輔仁大學 電機工程學系 大學部專題生

Investigate Partial CRC-32 Characteristic and Performance for Real-time Multimedia Streaming in 802.11 Wireless Mesh Networks

指導教授：莊岳儒 博士 學生：陳哲瑋、許家聲

Fig. 2. The procedure of generating n-checking bits

Abstract.

‧In this paper, we proposed a successively computing CRC mechanism (SCCM) to generate the low overhead field for each data frame of real-time multimedia streaming in a wireless mesh network. The field adopts a partial CRC-32 checksum, but can still possess efficient error detection ability. The SCCM performs the general CRC-32 calculation and can be implemented on the current hardware circuit. The corrupt and useless data frames can be found and discarded in the network early. Thus, the mSTA resources and wireless bandwidth utilization can be improved significantly.

The Successively Computing CRC Mechanism (SCCM)

‧The SCCM uses a checking table to record data frame status as shown in Table I.

‧N-checking Bits Generation: The procedure is shown in Fig. 2. As the first data frame is accessed processor and the checksum is calculated by the CRC-32 processor, the n-checking bits will be selected. They will be appended to the data frame. The CRC-32 of the first data frame will be reserved for next data frame calculation.

‧N-checking bits check: The procedure is shown in Fig. 3. When the first data frame is received, it will be accessed and calculated by the CRC-32 processor. After the CRC-32 is obtained, the n-checking bits will be selected. The selected n-checking bits are taken for comparing with n-checking bits that are appended on the first data frame.

‧The Chase Mechanism: If the error data frame occurs behind the correct data frame, the mSTA will not drop all the data frames which belong to the same UDP datagram in buffer. The mSTA generates a chase frame to notify the next mSTA. The mSTA will select the first data frame which belongs to the same UDP datagram in the buffer queue to be the chase frame. The other data frames which belong to the same UDP datagram will be cleared.‧Tunnel and De-tunnel: The data frames which include the n-checking bits should disguise as a normal Wi-Fi Mac data frame in heterogeneous network environment. In Fig. 4, each data frame with n-checking bits will be sent to the CRC-32 processor to generate a CRC-32. The CRC-32 will be appended to the data frame with n-checking bits. If the mSTA which supports the SCCM receives the tunnel frame, it will remove the tunnel information from the data frame which is shown in Fig 5.‧The Fake Ack: If the mSTA receives the frame and finds it corrupt. The mSTA still replies an ACK to pretend that it receives the data frame correctly. Thus the retransmission will not be executed.

To Apply the SCCM on Wireless Mesh Network‧Edge Mesh Station: The Edge mesh station (edge mSTA) is basically an AP to provide the normal station (STA) to connect to the mesh network. It can be seen as a bridge between 802.11 and 802.11 mesh network‧ Core Mesh Station: The Core mesh station (core mSTA) does not provide the bridge function to 802.11 and 802.11 mesh. It takes responsibility to forwarding the frame to the next mSTA. The core mSTA may operate the SCCM or not.‧In the 802.11 mesh standards [1], in the mesh header flag field, it has 4-bit size is reserved for future using. We adapt two reserved bits for SCCM. The figure is shown in Fig. 6. The first is SCCM enabled flag. This flag indicates the data frame that whether carries the n-checking bits or not. The tunnel flag indicates whether the frame is the tunnel data frame.

SA DA IP identifier Status CRC (32-bit)001D73341 001D857 50 clean 111…011

502E3511102E 9B5310DC 3760 clean 101…100… … … … …

3EF 9B7B 334 dirty 111…001

TABLE I. SCCM Checking Table

CRC-32 Processor

CRC-321

Payload1

CRC-32 Processor

CRC-322

Payload2CRC-321

CRC-32 Processor

CRC-32n

PayloadnCRC-32n-1

Payload1 Payload2H H PayloadnH

Payload1 Payload2 Payloadn

Wi-Fi MAC Data Frame

n-checking bit

Payload

n-bit n-bit n-bit

H H H

H H H

H

Fig. 3. The procedure of checking n-checking bits

CRC-32 Processor

CRC-321

CRC-32 Processor

CRC-322

Payload1 Payload2H H PayloadnH

Payload1 Payload2H H

CRC-321 CRC-322 CRC-32n

CRC-32 Processor

CRC-32n

PayloadnH

Wi-Fi MAC Data Frame

n-checking bit

PayloadH

Payload1 Payload2H H PayloadnH

n-checking bit

CRC-321 CRC-322 CRC-32n

CRC-321

Payload1 CRC-321

CRC-32 Processor

Payload2

CRC-32 Processor

CRC-321

CRC-32n-1 Payloadn

CRC-32 Processor

CRC-321

Control Signal Control Signal Control Signal

n-bitn-bitn-bit

H H H

Wi-Fi MAC Data FramePayloadH

Fig. 4. The procedure of generating n-checking bits Fig. 5. The procedure of checking n-checking bits

CRC-32 Processor

CRC-321

Payload1

CRC-32 Processor

CRC-322

Payload2CRC-321

CRC-32 Processor

CRC-32n

PayloadnCRC-32n-1

Payload1 Payload2 PayloadnH H H

Control Signal Control Signal Control Signal

n-bitn-bitn-bit

H HH

Wi-Fi MAC Data Frame

n-checking bit

PayloadH

MeshFlags

Mesh Time to live

(TTL)

Mesh Sequence Number

Mesh Address Extension

Address Extension

Mode

Mesh Power Saving Level

Receiver Service Period

Initiation

Octets: 1 1 1 0, 6, 12, 18

SCCM Enabled

Reserved

B0 B1 B2 B3 B4 B6 B7

TunnelEnabled

B5

destinationSTA

Source STA

MBSS(Mesh Network)

BSS1

BSS2

tunnelframe

tunnelframe

edge mSTA(with SCCM)

(SCCM source mSTA)

core mSTA(with SCCM)

core mSTA(without SCCM)

core mSTA(with SCCM)

edge mSTA(with SCCM)

(SCCM destination mSTA)

Fig. 6. The SCCM header information applied on the mesh header

Fig. 7. The heterogeneous mesh scenario

‧The Fig 7. is the real heterogeneous mesh environment. The edge mSTA which belong to the BSS1 will route a path for forwarding the data frame to destination. The path may have the core mSTA which is without SCCM.

Analyses and Results‧The performance of the proposed SCCM is investigated by the procedure described in section II. Due to the page limited, we only present that the bit error number range within 5 bits. ‧According to the results in Fig. 8, we can find that the error detection probability in each case is almost the same. Therefore, whatever the n-checking bits are selected from the least significant bit or most significant bit of the CRC-32, we will obtain the same performance. Besides, in Fig. 8, it also indicates that the more n-checking bits are adapted, the higher detection rate is. If we only adapt one bit from the first least significant bit or most significant bit, the error detection rate reaches to 50%. While using two checking bits, the error detection rate increases to 75%. Finally if we use 6 checking bits, the error detection rate will reach to 98 %.

(a) The n-checking bit from the least significant bit of the CRC-32 of 4-byte size data

(b) The n-checking bit from the least significant bit of the CRC-32 of 48-byte size data

(c) The n-checking bit from the most significant bit of the CRC-32 of 4-byte size data

(d) The n-checking bit from the most significant bit of the CRC-32 of 48-byte size data

Conclusion‧In this paper, we propose a low overhead error detection mechanism SCCM for real-time multimedia streaming in 802.11 wireless mesh networks. The SCCM is a concept of partial CRC-32 checksum. It only requires employing several sets of n-checking bits to reach efficient error detection ability and high dropping ratio for error and useless data frames.

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Fig 8. The scenario of the SCCM applying on the wireless mesh network