Transcript n - 輔仁大學電機工程學系

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
輔仁大學 電機工程學系 大學部專題生
Abstract.
H
Payload
Wi-Fi MAC Data Frame
n-checking bit
‧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.
H
Payload
Wi-Fi MAC Data Frame
H Payload1
H Payload2
CRC-321
CRC-32
Processor
CRC-32
Processor
CRC-32
Processor
CRC-321
CRC-322
CRC-32n
CRC-321
H Payload2
H Payload2
CRC-322
H Payloadn
CRC-32n
H Payloadn
H Payload1
H Payload1
The Successively Computing CRC Mechanism
(SCCM)
H Payload1
n-checking bit
CRC-322
H Payloadn
CRC-32n
CRC-321 H Payload2
CRC-32
Processor
CRC-32
Processor
CRC-32
Processor
CRC-321
CRC-321
CRC-321
n-bit
n-bit
n-bit
Control Signal
Fig. 4. The procedure of generating n-checking bits
CRC-32n-1 H Payloadn
Control Signal
Control Signal
Fig. 5. The procedure of checking n-checking bits
‧The SCCM uses a checking table to record data frame status as shown in Table I.
Wi-Fi MAC Data Frame
H
Status
clean
clean
…
dirty
Payload
n-checking bit
CRC (32-bit)
111…011
101…100
…
111…001
H Payload1
CRC-321 H Payload2
H Payloadn
CRC-32n-1 H Payloadn
H
H
Payload1
B0
Address
Extension
Mode
CRC-321 H Payload2
CRC-32n-1 H Payloadn
CRC-32
Processor
CRC-32
Processor
CRC-32n
CRC-32
Processor
CRC-321
CRC-322
CRC-32n
CRC-321
CRC-322
n-bit
n-bit
n-bit
n-bit
H
n-bit
Payload2
H
Payloadn
Fig. 2. The procedure of generating n-checking
bits
H Payloadn
Payload1
CRC-32
Processor
Control Signal
B1
B2
Mesh
Power
Saving
Level
B3
Receiver
Service
Period
Initiation
B4
B5
B6
SCCM
Enabled
Tunnel
Enabled
tunnel
frame
B7
Reserved
(without SCCM)
tunnel
frame
Fig. 6. The SCCM header information applied on the
mesh header
core mSTA
MBSS
(with SCCM)
(Mesh Network)
BSS1
‧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.
edge mSTA
(with SCCM)
(SCCM
source mSTA)
Source
STA
Fig. 7. The heterogeneous mesh scenario
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 %.
120
120
100
80
60
40
20
1 error
2 errors
3 errors
4 errors
5 errors
100
80
60
40
20
1 error
2 errors
3 errors
4 errors
5 errors
0
0
c0
c1
c2
c3
the checking bit position
c4
c5
c0
(a) The n-checking bit from the least significant
bit of the CRC-32 of 4-byte size data
c1
c2
c3
the checking bit position
c4
c5
(b) The n-checking bit from the least significant
bit of the CRC-32 of 48-byte size data
120
120
100
80
60
40
1 error
3 errors
5 errors
20
2 errors
4 errors
100
80
60
40
1 error
3 errors
5 errors
20
2 errors
4 errors
0
c32
c31
c30
c29
the checking bit position
c28
c27
(c) The n-checking bit from the most significant
bit of the CRC-32 of 4-byte size data
c32
c31
c30
c29
the checking bit position
c28
c27
(d) The n-checking bit from the most significant
bit of the CRC-32 of 48-byte size data
Fig 8. The scenario of the SCCM applying on the wireless mesh network
Conclusion
n-bit
Control Signal
destination
STA
core mSTA
0
H Payload2
CRC-32
Processor
Payload1
0, 6, 12, 18
(with SCCM)
Wi-Fi MAC Data Frame
CRC-32
Processor
H
1
core mSTA
n-checking bit
H Payload2
1
(with SCCM)
(SCCM
destination mSTA)
Accumulated Detection Rate (%)
IP identifier
50
3760
…
334
‧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.
H Payload1
Octets: 1
BSS2
edge mSTA
Accumulated Detection Rate (%)
DA
001D857
9B5310DC
…
9B7B
To Apply the SCCM on Wireless Mesh Network
Payload
Mesh Address
Extension
SCCM Checking Table
‧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 nchecking 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.
H
Mesh
Sequence
Number
Accumulated Detection Rate (%)
SA
001D73341
502E3511102E
…
3EF
Mesh Time to
live
(TTL)
Accumulated Detection Rate (%)
TABLE I.
Mesh
Flags
Control Signal
Fig. 3. The procedure of checking n-checking
bits
‧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.