1. ‘Express’ Forwarding for single-channel wireless mesh
Month Year
doc.: IEEE yy/xxxxr0
September 2007
‘Express’ Forwarding for single-channel wireless mesh
Date:
Authors:
Name
Address
Company
Phone
Mathilde Benveniste
233 Mt Airy Road
Basking Ridge, NJ 07920, US
Avaya Labs-Research
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Mathilde Benveniste, Avaya Labs
John Doe, Some Company

2. September 2007
‘Express’ Forwarding for Single-Channel Wireless Mesh Mathilde Benveniste Avaya Labs - Research
Mathilde Benveniste, Avaya Labs

3. September 2007
Introduction
VoIP cannot meet QoS requirements on a single-channel wireless mesh unless there is a way to reduce delay/jitter
End-to-end delay and jitter can be too high because of multi-hop transmissions
Delay/jitter contributes to the delay experienced by the end-user receiving QoS traffic
Jitter buffers remove short-term frame delay variations from received packet stream by inserting additional delay that allows the system to re-insert late-arriving packets in the proper time sequence before they are decoded and played out
Mathilde Benveniste, Avaya Labs

4. Excessive latency in single-channel meshes
Month Year
doc.: IEEE yy/xxxxr0
September 2007
Excessive latency in single-channel meshes
Multi-hop paths in single-channel meshes involve severe end-to-end latencies
Single-hop latencies compound at the least
Collisions due to “hidden nodes” increase latencies severely when EDCA (or CSMA/CA) is used to access a single channel
4 node mesh on a single channel
A and C cannot hear each other
B cannot receive when C transmits to D
Hidden node
ACK
(((( A B C D X Tx Tx
Mathilde Benveniste, Avaya Labs
John Doe, Some Company

5. Excessive latency in single-channel meshes – 2
September 2007
Excessive latency in single-channel meshes – 2
Hidden terminal collisions between two transmissions are likely to repeat
Single channel mesh
A and D cannot hear each other
C cannot receive when A transmits
B cannot receive when D transmits
Retransmission attempts likely to fail
Increased delay for successful transmission A B Tx X C D
((((
Interference
Mathilde Benveniste, Avaya Labs

6. Delay Budget September 2007
Recommended one-way total delay (ITU G.114) – 150 ms
Delay introduced in IP network and end system – 110 ms
IP network delay – sum of transmission and queuing delays traveling thru IP network (~50 ms)
End-system delay – sum of the encoding (20 ms), decoding (small), jitter buffer (~40 ms), and other data handling delays
The target for maximum latency in a wireless mesh should be 40 ms*
* In TGe, a target of 10 ms was used for WLAN delay in top-priority ACs
Mathilde Benveniste, Avaya Labs

7. Ways to reduce delay in a wireless mesh
September 2007
Ways to reduce delay in a wireless mesh
Capacity provisioning
The nodes and links of the mesh network must have sufficient capacity to prevent traffic buffer from building up anywhere in the network
Proper provisioning involves the use of multiple radios (and channels) at nodes of high traffic concentration to match traffic profiles
Congestion control
Even with proper provisioning, the stochastic nature of traffic may produce short term fluctuations which may cause congestion at certain nodes
Reducing load by rerouting traffic can alleviate congestion at a node, given the provisioning
Fast forwarding of QoS traffic
MAC layer prioritized transmission and retransmission of forwarded QoS traffic across the mesh helps reduce end-to-end delay along a multi-hop path, given congestion control and capacity provisioning
Mathilde Benveniste, Avaya Labs

8. September 2007
The role of EDCA
For fast forwarding, QoS traffic would require top priority access when forwarded on a multi-hop path
Lower priority access would be used for all other traffic
EDCA offers access prioritization on a single channel
Further prioritization is not possible with EDCA, however
Top-priority 11e traffic (VO/VI) already uses the top-priority access category (shortest AIFS)
A different mechanism is needed for forwarded QoS traffic
‘Express’ forwarding is therefore proposed
It is compatible with EDCA
It is an optional feature
Mathilde Benveniste, Avaya Labs

9. September 2007
‘Express’ forwarding
In order to reduce delay of a multi-hop path, ‘express’ forwarding enables a forwarding node to incur a shorter per-hop delay than a single-hop transmission
For a single-channel mesh
Forwarding delay is reduced by reserving the channel for a forwarded transmission longer, in order to enable the next forwarding node to seize the channel
Immediate access is thus given to nodes, other than the first node on a multi-hop path, forwarding QoS traffic
‘Time-critical’ frames can be exempted; they may be transmitted even before express-forwarded traffic
Note: There exist ways for express forwarding also on a multi-channel mesh (not covered here)
Mathilde Benveniste, Avaya Labs

10. Designating frames for ‘express’ forwarding
September 2007
Designating frames for ‘express’ forwarding
A frame to be express forwarded is designated as a ‘time-sensitive QoS’ (TSQ) transmission
A special flag may be necessary to mark a transmission as ‘express’ forwarded, depending on the criteria for a TSQ transmission and/or the information available along its path
Examples
The TSQ designation will depend on the type of traffic
For instance, a TSQ frame could be a frame of a specified user priority (e.g. VO)
The designation may be supplied by a forwarding node
For instance, express forwarding is engaged only or once a certain time-to-live remains
Each frame contains a TTL field that can give the # traversed hops:
HN = dot11MeshTTL – TTL
Mathilde Benveniste, Avaya Labs

11. Single channel ‘express’ forwarding
September 2007
Single channel ‘express’ forwarding
Mathilde Benveniste, Avaya Labs

12. Express Forwarding procedure
September 2007
Express Forwarding procedure
A known time increment DT0 (MIB variable Dot11MeshDTC) is added to the value of the Duration field when a TSQ frame is forwarded to a node other than the final destination
The Duration field of the ACK (if any) returned for the TSQ frame received at the destination node is set based on the Duration field of the received frame
All nodes that hear the transmission other than the receiving node set their NAV according to the Duration field of the received transmission
If the receiving node forwards the frame, it subtracts DTI (MIB variable Dot11MeshDEF) from the Duration value of a received frame before setting its NAV, and attempts transmission when its NAV expires
DT0 is at least one timeslot long; it would not be more than a couple of timeslots
DTI is shorter than DT0 by at least a time slot, if DT0 is nonzero
Mathilde Benveniste, Avaya Labs

13. Express Forwarding Illustration
September 2007
Express Forwarding Illustration
DT0
NAV setting at receiving node
DTI
NAV setting at all other neighbor nodes
Value in Duration field of TSQ frame
Channel
1-2
Frame
ACK
2-3
3-4
time 5
3-hop path 1-4 1 2 3 4
ANIMATED
The Duration field is set at value longer than usual when a TSQ frame is transmitted to a forwarding node of a multi-hop path; DT0 (> 50 s) added
The forwarding nodes, 2 and 3, adjust the Duration value on the received frame by subtracting the increment DTI when setting their NAV
The non-forwarding neighbor nodes (e.g. 5) set NAV according to Duration field
Mathilde Benveniste, Avaya Labs

14. Express Forwarding procedure - 2
September 2007
Express Forwarding procedure - 2
Contention is reduced when forwarding a TSQ frame for all forwarding nodes other than the first node on path
Neighbor nodes have their NAV still set to at least DT0 when the new forwarding node is ready to transmit (barring any independent NAV-setting request); thus they will not contend for the channel, letting that node transmit before any of them
A forwarding node performs CCA before attempting transmission
This avoids collision with another node that has not heard the received TSQ transmission to set the NAV accordingly
If the channel is busy, the forwarding node defers transmission with a random back off
Mathilde Benveniste, Avaya Labs

15. … with multiple fragments
September 2007
… with multiple fragments
When multiple fragments of a frame are transmitted, the express forwarding node will start transmitting only at the end of the last fragment
The Duration field value on each fragment of the frame to be forwarded is increased by DT0 and the fragments are flagged as TSQ
The Duration field on the ACK (if any) for each of the fragments received at the destination node is set based on received fragment duration (i.e., a SIFS and ACK tx time are subtracted)
The NAV at the node to which the fragments are forwarded is set by subtracting DTI (where DTI < DT0) from the Duration value of a received frame if the frame is to be forwarded
Forwarding of the received fragments starts when the NAV expires, which occurs once all the fragments have been received and acknowledged
Mathilde Benveniste, Avaya Labs

16. September 2007
… with TXOPs
TXOPs allow multiple frames to be transmitted with a single contention
TXOPs can be used together with express forwarding
The combination reduces contention and collisions along the forwarding path!
The Duration field value on each frame of a TXOP to be forwarded is increased by DT0 and the frames are flagged as TSQ
The Duration field on the ACK (if any) for each of the TXOP frames received at the destination node is set based on received frame
The NAV at the node to which the TXOP is forwarded is set by subtracting DTI (where DTI < DT0) from the Duration value of a received frame if the frame is to be forwarded
Forwarding of the received TXOP starts when the NAV expires, which occurs once the entire TXOP has been received and acknowledged
Mathilde Benveniste, Avaya Labs

17. … with RTS/CTS September 2007
RTS/CTS is the mechanism for reducing the impact of hidden terminals, which are common in a wireless mesh
RTS/CTS protection may be used in conjunction with express forwarding
The combination reduces contention and collisions on the forwarding path
The RTS of time-sensitive QoS frame is flagged TSQ and the Duration field on the RTS is increased by the increment DT0
The node to which the RTS is addressed responds with a CTS with Duration value set based on the Duration field of the received RTS.
If the node receiving the RTS must forward the RTS-protected frame(s), it sets its own NAV by subtracting DTI (where DTI < DT0) from the Duration value of the RTS and of all frames received subsequently as part of the same RTS-protected frame(s)
Forwarding of the received frame or TXOP starts when the NAV expires, which occurs when the frame or TXOP has been received and acknowledged
Mathilde Benveniste, Avaya Labs

18. Collisions, retransmissions, and ACK rate
September 2007
Collisions, retransmissions, and ACK rate
Mathilde Benveniste, Avaya Labs

19. Collisions caused by multi-hop flows
Month Year
doc.: IEEE yy/xxxxr0
September 2007
Collisions caused by multi-hop flows
If the ACK from 2 (or 3) causes collision at Node6, retransmission of frame from Node25 will wait till multi-hop TX P->Node5 completes
The sooner the latter completes, the sooner the transmission Node25->Node6 will complete
Express forwarding reduces time of transmission P->Node5, thus shortens delay for flow Node25->Node6
With Express Forwarding, the ACK from 2 prevents collision by Node25 for the remainder of multi-hop transmission P->Node5
Multi-hop flow from P to Node5 P 1 2 3 4 5
VIDEO (L)
VIDEO (H)
VOIP 17 18 21 13 25 24 11 20 6 7 8 22 16 10 12 19 14 9
ANIMATED
Mathilde Benveniste, Avaya Labs
John Doe, Some Company

20. (((( (((( Pair of hidden nodes X X Single channel mesh September 2007
Collisions between a pair of hidden nodes may repeat
Retransmission attempts likely to fail
They increase delay and may cause frames to be dropped
Large backoff values help break collision cycle
Prioritizing one of the transmissions helps break collision cycle
Single channel mesh
A and D cannot hear each other
C cannot receive when A transmits
B cannot receive when D transmits X Tx C D
((((
(((( A B X Tx
Mathilde Benveniste, Avaya Labs

21. Express Retransmission
September 2007
Express Retransmission
Retransmission typically involves backoff using a wider contention window
With express retransmission, a frame is retransmitted by dispensing with backoff and transmitting within DT0 following ACKtimeout
An express forwarded frame is less likely to collide on retransmission because of its prioritization
Only the first retransmission attempt receives priority treatment
Prevents two express-forwarded frames from colliding repeatedly
Subsequent retransmission attempts less likely to lead to collision when using large CWmax
Channel
time
ACKtimeout
TSQ
Frame
DT0
DTI
Retransmission
NAV setting at all other neighbor nodes
NAV setting at receiving node
Value in Duration field of TSQ frame
Mathilde Benveniste, Avaya Labs

22. Express retransmission illustration
September 2007
Express retransmission illustration
Express retransmission (X-RTX) enables a multi-hop transmission to complete faster
Express forwarding and X-RTX on single-channel mesh
The transmissions from A->B and F->E lead to hidden terminal collisions
Express retransmission enables the TSQ frame (A->B) to succeed upon retransmission
The ACKs sent by B and C protect the frame as it is forwarded on X
TSQ
((((
Interference
(((( Tx
Interference E F A B C D
X-RTX
TSQ
TSQ
ANIMATED
Mathilde Benveniste, Avaya Labs

23. Benefit of lower ACK rate
September 2007
Benefit of lower ACK rate
Reducing the ACK rate benefits express forwarding as ACK range increases
Nodes further away hear the TSQ ACK and set their NAV accordingly
The express-forwarded frame will be on the air when the neighbors’ NAV expires; collision is averted
Example: F can decode an ACK from B
B will be able to forward the received TSQ frame
without interference from F F
) ) ) )
ACK A B C D
TSQ
Mathilde Benveniste, Avaya Labs

24. Making room for time-critical transmissions
September 2007
Making room for time-critical transmissions
Mathilde Benveniste, Avaya Labs

25. Prevent delay of time-critical frames
September 2007
Prevent delay of time-critical frames
Time-critical (TC) frames = top-priority management frames, or top-priority frames with a delay in excess of a threshold (MIB variable Dot11MeshTCTrigger)
The MSDULifetime timer of EDCA can be used for TC frames
TC frames include frames sent on a single hop
Without TC provisions, express-forwarded traffic would delay the transmission of frames that are not forwarded on a multi-hop path
The ‘bypass’ feature eliminates for the time-critical frames access delay that could be caused by express forwarding
Mathilde Benveniste, Avaya Labs

26. Express Forwarding Bypass
September 2007
Express Forwarding Bypass
Nodes with time-critical (TC) frames are allowed to count down or transmit before a TSQ frame, according to EDCA rules
If a node with a TC frame hears a TSQ frame transmitted, it subtracts DT0 from the frame’s Duration value when setting its NAV
When the NAV expires the node counts down backoff or attempts transmission if backoff expires
Because DT0 exceeds DTI by at least one time-slot, the node with the forwarded frame will sense the channel busy when a TC frame with expired backoff is transmitted
The forwarding node will back off, using a random delay
EF Bypass for TC frames can be cancelled by setting DT0 = DTI
Mathilde Benveniste, Avaya Labs

27. Postponing express forwarding
September 2007
Postponing express forwarding
‘Express forwarding’ may be postponed until a specified number of hops has been traversed
Postponing EF better balances the (express forwarded) traffic on long multi-hop paths with traffic on the shorter paths (which is not express forwarded)
A hop counter HN is carried on the frame; a forwarded frame is not marked TSQ, and its Duration value is not increased, until HN reaches a designated value EFNH (MIB variable dot11MeshEFNH)
TTL [# of remaining hops, TGs draft] is the hop counter set on frame initiation
HN= dot11MeshTTL – TTL
Hence, the TSQ flag is set when TTL  dot11MeshTTL – EFNH
Mathilde Benveniste, Avaya Labs

28. Month Year
doc.: IEEE yy/xxxxr0
September 2007
Conclusion
‘Express’ forwarding can reduce channel access delay along multi-hop mesh paths
VoIP can thus use a wireless mesh this way
One can specify the trigger hop count when express forwarding takes effect
Increasing the trigger hop count makes express forwarding less aggressive
Fairness to all frames; Time-Critical frames get through without delay
No time-sensitive frame builds excessive delay, whether forwarded or not
Express Retransmission preserves priority in case of hidden terminal collisions and helps reduce overall collision rate
Low ACK rates and large CWmax limits enhance express forwarding by reducing the collision rate
It is compatible with stations/APs observing any of the existing MAC protocols
The features are optional
Mathilde Benveniste, Avaya Labs
John Doe, Some Company

29. Month Year
doc.: IEEE yy/xxxxr0
September 2007
Motion
Move to adopt the changes in Doc r0 into the TGs Draft D1.04
Mathilde Benveniste, Avaya Labs
John Doe, Some Company