Fault Tolerance for WLAN

Speaker ： Mark Yang

93.04.27

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Outline

Hardware Fault Tolerance Dependability enhancement for IEEE 802.11 wireless LAN

with redundancy techniques Tolerance to Access-Point Failures in Dependable Wireless

Local-Area Networks Comparison

Software Fault Tolerance TCP-DCR: A novel protocol for tolerating wireless channel

errors Implementation of Explicit Wireless Loss Notification Using

MAC-Layer Information Comparison

Simulation Conclusion

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Hardware (1)

Dependability enhancement for IEEE 802.11 wireless LAN with redundancy techniques

Dependable Systems and Networks, 2003. Proceedings. 2003 International Conference on , 22-25 June 2003 Pages:521 -

528

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Hardware (1) – Abstract

Propose the alternate approach of tolerating the existence of

"shadow regions" as opposed to prevention in order to enhance

the connection dependability.

A redundant AP is placed in the shadow region to serve the

mobile stations which roam into that region. The secondary AP is connected to the same distribution system

as the primary AP. (DS configuration)

The secondary AP acts as a wireless forwarding bridge for traffic

to/from the mobile stations in the shadow region to the primary

AP. (Forwarding configuration)

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Hardware (1) – DS configuration

An additional AP is placed in the shadow area with the same frequency as the primary AP. The secondary AP forwards the data between the mobile stations in the shadow area and the primary AP. The two APs communicate over the distribution system.

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Hardware (1) – Forwarding configuration

The secondary AP is placed at a certain location where it could communicate with both the mobile terminal in the shadow area and with the primary AP. The secondary AP thus could forward the packet transmissions in both directions.

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Hardware (1) – Aspects

Since the beacon interval is less than 100ms, the maximum detection delay for link failures is 100ms A mobile station in the shadow region may only transmit data when it is granted a TXOP by the secondary AP

The primary AP sends the specification of TXOP to the secondary AP. Simultaneously, the primary AP also broadcasts the same channel reservation message in the cell in the form of a QoS-Poll frame. All stations in the non-shadowed area will receive the QoS-Poll frame and defer any transmission attempt until the channel reservation time is over. With channel reserved, the secondary AP then sends the QoS-Poll frame to mobile stations in the shadow region sequentially so that they may send their data packets free of collisions.

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Hardware (1) – Fault models

Reliability:

Availability:

Survivability:

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Hardware (1) – Numerical examples

Availability = 1-10-2 = 0.99

1 /λos ≈ 2.8 hours

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Hardware (2)

Tolerance to Access-Point Failures in Dependable Wireless Local-Area Networks

Object-Oriented Real-Time Dependable Systems, 2003. Proceedings. Ninth IEEE International Workshop on , 1-3 Oct. 2003

Pages:136 - 143

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Hardware (2) – Abstract

Enhancing the dependability of wireless networks by focusing on tolerating AP failures and develop and evaluate a new fault-detection approach, based on signal-to-noise ratio. Detection of AP Failures

Beacon-frame monitoring Signal-to-noise ratio

Three techniques to recover from AP failures: Access-Point Replication Overlapping-Coverage Link-Multiplexing

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Hardware (2) – Beacon-frame monitoring

Handoff mechanism in 802.11 WLAN Passive Scanning: A mobile station sweeps from channel-to-channel to detect the presence of Beacon frames which are periodically transmitted by the APs. Active Scanning: A mobile station actively seeks out APs by broadcasting Probe Request frames on every channel.

Need to distinguish between user mobility and AP's failure. User mobility:

• A few users trying to handoff to a new AP due to user mobility at a given point of time.• Employ active scanning to discover new APs.

AP's failure:• The number of users trying to handoff to a new AP could be relatively large.• Using a passive scanning method instead to detect the presence of new AP.

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Hardware (2) – Signal-to-noise ratio

Using the strength of the signal that a mobile station receives from an AP, as an indicator of the AP's "up/down" status. Initial fault recovery mechanism if the signal-to-noise ratio (SNR) drops suddenly.

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Hardware (2) – Access-Point Replication

Using an additional AP that is designated as a backup, and that can be activated once the primary AP fails. Drawback:

The latency involved in detecting AP failures and performing the fail-over (authenticate ACK re-association request re-association response) is relatively large (7.03 seconds). Additional infrastructural costs – might not necessarily be actively used under fault-free conditions.

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Hardware (2) – Overlapping-Coverage

If one AP fails, mobile stations associated with that AP can be transferred over to another AP whose coverage area intersects with that of the failed AP. In IEEE 802.11, the channels used by neighboring AP be separated by

at least five channels, this limited availability of channels can result in shadow areas. Drawback:

Requires that some spare capacity be reserved at each AP to take over the additional users that the AP will have to support in case a neighboring AP (with overlapping coverage) fails. The latency involved in detecting an AP failure and switching to a functional AP is relatively large.

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Hardware (2) – Link-Multiplexing

Using redundant communication paths from a mobile station, with

each path connecting a distinct wireless network-interface card at

the mobile station to a distinct AP.

Using link-multiplexing over link-replication

Total bandwidth used for communication can remain the same as that

used by a single link. Increase in the amount of average delay in message transmission due

to the multiplexing and demultiplexing.

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Hardware (2) – Link-Multiplexing (cont.)

Requires additional software be installed at the client & server.

Utilize a library interpositioning (interceptor) approach to capture

the network layer calls made by the application, and can be embedded

inside a middleware layer at both the client and the server.

Fault-detection. Intercepting network layer calls made by the application. Multiplexing/de-multiplexing data from/to the application.

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Hardware – Comparison

Hardware (1) Hardware (2)

Measuresreliability, availability and survivability

delay in detecting AP failure and switching

Extra cost redundant APmultiple network-interface card

SoftwareStandard

802.11e (channel reservation)

interceptor installed at client and server

Recover time longer shorter

Otheronly for shadow regions

mobile station to consume more battery power

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Software (1)

TCP-DCR: A novel protocol for tolerating wireless channel errors

Accepted for publication in IEEE Transactions on Mobile Computing (February 2004)

http://www.crhc.uiuc.edu/wireless/groupPubs.html

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Software (1) – Abstract

TCP-DCR delay the triggering of congestion response algorithms for a

small bounded period of time T to allow the link level retransmissions

to recover the loss due to channel errors. If the packet is not recovered by link level retransmission by the end of

the delay period, TCP-DCR protocol triggers the congestion recovery algorithms of fast retransmission and recovery. Through simulations, TCP-DCR

Does not impact the fairness towards the native implementations of TCP. Significantly better performance when channel errors contribute more towards packet losses in the network.

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Software (1) – Behavior

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Software (1) – Choice of T

t0 t0+(RTT/2 – rtt/2) t0+RTT/2

t0+RTT/2+rtt/2 BS receives indication that the packet is lostt0+RTT/2+rtt Packet is recovered at receivert0+RTT+rtt Sender receives an ACK for the packetSender would have to delay the congestion at least: (t0+RTT+rtt)-(t0+RTT) = rtt The interpacket delays are non-zero and the TCP sender may not know the value of rtt The lower bound of T is one RTT

Retransmission timeout is usually set to RTT + 4 times.The choice of T should be such that unnecessary retransmission timeouts are avoided. The upper bound of T is one RTT.

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Software (1) – Simulation No Congestion Losses

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Software (1) – Simulation (cont.)

Only Congestion Losses

10Mbps5ms

congestion

12 TCP-SACK flows & 12 TCP-DCR flows

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Software (1) – Simulation (cont.)

Channel Errors & Congestion Losses

12 TCP-SACK flows & 12 TCP-DCR flows

TCP-DCR flows can make use of the link bandwidth not utilized effectively by the TCPSACK flows.

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Software (2)

Implementation of Explicit Wireless Loss Notification Using MAC-Layer Information

Wireless Communications and Networking, 2003. WCNC 2003. 2003 IEEE , Volume: 2 , 16-20 March 2003 Pages:1339 - 1343

vol.2

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Software (2) – Abstract

TCP suffers a significant degradation in performance over wireless networks because it does not distinguish wireless link loss from congestion loss. To overcome this problem, the Explicit Wireless Loss Notification (EWLN) scheme is proposed to explicitly inform wireless link loss to the TCP sender. EWLN scheme that deploys the information from the MAC layer and takes into account the interplay with the error recovery mechanism at the link layer. The sender's congestion control mechanism can be decoupled from the retransmission mechanism and set to react only to congestion related losses.

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Software (2) – MAC Protocol

MAC Protocol

Link-level retry

Comparing the seqNo of the current and buffered packets

To mobile terminal

To next node

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Software (2) – Receiver

Ewln_flag = 1 Send duplicate ACKEwln_flag = 0 Congestion error

Normal

Wireless link errorand link-level can't recovery

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Software (2) – Sender

Retransmit the packet upon receiving the first ACK with EWLN set.

To avoid transmission duplication, retransmit only when the first ACK + EWLN

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Software (2) – Example

Two packet losses occur over a wirless link in a single transmission window

Receive 1

Not receive 2

Not receive 2

Not receive 4

Receive 6

Not receive 4

Not receive 4

Not receive 4If lose again?

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Software (2) – Simulation

No wirelesslink error

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Software (2) – Simulation (cont.)

congestion

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Software – Comparison

Software (1) Software (2)

Measures throughput number of sequence

Modification sender sender, receiver and BS

Requirement receiver bufferbuffering technique in MAC protocol

Assumption link level retransmission 1. link level retransmission2. packet corruption only in the payload part

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Simulation – Environment

Paper : TCP-DCR: A novel protocol for tolerating wireless channel errors

Software : Linux 9 + NS 2.26 (DCR: modify tcp-sack1.cc)

Topology :

Tcl (additional setup): Error Model (exponential) Link Level Retransmission (LL/LLSnoop)

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Simulation – DCR code verify

ack no 1417 received at 87.2862, cwnd=22ack no 1417 received at 87.4168, cwnd=22dcr start at 87.4168 [ack no=1417, delay time=0.849]ack no 1429 received at 91.4158, cwnd=1dcr cancel at 91.4158 [ack no=1429]ack no 1429 received at 91.4769, cwnd=2

ack no 1131 received at 69.7674, cwnd=19ack no 1131 received at 69.9014, cwnd=19dcr start at 69.9014 [ack no=1131, delay time=0.553]ack no 1131 received at 70.2204, cwnd=19ack no 1131 received at 70.2242, cwnd=19delay fast recovery at 70.2242! [ack no=1131]ack no 1140 received at 70.7637, cwnd=19dcr cancel at 70.7637 [ack no=1140]

1st dupack

LL retransmission

ack no 4128 received at 85.4946, cwnd=19ack no 4129 received at 85.4962, cwnd=19ack no 4129 received at 85.4978, cwnd=19dcr start at 85.4978 [ack no=4129, delay time=0.144]ack no 4129 received at 85.6545, cwnd=19ack no 4129 received at 85.656, cwnd=19fast recovery begin at 85.656, dcr cancel! [ack no=4129]ack no 4142 received at 85.6816, cwnd=9

Time out

70.2242-69.9014< 0.553

85.656-85.4978> 0.144

Fast recovery

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Simulation – Performance (1)

Some of "Fast-recovery" & "Timeout" events stall happen.

Almost all errors are recovered by "Link level retransmission“.

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Simulation – Performance (2)

Some of "Fast-recovery" & "Timeout" events stall happen.

Almost all errors are recovered by "Link level retransmission“.

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Simulation – Performance (3)

Some of "Fast-recovery" & "Timeout" events stall happen.

Almost all errors are recovered by "Link level retransmission“.

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Conclusion

Papers for WLAN Fault Tolerance Hardware Fault Tolerance : less

Software Fault Tolerance : more

Simulation / Experiment method Hardware : Numerical examples or Experiment

Software : NS (Network Simulation tool)