An Almost Overhead-Free Error Control Scheme for ... - IEEE Xplore

An Almost Overhead-free Error Control Scheme for IEEE 802.16-based Multi-hop Networks Yue-Ru Chuang§ , Hsueh-Wen Tseng , Shiann-Tsong Sheu‡ , and Chih-Wei Su†§ §Department of Electronic Engineering, Fu Jen Catholic University, Taiwan, R.O.C. Cameo Communications, Inc., Taiwan, R.O.C. ‡Department of Communication Engineering, National Central University, Taiwan, R.O.C. †§ Networks and Multimedia Institute, Institute for Information Industry, Taiwan, R.O.C.

Abstract—Based on IEEE 802.16 specifications, the convergence sublayer (CS) of 802.16-based networks provides the function for heterogeneous upper layer protocols. The CS can encapsulate such as IP packets with variable lengths or ATM cells with fixed length in its MAC PDU and usually adds a CRC-32 field to the rear of the MAC PDU. Based on the CRC-32, each 802.16 relay node or BS can detect error occurrence on a MAC PDU and use ARQ mechanism to retransmit the corrupt MAC PDU. However, for real-time streaming (e.g., UDP streaming), the ARQ mechanism is not required and the CRC-32 is also removed. Once CRC32 is removed from the MAC PDUs, PDU error will not be detected by the intermediate nodes in time and these corrupt IP packets or ATM cells will be still forwarded in the networks to waste radio bandwidth. Hence, the error detection mechanism could be maintained, but the large overhead of CRC-32 should be reduced for real-time streaming. In this paper, we propose an efficient discreteerror-checking scheme (DECS) to reduce the overhead of error detection for real-time streaming. The proposed DECS only needs a few checking bits inserted in the MAC PDU, and it can efficiently provide the error detection function. Moreover, a chase mechanism concurrently works on each relay node and BS to early discard these corrupt and useless IP packets or ATM cells. The concept of the DECS including the chase mechanism can be extensively implemented in many wire and wireless multi-hop networks to further improve bandwidth utilization.

I. I NTRODUCTION An emerging wireless metropolitan network (i.e., IEEE 802.16-2004 (fixed) [1], 802.16e (mobile) [2], 802.16j (relay) [3] and 802.16m [4]) is one of the most exciting technology today. IEEE 802.16-based (also named WiMAX) multi-hop networks provide an attrac-

tive communication technology for fixed, nomadic and mobile users to access different quality services via networks. Besides the multi-hop relay architecture of 802.16 networks, the heterogeneous network integration (such as 802.11 WiFi network integrates with 802.16 WiMAX network [5]) also provides and extends the multi-hop network topology.In this sort network, an end user can access data from a BS (base station) via RS (relay station) and SS (subscriber station)/CPE (customer premise equipment). The radio paths between a BS and end users may pass many intermediate nodes, thus packet loss and error probability will increase. From 802.16 specifications, the MAC PDU (protocol data unit) can encapsulate IP packets or ATM cells into its payload and attach a CRC-32 (32-bit cyclic redundancy check) field in the rear of the MAC PDU for error detection. CRC adopts a kind of hash functions to produce a checksum (i.e., redundant information) against a block of data. Before the sender transmits data to its corresponding receiver, the checksum will be generated based on the original data, appended to the original data, and packed in a frame. The generated redundant information is also known as frame check sequence (FCS) in some contexts. When the receiver receives the entire frame, it can determine the correctness of the frame by checking the FCS. CRC has been shown to be easily implemented with strong error detection [6]. IEEE 802.16-based networks use the CRC-32 to detect bit errors and accompany with the automatic repeat request (ARQ) mechanism to retransmit the corrupt data to provide a reliable communication.

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

In IEEE 802.16j, two transmission modes are used for a RS, which can run in the transparent mode (without sending relay downlink map and relay uplink map) or the non-transparent mode (with sending relay downlink map and relay uplink map). If the RS runs in the non-transparent mode (normally), it can decapsulate the received WiMAX MAC PDUs, discard some corrupt packets or cells, re-encapsulate new MAC PDUs and forward them to destinations [3]. In a destination node, if IP packets are received, they will be first gathered to form a segment and then passed to Transport layer. The 16-bit/32-bit checksum filed (TCP and UDP is 16bit, and SCTP is 32-bit) is inserted in the segment for detecting segment error, as shown in Fig. 1(a) [7]. The segment error may be caused by packet loss, bit error or both. Once the segment error is detected, all packets belonging to the same segment (including error and correct packets) will be discarded. Similarly, if ATM cells are received, CRC-32 field is also provided in the CPCSPDU of AAL (Common Part Convergence SublayerPDU, and ATM Adaptation Layer) for error detection, as shown in Fig. 1(b) [8]. If a CPCS-PDU error is detected, all ATM cells belonging to the same CPCS-PDU will be also discarded. These discarded packets or cells without any error in them are called useless packets or cells. These useless packets or cells are still transmitted in networks but dropped by the destinations. Thus, they will significantly cause waste of network bandwidth. Recently, multimedia applications become more and more popular. Real-time traffics (such as video streaming, IPTV streaming, and VoIP streaming of MSN and Skype, etc.) occupy an important proportion of network traffics. ARQ does not suitably work for these real-time applications due to their time constraints. The CRC-32 hence is specified an option in the 802.16 specifications to reduce the overhead of a WiMAX MAC PDU. Without CRC-32, error detection can be only performed in destination nodes, other intermediate relay nodes cannot find error occurrence and still forward all data to destinations. However, the effect of corrupt IP packets or ATM cells not only will cause an error or useless WiMAX MAC PDU, but also may cause consecutively error or useless WiMAX MAC PDUs. Thus, the bandwidth waste mentioned above will greatly degrade system bandwidth utilization. Therefore, these error and useless packets or cells should be found and dropped from networks as early as possible to avoid bandwidth waste. To solve the above problems, we propose an efficient discrete-error-checking scheme (DECS) in this paper for real-time traffics. In the scheme, several sets of partial CRC-32 checking bits are used instead of the original

Transport Layer Segment

(Data Checksum 16/32-bit)

SDU PDU IP Packet H Data Checksum Packet 1 H H

H

H

H

H

Packet 2

Packet 3

Packet 4

Packet 5

H

H

H H

CRC

WiMAX MAC PDU1

H

CRC

WiMAX MAC PDU2

(a) Checksum in Transport layer segment.

AAL Layer

SSCS Sublayer CPCS Sublayer

CS Sublayer SAR Sublayer ATM Layer

H

CRC-32

H ATM Cell1

H H

H

H ATM Cell2

H

WiMAX MAC PDU1

H ATM Cell3

CRC

H H

H ATM Cell4

ATM Cell5

H

CRC

WiMAX MAC PDU2

(b) CRC-32 in AAL CPCS-PDU. Fig. 1.

Error detection mechanisms in upper layer.

CRC-32 in WiMAX MAC PDUs. These sets of checking bits will be separately inserted into the IP packets of the same Transport layer segment or the ATM cells of the same CPCS-PDU, as shown in Fig. 3. Using these partial CRC-32 checking bits, these intermediate nodes (e.g., RSs and BS), which execute the DECS, will have the chance to detect bit error or data loss earlier. Obviously, the best error detection ability is to use 32 checking bits (i.e., using CRC-32). However, the overhead on a MAC PDU is large as mentioned before and it has become an option in 802.16 specifications for real-time traffics. Actually, the suitable number of checking bits should be based on the radio channel condition. The channel condition is worse (better), more (less) checking bits are required. From simulation results, we find that the error detection performance of the DECS is well when only a node runs the DECS and uses 4 checking bits. Besides, because these sets of inserted checking bits are generated via calculating the total IP packets of the same Transport layer segment or the total ATM cells of the same CPCSPDU, a node hence has ability to detect error occurrence caused by bit error or data loss. Then it can find out most of the error and useless packets or cells, and use the proposed chase mechanism to discard them in a network early. The rest of this paper is organized as follows. In Section II, the proposed discrete-error-checking scheme


Total IP packets of the same Transport layer segment or total ATM cells of the same AAL CPCS-PDU

Input Bits

C -1

+

C -2

+

x

an- 2

x

+

C1

+

C0

x

a2

x

a1

+

Checking Bits Data Blocks H WiMAX MAC PDU1

H H H WiMAX MAC WiMAX MAC WiMAX MAC PDU2 PDU4 PDU3

Fig. 2. Appending checking bits to data blocks and encapsulating them into WiMAX MAC PUDs.

(DECS) is described explicitly. In Section III, the simulation model and results are presented. Finally, some conclusions are remarked in Section IV. II. T HE D ISCRETE -E RROR -C HECKING S CHEME (DECS) From IEEE 802.16 specifications, the WiMAX MAC PDU can be usually classified into three parts: the MAC header, payload, and CRC-32. The CRC-32 is an optional field, if it appears in the rear of the MAC PDU, the CI (CRC Indicator) bit in the MAC header is set to one, otherwise is set to zero. A WiMAX MAC PDU usually carries one or more IP packets or ATM cells with the same CID (connection identifier). Based on WiMAX packing and fragmentation functions, these carried IP packets or ATM cells may belong to the same Transport layer segment or CPCS-PDU, as shown in Fig. 1. For real-time traffics, we use the UDP segment instead of the Transport layer segment in the following description. The checking bit generation of DECS still follows the general CRC calculation procedures, as shown in Fig 3.[9]. A source node running the DECS will divide the all IP packets or ATM cells, which belong to the same UDP segment or CPCS-PDU, into several data blocks, as shown in Fig. 2. The DECS can be implemented on both downlink (DL) and uplink (UL) transmissions, and the operations of DECS on these two ways of transmission are the same. For UL transmission, the source node can be the SS/CPE. For DL transmission, the source node can be the BS. The lengths of these data blocks can be equal or non-equal. The number of divided data blocks and the lengths of these data blocks can be decided from the aspect of approaching the maximum dropped ratio. In the DECS, the chase mechanism concurrently works to chase and discard the useless IP packets or ATM cells. Hence, we have analyzed the optimal locations to separate the data blocks and attach the checking bits in their rears. Thus, we can obtain the maximum dropped ratio of these error and useless IP packets or ATM cells. However, the analyses do not present in this paper due to the constraint of the number of pages.

an-1

Fig. 3. A general CRC calculation architecture using the polynomial divisor of 1 + a1 x + a2 x2 + ... + an−1 xn−1 + xn .

The rear of each data block will be attached the n checking bits. Then, each data block will be encapsulated or fragmented into one or more WiMAX MAC PDUs, as shown in Fig. 2. Hence, some MAC PDUs carry the checking bits, but others do not. The number (n) of checking bits is usually smaller than that of CRC32, thus the overhead of error control a WiMAX MAC PDU can be significantly reduced. Based on the 802.16 standard, if the n checking bits appear in the rear of an MAC PDU, then the CI bit in the MAC header is set to one. Otherwise the CI bit is set to zero. The fragmentation of a data block is based on the available WiMAX bandwidth. The transmission bandwidths and timing are scheduled by BS, and they are announced via the DL-MAP (downlink map) and UL-MAP (uplink map) from BS. It is a request-grant behavior. For real-time UGS (unsolicited grant service) traffics, BS assigns the bandwidth with fixed size periodically to the sender. For real-time rtPS (real-time polling service) or ertPS (extended real-time polling service) traffics, sender requires requesting bandwidth for each time of data transmission. The sender then fragments the fit data and encapsulates its WiMAX MAC PDU based on the acquired bandwidth. In DECS, the CRC calculation circuit successively processes all the IP packets or ATM cells, which belong to the same UDP segment or CPCS-PDU, from the first to the last. When CRC-32 circuit calculates at the end of a data block, the intermediate checksum value (ICV) will be accessed from the CRC-32 circuit. The n most significant bits (n MSBs) of the ICV will be selected to be the n checking bits and attached on the rear of the data block. Hence, the n checking bits are the intermediate outcome of CRC calculation. Similarly, the CRC-32 circuit then continues to calculate the following data blocks based on the pervious ICV generate new ICV values for the next data blocks. Namely, the CRC32 circuit does not resume calculating the checksum for the next data block. The circuit continuously processes the data blocks and subsequently generates the second set, the third set of n checking bits, and so on. Hence,


TABLE I T HE MAINTAINED ICV LOOKUP TABLE FOR DECS CID

ConnS

PS

CMS

ICV (32-bit)

20000

Beginning

Clean

Working

101...011

23000

Continuation

Clean

Suspend

NULL

31500

Continuation

Clean

Working

100...010

34000

Continuation

Dirty

Working

110...111

...

...

...

...

...

62300

Continuation

Dirty

Working

110...100

100 90

) % (L /xa Sm

80 70

m=3 m=4

60

m=5 m=6

74

m=2

50 40 30 20 10 0 0

10

20

30

40

50

60

70

X)

Division Location (

80

90

100

(data units)

Fig. 4. The optimal division locations of the data blocks and the dropped percentages (Smax /L) of error and useless data units.

a set of n checking bits can provide the bit error and loss detection from the first data block to the data block the bit error and loss checking bits. Hence, the DECS is very different from the conventional 802.16 CRC-32 process. The conventional CRC-32 calculation on each MAC PDU is independent. That is, the CRC-32 circuit will be resumed for each calculation on each MAC PDU. Thus, using the standard 802.16 CRC-32 checksum, a node cannot be aware of packet or cell loss, it can only detect the bits error on each MAC PDU. On the other side, when the payload of a received MACPDU is stored in the buffer, the node begins to execute the DECS (the CRC-32 calculation process) on the payload, and then records the ICV to an ICV lookup table, as shown in Table I. The 32-bit ICV is obtained from the CRC-32 circuit, as shown in Fig. 3. If n checking bits are carried in this MAC PDU, the node compares the n MSBs of the ICV with the n checking bits. If the values of both are the same, the received MAC PDU is considered to be correct and there is no loss. Similarly, when the following MAC PDU, which still carries the IP packets or ATM cells of the same UDP segment or CPCS-PDU, arrivea at the node, the stored ICV will be first reloaded to the CRC-32 circuit. Then the CRC calculation process is executed continuously to get the new ICV. This is the major concept of the discrete-error-checking scheme.

The range of n is from 1 to 32 (bits), and the node can correctly detect the packet/cell error or loss with the probability of (1 − 1/2n ). How many checking bits do we need? And where are the optimal locations to divide the total IP packets or ATM cells into several data blocks? We have done the mathematic analyses. Based on the analyses, Fig. 4 presents a part of the analytical results. Here, we use the data unit with mean length (k bytes) to represent an IP packet or ATM cell, and the k is set to 53 in this figure. Other analytical parameters are described as follows. L means the total length of the total IP packets or ATM cells of the same UDP segment or CPCS-PDU, and it is assumed 100 data units. The S denotes the total number of data units, which are discarded by a node based on the DECS and chase mechanism. The discarded data units include the error and useless data units. The L data units will be divided into m data blocks, and hence there are m sets of n checking bits appended to these data blocks. The number of checking bits is n, and it here is 4. The bit error rate (BER) in a wireless environment is assumed 10−4 [10] and only a node performs the DECS in this model. From the analytical result, we can find that if the number of divided data blocks is two (m = 2) and the derived division location (x) is at the 33-th data unit, the maximal dropped percentage (Smax /L) is about 45.1244%. From our analyses, considering different fragmentation number (m) of data blocks, the upper bound (Sub /L) of the maximal dropped percentage is about 74.0268%. Thus, the DECS only needs to divide four data blocks (m = 4) and four checking bits (n = 4), the obtained dropped percentage (=64.7092%) can approach almost 90% of the (Sub /L). The totally required overhead is 16-bit (4 data blocks4 checking bits), it is very small. In the ICV lookup table, the ICV of each connection (with a CID) is always updated whenever the CRC circuit completes to process a WiMAX MAC PDU. The table also contains other information to assist each node to run the DECS, as shown in Table I. First, the CID is the ”Transport CID”, which is assigned by a BS and is dedicated for some data connection. The item ConnS means ”Connection State”. It is used to indicate whether the first IP packet or ATM cell of a UDP segment or CPCS-PDU has been forwarded by this node or not. If the first IP packet or ATM cell has been forwarded, the ConnS is recorded as ”Continuation” state, otherwise is recorded as ”Beginning” state. Recall that the checking process of DECS must be performed throughout the entire UDP segment or CPCS-PDU. If any IP packet or ATM cell was passed through the node without being executed the checking process, then the subsequent IP


packet or ATM cells of the same UDP segment or CPCSPDU will not require being checked any more, because the accumulated ICV has become incorrect. The PS means ”PDU Status”, which records whether the current UDP segment or CPCS-PDU is corrupted or not according to the result of checking process. For a data connection, the PS is initially set to ”Clean”. Once the node finds that the n checking bits attached behind an MAC PDU are not equal to the n MSBs of the newly calculated ICV, then the node will regard this received MAC PDU as corruption and the corresponding PS will be set to ”Dirty”. Then the subsequently arrival MAC PDUs, which encapsulate the packets or cells of the same UDP segment or CPCS-PDU, will be discarded immediately without any checking process. There are two cases causing an MAC PDU corruption, one is packet or cell loss and the other is bit error. When a node detects a corrupted MAC PDU, it will first scan its buffer to find out the first outgoing packet or cell of the same UDP segment or CPCS-PDU, set it to be the last packet or cell of this UDP segment or CPCS-PDU, and attach a set of wrong checking bits (can be randomly generated), then re-encapsulate it to form a new MAC PDU and forward it. For an ATM cell, the AUU (ATMuser-to-ATM- user) bit in the PTI field of the ATM cell header can be used to indicate the last cell. For an IP packet, a predefined bit can be designed in the option field of the IP header to indicate the last packet. Finally, the node removes all other packets or cells of the same UDP segment or CPCS-PDU from its buffer. Similarly, the next node running the DECS will receive this special MAC PDU (carries the last packet or cell) and find the wrong checking bits. The same process will be performed again. The first outgoing packet or cell of the same UDP segment or CPCSPDU in its buffer will be also set to be the last, and others will be discarded from the buffer. This is the chase mechanism. The chasing and dropping processes are forward propagated until all the error and useless packets or cells in the multi-hop network are cleared or the following node does not support the DECS. Based on this mechanism, more useful buffer space and radio bandwidth can be available. The CMS in the ICV lookup table means ”Checking Mechanism State”. It is used to indicate whether the checking process of this connection requires being performed or suspended. WiMAX transmission requires maintaining accurate transmission timing based on the DL-MAP and UL-MAP from a BS. Hence, once a node finds that its DECS process cannot match the WiMAX transmission schedule, it first suspends the

SS A

λA

... RS Buffer (Switch 1)

Fig. 5.

λB SS B

... RS Buffer (Switch 2)

...

λN

SS N

...

BS

RS Buffer (Switch N)

The simulation model of a multi-hop relay network.

DECS process of the connections, which are marked as ”Clean” state. If the schedule still cannot be matched, it will further suspend the DECS process on the connections, which are marked as ”Dirty” state. On the other side, if the traffic load in the network is light, the DECS process may be also suspended to reduce the workload of a node, because the bandwidth utilization is not critical at this moment. In the ICV lookup table, whenever a node completes the checking process on the last packet or cell of a UDP segment or CPCS-PDU, the values of ConnS, PS and ICV of this connection will be reset for the following packets or cells of other UDP segment or CPCS-PDU. III. S IMULATION M ODEL AND R ESULTS The performance of the proposed DECS was investigated by simulations. The simulation model is presented in Fig. 5. It is an 802.16-based multi-hop relay network and allocates N RSs to extend the radio path. Thus, we can obviously observe the effect of DECS. The DECS can run in SS, RS and BS, but we here only focus on RS. For simplicity, we use the data unit with mean length (k bytes) to represent an IP packet or ATM cell, and the k is set to 53, as mentioned above. Each RS maintains a buffer to store all received data units, and the buffer size is infinite. That is, we only consider the effect of bit error, and do not consider the error caused by the packet or cell loss. Besides, there are N SSs (users) located in the system, and each of them provides traffic from SS to BS. In this simulation, we only focus on the data traffic sent from the SS A, and other SSs will send the background traffics. The traffic arrival rates in these SSs follow a Poisson distribution with a mean λ, where the λA = 0.5 and others (λB , ..., λN ) allot evenly the remainder of 0.5. The channel BER is still assumed 10−4 [10]. Each UDP segment or CPCS-PDU consists of 100 data units (L = 100), and can be divided into m data blocks. That is, there are m sets of n checking bits separately attached on the rears of these m data blocks. The number of RS is 6 (N = 6). To investigate the effect of the proposed DECS, the total number of dropped data unit (DDU) is the measured metric. The simulation run is 106


time slots, and the system will totally generate 106 data units in this simulation based on the Poisson distribution. Therefore, a good chasing and dropping mechanism is expected to have a high DDU value. 300000 250000 200000 .

RS1 (m,n) RS2 (m,n) RS3 (m,n) RS4 (m,n) RS5 (m,n) RS6 (m,n)

150000

U D D

100000 50000 0

1

2

4 8 Checking Bits (n)

16

IV. C ONCLUSION 32

(a) Two data blocks of division (m = 2). 300000 250000 200000 .

RS1 (m,n) RS2 (m,n) RS3 (m,n) RS4 (m,n) RS5 (m,n) RS6 (m,n)

150000

U D D

100000 50000 0

1

2

4 8 Checking Bits (n)

16

four or more checking bits (n = 4 or more) behind each data block, the ratio of DDU on the first RS can reach more than 90%. It is very useful to implement the DECS in an 802.16-based multi-hop network, because the recommended number of RS between SS and BS is one in the IEEE 802.16j specification. Moreover, if we use only four checking bits, the overhead of a WiMAX MAC PDU could be very small and the error detection function can be also maintained.

32

In this paper, we proposed an almost overhead-free error control mechanism for the 802.16-based multi-hop relay networks. It can effectively reduce the redundant overhead of CRC-32 in WiMAX MAC PDU, and still possesses the ability of early detecting and discarding the error and useless data during transmission. Thus, the limited radio bandwidth can be used more effectively. From the simulation results, we can also demonstrate the good effect of the proposed DECS. Importantly, the scheme can not only be used in the 802.16-based multihop relay network, but also be applied in different kinds of multi-hop wireless networks to improve bandwidth utilization.

(b) Four data blocks of division (m = 4).

ACKNOWLEDGMENT

Fig. 6. The DDU of SS A on each RS, where N is 6 and n is from 1 to 32.

This work was supported by National Science Council under contract NSC 97-2221-E-008-055-MY2.

Figures 6 show the individual contribution of each RS with DECS via the obtained DDU. In this simulation, the SS A will generate about 5 × 106 (= λA × 106 ) data units sent to BS. Because the BER is 10−4 , L is 100 and k is 53, the error probability (p) of a data unit is about 4.15% (= 1 − (1 − 10−4 )53×8 ), and the error probability of a UDP segment or CPCS-PDU is about 98.56% (= 1 − (1 − 10−4 )53×8×100 ). Hence, the data units of SS A can be divided into 5000 UDP segments or CPCS-PDUs, and the possible number of error UDP segments or CPCS-PDUs is about 4928, and the maximal number of DDU in the model should be 492800 (data units). In Fig.6, a UDP segment or CPCS PDU is divided into two data blocks (m = 2) and uses two sets of n checking bits. The used numbers of checking bits (n) are from 1 to 32 bits (i.e., using CRC-32). In Fig.6(b), a UDP segment or CPCS PDU is divided into four data blocks (m = 4) and uses four sets of n checking bits. From these results, we can find that the most of error and useless data units will be detected and discarded by the first RS. After the second RS, the effect of chasing process is trivial in a multi-hop network. Especially, if we use

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