Error Control for Integrated Wireless and Wireline Networks

Error Control for Integrated Wireless and Wireline Networks  James X. Qiu and Jon W. Mark Centre for Wireless Communications Department of Electrical and Computer Engineering University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Abstract { To support multimedia applications, future

PCS networks will likely involve an advanced interconnected wireless and wireline network. This paper focuses on how a PCS network can compensate for and mitigate the eects of wireless channel errors. An error control architecture for wireless ATM that utilizes interleaving in the physical layer, forward error correction (FEC) code in the wireless ATM adaptation layer (AAL), and Selective Repeat automatic repeat request (ARQ) in the network layer is proposed. Moreover the FEC scheme in the AAL not only provides header error correction, but also includes error protection for the payload, which is a strong distinction from the traditional ATM implementation to combat channel error.

ultimately mitigate the adverse eects of wireless channel impairments on the QoS of a wireless ATM (WATM) network is also proposed. Section 2 brie y discusses how existing ATM networks treat channel errors, and why existing error control algorithms are ineective for wireless channels. Section 3 discusses interleaving and forward error correction (FEC) as candidates for combating channel errors. In section 4, the de ciency of the existing TCP implementation over heterogeneous networks is presented along with two possible solutions. In section 5, we propose an error control architecture that would facilitate the integration of wireless and ATM networks with respect to error performance. Concluding remarks are given in section 6.

1 Introduction and Motivation

2 ATM Error Control Performance on Wireless Channels

In recent years, the vision of Personal Communication Systems (PCS) has attracted a broad range of research in support of multimedia applications over the wireless medium. In particular wireless ATM has been proposed as a viable solution for integrating wireless and wireline networks. However, some of the most important ATM mechanisms have been designed with the assumption that transmission bit errors are rare and randomly distributed, which is not valid in the wireless domain. Thus error control for QoS provisioning in the integrated wireless ATM network is imperative. In this paper we assume the structure of WATM to consist of mobiles connected to the base stations through wireless links, and that base stations are connected to an ATM network via wired links. The goal is to examine the eects of wireless channel characteristics and schemes to increase the performance of wireless ATM in the presence of channel error. QoS requirements of typical ATM services and existing network protocols will be analyzed. Analytical results indicate that interleaving, forward error correction (FEC) coding, and selective repeat for mobile TCP can signi cantly reduce the eects of channel error. An error control architecture to minimize and  This work has been supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada under Grant no. A7779, and a postgraduate scholarship.

An ATM cell consists of 48 bytes of payload and 5 bytes of header for a total of 53 bytes. Current implementation of error detection and correction are only provided for the 5-byte header. A Hamming code on a 40-bit number only requires 5 bits, so with 8 bits a more sophisticated code can be used. The ATM header error coding (HEC) decoder uses a two state correction/detection process [1]. The standard correction code for ATM can correct either all single bit errors in the correction mode, or can detect about 90% of all multi-bit errors in detection mode. Various studies [9] have shown that the vast majority of errors on optical links are random single bit errors. In the detection mode, a detected error causes a cell to be discard. Because the channel quality of ber optics is very high, the bit error probability, and thus the cell loss ratio, is relatively low. With current radio frequency (RF) technology and mobility of mobile stations, the mobile can experience severe noise interference, multipath fading and shadowing eects. As a result, the bit error rate (BER) can be signi cantly higher than its wireline counter part. Moreover, the bit errors are mostly burst errors which can result in a block of information being aected. For these reasons, using the standard ATM cell format and error control algorithm for wireless packets will likely incur high packet loss, and low trans-

mission eciency. Excessive retransmission will severely degrade the channel throughput and other trac QoS parameters, such as delay and delay jitter. Another impairment associated with using ATM cell structure for wireless transmission is that no error detection and/or correction are provided for the payload. It is up to the upper layer functionalities to detect or correct any error. By doing so, the upper layer applications will need to be modi ed to implement some kind of error correction scheme to minimize retransmission. However, such practice is highly undesirable, since it violates the semantics of the layered protocol architecture and requires close interaction between protocols at dierent layers. A desired solution is to implement error correction at the ATM layer. To address and overcome the ineciency associated with a straightforward implementation of ATM over wireless channels, in the proposed error control architecture, the functionalities of the AAL are modi ed and enhanced. It provides strong error protection to the packet header and variable rate protection for the payload based on the required QoS parameters. Moreover, the physical layer implements interleaving to randomize the burst error.

3 Wireless ATM Error Control In the analysis, we assume that the ber-optic based ATM backbone network is error free. So error control will mainly be focused on the wireless network. A suitable functional block diagram of the transmitter and receiver system is shown in Figure 1. In this section we wireless packets

FEC Encoder

Channel Interleaver

Transmitter

wireless and ATM channel

Channel De-interleaver

FEC Decoder

wireless packets

Receiver

Figure 1: Transmitter and Receiver System for WATM will concentrate on the interleaver and the FEC in the transmission system; in particular their purposes, usages and advantages are emphasized. 3.1

Interleaving

Most of the well known channel codes are eective against random errors. However, most of the error control codes do not perform well in the presence of burst errors. An eective method for dealing with burst error is to use interleavers to randomize otherwise burst errors. In choosing the types of interleaver, we need to consider tradeos of periodic versus pseudo-random and block versus convolutional interleavers. Base on the QoS of dierent trac types, it is recommended [4] to use a convolutional interleaver with a short interleaving span for CBR and RT-VBR services, and a block interleaver with a long interleaving span for NRT-VBR, ABR and

UBR services. Because the mobile station should be low powered and relatively simple, the designer of such mobile devices would normally build one type of interleaver which suits the majority of the intended purposes. 3.2

Forward Error Correction

As discussed in the introduction, for mobile wireless ATM networks, error control beyond that provided by the current ATM standards is required. The header of the ATM cell should be protected by a relatively powerful FEC code to ensure proper delivery and low CLR. Also, unlike the retransmission approach taken by the ATM standard, dierent error correction techniques applied to the ATM payload according to the required QoS de ned by the service parameters (i.e., CBR, ABR and VBR etc.) should be bene cial. By using only as much error protection as required by the application, a more ecient use of the allocated bandwidth can be realized.

3.2.1 ATM Header Error Protection

An ATM header has 32 bits of information which needs to be protected. Because the header contains the routing and control information, it is critically important that the header bits be strongly protected. For illustration purposes, consider binary phase shift keying (BPSK). The average bit error rate (BER) forqcoherent BPSK over an AWGN channel is Pb = 21 erfc( NEob ) where 2NEob is the signal-to-noise ratio (SNR). Because, in the transmission structure of Figure 1, both the transmitter and the receiver inputs are properly interleaved, our assumption of AWGN channel is reasonable. For an uncoded header, the probability of correct reception is shown in (1), where N is the size of the header in bits. Puncoded(C) = (1 ? Pb )N : (1) If an (n; k) t-error correcting FEC code is applied to the header, the probabilityqof correct reception q 2kE is given by kE 1 0 b (2), where Pb = 2 erfc( nNo ) = Q( nNob ). t n X (1 ? Pb0)n?iPb0 i: (2) Pcoded (C) = i i=0

The average probability of packet or cell loss, PCL, is given by PCL = 1 ? P(C): (3) In Figure 2, we compare the current ATM cyclic code with the Bose-Chaudhuri-Hocquenghen (BCH) code [6], which is a widely used technique for providing highperformance FEC with reasonable complexity. It can provide a signi cant advantage for wireless ATM. BCH cyclic codes are among the most important block codes, since they exist for a wide range of rates, achieve signi cant coding gains, and can be implemented even at high

in header size to accommodate the FEC code can greatly reduce the probability of cell loss. Because of this relatively small tradeo for a huge gain, we strongly recommend the use of eective error correction coding techniques to protect the header information from being corrupted in a high error and noisy wireless channel.

Average CLR vs. SNR using BPSK

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Figure 2: Probability of Cell Loss versus signal to noise ratio, SNR Average CLR vs. Average Bit Error using BPSK

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Figure 3: Probability of Cell Loss versus bit error rate, BER speeds [6]. For example, at a SNR of 8 dB, the probability of cell loss of the (n = 63; k = 45) BCH is PCL = 1:8810?6, but for normal ATM, it is PCL = 4:2010?4. The improvement is almost 2 decades for an increase in header size of only 57%. The same improvement is also supported by the PCL versus bit error plot (Figure 3). For a bit error probability of Pb = 5  10?4, the probability of cell loss of (n = 63; k = 45) BCH and normal ATM are PCL = 2:99  10?5 and PCL = 2:03  10?3 respectively. This shows that using a FEC of (n = 63; k = 45) BCH can achieve an improvement in PCL of 2 order of magnitude with the same average bit error rate. Even more advanced coding (e.g., concatenated outer Reed-Solomon and inner convolutional code) techniques can be used to ensure the integrity of the header information. However, as Figures 2 and 3 suggest, the improvement in probability of packet loss becomes diminishingly smaller, as more advanced coding techniques are applied. Because of the practical issues in designing mobile units, the designers must balance the relative tradeos of each implementation. From the above analysis, we see that a small increase

The proposed error control architecture also incorporates payload error protection. Future PCS is expected to support dierent types of services such as voice, video and data, etc. Each type of service has a dierent error rate requirement. For a wired ATM network, because the bit error rate is very small, in order to achieve high speed transmission there is no need to do error detection and correction for the payload at the ATM layer. If an application needs very low error rate, error control can be done at a higher layer. However, for the error prone wireless channels, such practice would cause too many retransmissions, thus it is desirable to apply error control at the ATM layer level using FEC. Ideally we can build a separate codec for each type of service, but in reality this strategy would be both cost inecient and ergonomically unsuitable for mobile use (i.e., weight and power consumption). To overcome this shortcoming, Moore and Rice [7] propose a variable rate coding scheme based on the concept of multiple shortened code developed in [10]. The multiple shortened code works as follows: let g(x) be a binary polynomial that generates an (n; k) t-error correcting cyclic code, C. For any l < k, g(x) can be used to generate a (n ? l; k ? l) shortened code, Cs (l). Any shortened code Cs(l) can correct at least as many errors as t [10]. Because g(x) is used to generate both C and Cs (l), the same encoding/decoding circuits used for C can be used for any Cs(l). Variable code rates are realized by selecting dierent values of l and applying the resulting shortened code multiple times to the payload. In this way, the \mother code" remains xed so the same hardware can be used to generate and decode the various rate codes. Variable rate coding schemes can be used to provide dierent error protections for dierent QoS parameters using the same code generator. The procedure to apply multiple shortened codes is described as follows (see Figure 4): Suppose there are N information bits to be encoded and N

segment 1 segment 2 segment 3

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Probability of Packet/Cell Loss

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Figure 4: Variable Rate Coding Algorithm let m1 ; m2; : : : ; mb be the integer divisors of N, where mi > mi+1 . Partition the N information bits into mi

segments (1  i  b). Each N=mi bit segment is encoded using the (n ? li ; k ? li ) shortened code Cs (li ) where li is determined by k ? li = mNi . The encoder/decoder have b + 1 possible error control states (ECS), S0 ; S1 : : :Sb , de ned as follows:  ECS S0 : no error protection is applied.  ECS Si : the data block is divided into mi segments. Code Cs(li ) is applied mi times to the payload, where the value of mi corresponds to a given QoS. In this case, the bigger the mi , the stronger the error correction ability which corresponds to higher QoS requirements. A recommendation of how wireless ATM could seamlessly incorporate this variable rate coding algorithm will be illustrated in section 5. In this section, we have covered a number of techniques to reduce the average packet or cell loss ratio and increase transmission eciency due to header corruption and payload data errors. However, in deep fading channels, occasionally an entire wireless packet is covered by the length of the error burst which causes packet loss or data error. When it happens some of the existing transfer protocols make incorrect assumptions about the heterogeneous network trac characteristics that are only correct for wired channels. In particular, we will look at one of the most popularly used transport protocol, namely TCP.

4 TCP Over Heterogeneous Networks In theory, transport protocols should be independent of the technology of the underlying network layer. In particular, TCP should not care whether IP is running over ber or radio link. In practice, it does matter, because most TCP implementations have been carefully optimized based on the assumption that are true for wired networks but which fail for wireless networks. The principal problem is the congestion control algorithm. Nearly all TCP implementations nowadays assume that timeouts are caused by congestion, not by lost packets. Consequently, when a timer goes o, TCP retransmits the packet, but at the same time invokes congestion control measures by reducing the TCP window size and throughput. The other problem is that higher link error rate can reduce the eciency of the protocol signi cantly since, with the Go-Back-N [9] algorithm, a detected packet error is followed by retransmission of all the packets after the timeout packet. These two main issues will be explored in this section; possible solutions will be presented. The proper approach in dealing with lost packets over a wireless link is to send them again, and as quickly as possible. Slowing down just makes matter worse. In wireless ATM, the path from sender to receiver is often heterogeneous. When a packet is lost on a wired

network, the sender should slow down as usual. When one is lost on a wireless network, the sender should try harder. When the sender does not know what part of the network caused the packet loss, it is dicult to make the correct decision. A solution proposed by Bakne and Badrinath [2], called indirect TCP, is to split the TCP connection into two separate connections. The rst connection goes from the sender to the base station. The second one goes from the base station to the receiver. The base station simply copies packets between the connections in both directions. The advantage of this scheme is that both connections are now homogeneous. Timeouts on the rst connection can slow the sender down, whereas timeouts on the second one can speed it up. Other parameters can also be tuned separately for the two connections. The disadvantage is that it violates the semantics of TCP. Since each part of the connection is a full TCP connection, the base station acknowledges each TCP segment in the usual way. Only now, receipt of an acknowledgment by the sender does not mean that the receiver got the segment, only that the base station got it. Confusions could also arise when the base station nds out the mobile is inaccessible (maybe by repeated timeouts) after it has already sent acknowledgment to the sender. A dierent solution, due to Balakrishnan et al. [3], does not break the semantics of TCP. It works by making several small modi cations to the network layer code in the base station. One of the changes is the addition of a snooping agent at the base station that observes and caches TCP segments going out to the mobile host, as well as acknowledgment coming back from it. When the snooping agent sees a TCP segment going out to the mobile host but does not see an acknowledgment coming back before its (relatively short) timer goes o, it just retransmits that segment without telling the source that it is doing so. It also generates a retransmission when it sees duplicate acknowledgments from the mobile host passing by, invariably meaning that the mobile host has missed something. Duplicate acknowledgments are discarded on the spot to avoid having the source misinterpret them as a sign of congestion. One disadvantage of this transparency, however, is that if the wireless link is very lossy, the source may time out, waiting for an acknowledgment and invoke the congestion control algorithm. With indirect TCP, the congestion control algorithm will never be started unless there really is congestion in the wired part of the network. Both schemes require the base station to buer large amount of data if the wireless channel is very lossy and requires repeated transmission. It is the tradeo one must made to obtain higher throughput and QoS. To solve the problem of unnecessary retransmission with the Go-Back-N approach, the implementation of TCP automatic repeat request (ARQ) at the base station can use Selective-Repeat protocol instead. Selective-

Mobile Station Protocol Stack Application Layer Transport Layer AAL Layer

Upper layer applications TCP or UCP etc.

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Figure 5: Mobile Station and Base Station Protocol Stack Repeat works by forwarding in-order packages and buering out-of-order packets and individually requesting retransmission of erroneous packets. Upon successful retransmission, in-order packets are forwarded and occupied buers are released. Therefore when the base station notices a gap in the inbound (from the mobile station) sequence numbers, it generates a request for a selective repeat of the missing packets using a TCP option. The same procedure applies to mobile stations receiving packets from the base station. By using a combination of Selective-Repeat protocol and either the indirect TCP or snooping agent approach, the wireless link is made more reliable in both directions, without the source knowing about it.

5 A Proposal for Wireless ATM Error Control Architecture In this section, we will study how the dierent schemes for error control discussed in the previous sections merge together to form an integrated system to combat channel errors associated with wireless ATM networks. We propose an architecture which integrates header error correction code, payload variable rate coding technique and TCP selective reject ARQ into one error control system to provide the QoS required by future PCS applications. The proposed architecture of the integrated network is illustrated as follows. Figure 5 shows the protocol stack for mobile stations and base stations. For illustration purposes, a particular data path is described as follows: at the mobile end, a data message is generated from the application layer (e.g., FTP or Netscape) and passed down to the transport layer. The transport layer protocol (e.g., TCP and UCP) provides transparent transfer of data between end systems, relieving the upper layers from any concern with providing reliable data transfer. The normal task of ATM Adaptation Layer (AAL) is to break up large messages into smaller and manageable chunks (e.g., 48 bytes ATM cell). In the proposed architecture, AAL also needs to implement the Variable Rate

Coding algorithm. Based on the QoS requirement from the Transport Layer, the Convergence Sublayer (CS) will assign QoS identi ers to the message, and delaminate the message by attaching a CS Header and CS Trailer (Figure 6). The Segmentation and Reassembly Sublayer (SAR) partitions the large message into smaller chunks of 44 bytes to 48 bytes. Based on the QoS identi er, these chunks are then further segmented according to the Variable Rate P Coding Scheme discussed in section 3.2.2, such that mi=1 segmenti = 44 to 48 bytes1 , as shown in Figure 6. The preservation of 44 to 48 bytes total data size facilitates reassembly of wireless packets into ATM cells at the base station. The segmented data and corresponding FEC code is then passed to the Wireless ATM (WATM) Layer. The WATM Layer is responsible for attaching the ATM header and calculating and adding header FEC code (e.g., BCH t-error correction code) as suggested in section 3.2.1. The resultant wireless packet is then interleaved before it is sent to the Physical Layer for transmission. Output by transport layer Output by Convergence Sublayer

message generated by applications Q CS header O S

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Figure 6: Mobile Station Wireless Packet Formation 1 Because some ATM implementations reserve 4 bytes in the payload for other use, the size of actual segments are implementation dependent.

44 - 48 byte pay load

TCP check ok or non-TCP transport protocol

Output by SAR Sublayer Pass checking for payload error S A R

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From Physical Layer (wireless) after deinterleaving

Figure 7: Base Station Wireless Packet to ATM Cell Formation At the base station, the reverse process is implemented as shown in Figure 7. A wireless packet is rst checked for header error using perhaps a BCH t-error decoder; if error is found, it is corrected on-the- y, provided that the burst error is shorter than t. Then the payload errors are checked and corrected with the Variable Rate Decoder. If the wireless packet passes both check, it is considered \clean" and both the header and payload are forwarded with the extraneous FEC codes removed; otherwise retransmission is required. If the packet is the rst of a TCP/IP message, then the snooping agent or other error control implementation of TCP will look at the data (header) and try to determine whether there are lost packets. If the sequence number shows a gap, the snooping agent will use selective repeat ARQ to request retransmission of the missing packets. If there is no packet loss or the retransmission is successful or the transport protocol is not TCP, then the wireless packet is quickly reassembled (by adding HEC for the ATM header only, since the payload is already of the right size) into an ATM cell and passed to the physical layer for transmission. Figure 7 illustrates this receiver process at the base station. This architecture provides transparent integration of wireless PCS networks into an ATM network with respect to error performance. The ATM network needs only to know little about the wireless channel characteristics and the error properties associated with it.

6 Conclusions In this paper, we rst describe how the current ATM network controls channel error, and why it is insucient and inecient for an integrated PCS network. Then we illustrate how the use of interleaver, FEC in the header protection and variable rate code in payload protection can help to reduce the probability of packet loss and increase channel throughput. Thirdly we discuss and provide three solutions to the problems of running TCP over integrated wireless and wireline networks. Finally

we propose an error control architecture to incorporate the dierent error control schemes into an integrated system to mitigate the adverse eects of channel error associated with wireless connections. The eects of channel error caused by multipath fading, Doppler spread, none-line-of-sight shadowing, and interference severely hinder the success of an integrated wireless and wireline PCS network. Any QoS guarantee and provisioning algorithms2 which do not take into account the wireless channel characteristics are not practical. This paper shows that by using interleaving, we can randomize a burst error and achieve optimal error detection and correction; by adding FEC to protect both the header and the payload of a wireless packet, we can reduce packet loss ratio, retransmission and increase throughput; by making some modi cations to the TCP implementation at the mobile and base station, we can minimize the impact of the incorrect assumptions of TCP ow control algorithm.

References [1] \ATM User-network Interface Speci cation", The ATM Forum. Englewood Cli, Prentice-Hall, 1993 [2] A. Bakne, B. R. Badrinath, \I-TCP: Indirect TCP for Mobile Hosts", Proc. Fifteenth Int'l. Conf. on Distr. Computer Systems, IEEE, pp. 136-143, 1995 [3] H. Balakrishnan, S. Seshan, R. H. Katz, \Improving Reliable Transport and Hand-o Performance in Cellular Wireless Networks", Proc. ACM Mobile Computing and Networking Conf., ACM, pp. 2-11, 1995 [4] J. B. Cain, D. N. McGregor, \A Recommended Error Control Architecture for ATM Networks with Wireless Links", J. Select. Areas Comm., IEEE, vol. 15, pp. 16-28, January 1997 [5] Shu Lin and Jr. Daniel J. Costello, Error Control Coding: Fundamentals and Applications, PrenticeHall, 1983 [6] A. M. Michelson, A. H. Levesque, Error-Control Techniques For Digital Communication, John Wiley & Sons Inc., 1985 [7] D. Moore, M. Rice, \Variable Rate Error Control for Wireless ATM", Proc. ICC, IEEE, vol.2, pp. 988991, 1995 [8] John Proakis, Digital Communications, 2nd Ed, McGraw-Hill, 1989 [9] A. S. Tanenbaum, Computer Networks, 3rd Ed, Prentice Hall, New Jersey, 1996 [10] Kaizhen Wu, Shu Lin, and Micheal Miller, \A Hybrid ARQ Scheme Using Multiple Shortened Cyclic Codes", Proc. GLOBECOM, IEEE, pp. C8.6.1C8.6.5, 1982 2 To guarantee QoS parameters, a combination of scheduling, call admission control, ow control, and congestion control can be used.