. The forth group errors could not be recovered by 2 R-blocks, so it has to wait a NAck message to be corrected. V.

Simulation results

The utility of the additional proposed mechanism seems more significant for applications containing infrequent transmission of larger volumes of data than frequent transmission of small volumes of data. It is, apparently, more important when the channel quality is time varying. The additional mechanism yields, as shown in Fig. 3, to limit the number of control blocks (acknowledgment messages). Therefor, the performance criteria analyzed, in this section by simulation methods, is the average value of Ack/NAck number sent by Rx to receive correctly a TBF. The block error rate, ρ, where a block is considered as erroneous if at least one bit error occurred, is calculated in function of Carrier-to-Interference Ratio (C/I) in [11]. In these simulations the value of C/I is considered constant within a TBF transmission. The traffic load model is the FUNET model, which is based on statistics collected on email usage from the Finnish University and Research Network. The probability distribution function for this model can be approximated by a Cauchy distribution, Cauchy(0.8,1), with a maximum TBF size of 10 Kbytes. This model is given in [12]. Cauchy (0.8, 1) = f (x) = 1/(π × (1 +(x – 0.8)2)).

Ack/NAck number

Channel coding scheme CS-1

10 9 8 7 6 5 4 3 2 1 0

present new

2

4

6

8 10 log (C/I)

10

12

14

Fig. 4. Acknowledgment messages number versus channel conditions for CS-1 Channel coding scheme CS-2

Ack/NAck number

20

present new

15 10 5 0 2

4

6

8

10 10 log (C/I)

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14

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Ack/NAck number

Fig. 5. Acknowledgment messages number versus channel conditions for CS-2 Channel coding scheme CS-3

50 45 40 35 30 25 20 15 10 5 0

present new

2

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10 log (C/I)

Fig. 6. Acknowledgment messages number versus channel conditions for CS-3 Channel coding scheme CS-4

80

Ack/NAck number

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present new

60 50 40 30 20 10 0 6

8

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18 20 10 log (C/I)

22

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Fig. 7. Acknowledgment messages number versus channel conditions for CS-3

The graphs in figures 4, 5, 6 and 7 represent the average number of Ack/NAck necessary to receive correctly a TBF versus the C/I (or channel quality). Each graph corresponds to one of the four channel coding schemes proposed in [5] and noted CS-1, CS-2, CS-3 and CS-4. We notice that the average value of Ack/NAck number decreases considerably when the channel coding rate decreases. The proposed additional modification, exposed in IV, compatible with the present acknowledgement mechanism, presented in III, and combining FEC and ARQ mechanism at RLC level decreases, notably, the average value of Ack/NAck number. The benefit drawn from this modification depends on the channel coding used and on the channel quality e.g., It seems very significant for CS-3 at low C/I. The profits obtained on Ack/NAck number, in principle, economize feedback radio resources, hence increase the throughput of opposite direction traffic, and reduce the delay of data delivery. The increase of throughput may be more considerable when the channel quality is time varying than for "constant quality" channel. VI.

Conclusion

This paper studies the current acknowledgment mechanism used in GPRS RLC-level and proposes a complementary scheme to improve the performance of this sublayer. The new mechanism can be seen as a possible implementation of the Non Selective Repeat (NSR) mechanism, initially conceived for point-to-multipoint communications ([3]), for point-to-point communications. Simulations results have shown the capacity of the modification to decrease the number of control messages (Ack/NAck). These profits, obtained on Ack/NAck number, reduce the delay of data delivery. ACKNOWLEDGMENT The authors would like to express their gratitude to A. Haro, I. Buffet and L. Le Bris at Bouygues Telecom for the many discussions and the provided references this article is based on. R EFERENCES [1] G. Brasche and B. Walke, “Concepts, Services and Protocoles of the New GSM phase 2+ (GPRS)”, IEEE Comm. Magazine, pp. 94-104, August 97. [2] J. Cai and D. Goodman, “General Packet Radio Service in GSM”, IEEE Comm. Magazine, pp. 122131, October 97. [3] H. DJANDJI, “An Efficient Hybrid ARQ Protocol for Point-to-Multipoint Communication and its

Throughput Performance” to be published in IEEE Trans. Veh. Tech. [4] ETSI document. European Standard, GSM 03.60: “General Packet Radio Service (GPRS) Service Description”, ver. 6.3.2, 1999-07. [5] ETSI document. Technical Specification, GSM 03.64: “Overall Description of the GPRS radio interface”, ver. 6.2. 0. 1999-05. [6] ETSI document. Technical Specification, GSM 04.64: “Logical Link Control (LLC) layer specification”, ver. 6.3.0, 1999-03. [7] ETSI document. Technical Specification, GSM 04.65: “SubNetwork Dependent Convergence Protocol layer specifiaction”, ver. 6.3.0, 1999-03. [8] D. J. Goodman, R. A. Valenzuela, K. T. Gayliard, and B. Ramamurthi, “Packet reservation Multiple Access for Local Wireless Communications”, IEEE Trans. Com. August 1989 pp. 885-888. [9] D. Turina, R. Ludwig, “Link Layer Analysis of the General Packet Radio Service for GSM”, ICUPC, San Diego, October 1997 [10] G.COHEN, J-L. Dornstetter and P. Godlewski, "Codes Correcteurs d'Erreeurs", Masson, paris 1992. [11] P. Schramm et al, “Radio Interface Performance of EDGE, a Proposal for Enhanced Data Rates in Existing Digital Cellular Systems”, Proc. IEEE VTC'98. [12] ETSI SMG2 document GPRS Ad-Hoc, “Evaluation Criteria for the GPRS radio Channel”, Windsor, England, 1996. [13] T.S. Rappaport, “Wireless Communications: Principles and Practice”, Edition Prentice Hall, New York, USA, 1996