MACSCC: A Medium Access Control Protocol with Separate Control Channel for Multihop Wireless Network

1
MAC-SCC A Medium Access Control Protocolwith
Separate Control Channel for Multi-hop Wireless
Networks

• Yijun Li, Hongyi Wu, Nian-Feng Tzeng,
• Dmitri Perkins, and Magdy Bayoumi
• The Center for Advanced Computer Studies (CACS)
• University of Louisiana at Lafayette

2
Outline

• Introduction
• Motivation for this work
• Proposed MAC-SCC
• Experimental Results
• Conclusions

3
Introduction

• Infrastructure-based network
• WLAN we are using
• Based on Access Points (AP)
• Ad-hoc network
• Infrastructure is not possible or expensive
• Military application, rescue, sensor network

4
Ad Hoc Network-(multi-hop)

• Collection of self-configured nodes
• Each node works as host and router

Destination
Source
5
Review of TCP/UDP
TCP
ACK 2
Timeout
Receiver
Sender
Sliding window Min( receiver window, congestion
window)

• Congestion Control
• Slow start for congestion window size
• Slow Start when timeouts. If timeout frequently
  happens, TCP throughput will be low.

• Flow Control
• Receiver buffer not overloaded.
• Advertises available buffer size in ACK

UDP

• Whenever packet is ready, just send it
• Potential congestion without any control
• Unreliable without ACK.

6
Review of IP layer

• TCP/UDP only consider sender and receiver. IP
  layer will take care of how to make it
  transparent in multi-hop network and instruct how
  packets can send/receive between sender and
  receiver.
• Before a data packet from TCP/UDP can be sent
  from source to destination, a route from source
  node to destination should be discovered first.

Two type of routing schemes

• Proactive routing scheme
• Periodically send hello messages to direct
  neighbors to maintain the topology information.
• Two much control traffic.

• Reactive routing scheme
• Whenever source node wants to send a data packet,
  it broadcasts route request packets to whole
  network until destination is founded
• Destination node sends back a route reply packet.
  
• The route information will be contained in the
  packet header.
• Need a long delay to find a route.

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MAC/PHY Layer

• MAC layer will try to avoid the collision of
  access to the wireless media
• PHY layer will transmit and receive bit-stream.
  From hardware view, base-band processing and
  frond-end transceiver

• 802.11x MAC
• 802.11 infrastructure-based and peer-to-peer
  (ad-hoc)
• 802.11eMAC Enhancements for QoS to improve QoS
  for better support of audio and video (such as
  MPEG-2) applications.
• 802.11i Medium Access Method (MAC) Security
  Enhancements enhance security and authentication
  mechanisms.
• 802.11x PHY
• 802.11 2Mbps (Proposed in 1997)
• 802.11b 1, 2, 5.5 and 11 Mbps, 100mts. range
  (product released in 1999, no product for 1 or 2
  Mbps)
• 802.11g 54Mbps, 100mts. range (uses OFDM
  product expected in 2003)
• 802.11a 6 to 54 Mbps, 50mts. range (uses OFDM)
• 802.11n MIMO, LDPC and OFDM in PHY to increase
  data-rate more than 100Mb

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Main problems for MAC design

• Hidden station outside senders transmission
  range and in the receivers transmission range.
  It will disturb senders transmission if it
  transmits, due to it cannot detect the senders
  transmission.

A
B
C
D

• Exposed Station in the senders transmission
  range, but it misunderstand that it cant
  transmit a packet

A
B
C
D
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RTS/CTS scheme for MAC in 802.11x
Send RTS, Reply with CTS

• Red machines receive RTS/CTS, get the period they
  should keep silent. This period is NAV.

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RTS/CTS scheme for MAC in 802.11x
Send Data, Reply with ACK
After RTS/CTS reservation, red machines will keep
silent until the current data transmission is
finished. If red machines have packets to send
during reservation, they have to back-off
sometime after media is free to access
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Questions?
Is 802.11 MAC perfect?
NO
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Channel efficiency
NAV
Problem 1 in 802.11 MAC
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Link failure instability in TCP

• Collision in node C
• Several retries failed, report a link failure to
  IP layer, then have to find a new route

Problem 2 in 802.11 MAC
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Unfairness in TCP

• In two-hop session, only communication between 4
  and 5 can affect one-hop session.
• In one-hop TCP session, the available interval
  between packet transmission is larger than that
  of the two-hop TCP session, which gives the
  one-hop session more chances to transmit data.
• Also, random back-off actually favors the last
  succeeding transmission.
• As the results, one-hop session will occupy the
  entire wireless medium due to its unauthorized
  priority

Problem 4 in 802.11 MAC
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Our proposed MAC-SCC

• The available bandwidth is partitioned into two
  channels a data channel and a control channel,
  each associated with a network allocation vector
  (NAV).
• The station transmits or receives on one channel
  only at any given time.
• During the current data transmission, the next
  data frame can be pre-scheduled via the separate
  control channel, and thus reducing the frame
  collision probability and the bandwidth wasted
  during back-off.
• Moreover, the use of the separate control channel
  helps to achieve fair medium access and solve the
  instability problem resulted from frequent link
  failures.
• The optimal bandwidth partitioning between the
  two channels is analyzed via a statistical model

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MAC-SCC protocol

• CH A Data channel, CH B Control channel
• S and D (enhanced RTS/CTS handshake)
• If CH A, CH B are idle, send RTS on CH A. If CH B
  is idle only, send RTS on CH B. Otherwise,
  back-off.
• When RTS/CTS handshake occurs in CH B, send
  SRTS/SCTS on CH A to reconfirm the reservation
• D receives RTS/SRTS, replies with CTS/SCTS,
  respectively
• After handshake, S sends DATA, D replies with
  ACK.
• Other nodes
• contains NAVa, NAVb for CH A, CH B respectively.
• Update NAVa, NAVb
• When NAVa 0, convert NAVb into NAVa, release CH
  B

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Example of MAC-SCC
No back-off here
DIFS
Pre-schedule the next data transmission
during the current data transmission
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Simplify hardware design
scheduling channel usage

• Two NAVs for two channels
• Listen to two channels, but only allow
  transmitting or receiving at one channel at the
  same time

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Further discussion on related work
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Issues in MAC-SCC

• How to partition two channels?
• What is system throughput?
• How about control packets overhead and link
  failure?
• How MAC-SCC affect TCP/UDP performance?
• Fairness in TCP
• UDP system throughput
• Two different ways to evaluate MAC-SCC
• Stand-alone simulation in Parsec
• Whole protocol stack simulation in Qualnet

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Analytical Bandwidth partitioning
Assume RTS arrival is Poisson distributed with
rate
Optimal D is 10
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Simulation Setup

• The traffic load (G) is defined to be the number
  of frames per frame time
• The bigger G, the more packets to send

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Stand-alone Simulations

• PARSEC A Parallel Simulation Environment for
  Complex systems
• Parsec is an parallel programming language based
  on C.
• We write PARSEC code to simulate and compare the
  MAC-SCC protocol and 802.11
• System throughput, link failure probability, and
  optimal bandwidth partitioning are studied

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Bandwidth Partitioning

• D Bandwidth of data channel / Bandwidth of
  control channel
• D gtgt 10, control channel ? bottleneck
• D ltlt 10, data channel ?bottleneck
• Optimal D is 10 and verifies the analytic results

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Throughput Comparison in Parsec
MAC-SCC keep flat in high traffic load
In high traffic load, MAC-SCC works much better
than 802.11 Gain is up to 60
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Link failure probability
Less failure in MAC-SCC
MAC-SCC always has lower link failure probability
due to scheduling packet
27
Comprehensive Simulations

• Qualnet is a commercial simulator from
  www.scalable-network.com
• It contain whole protocol stack
• Deployed FTP sessions in application layer to
  study TCP fairness
• Deployed VBR in application layer to study
  bandwidth partitioning under non-Poisson
  distributed traffic
• Used UDP to study system throughput in high
  traffic load

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Fairness in two TCP sessions
Similar

• String topology, session 1 is one hop, session 2
  is 1, 2, 3, or 4 hops
• MAC-SCC get the similar throughput with the ideal
  case
• MAC-SCC total system throughput is a little bit
  lower, due to TCP congestion control? no high
  traffic load

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Fairness in Multiple TCP Sessions
Node Topology

• Study TCP fairness in more than 2 TCP sessions
  (3, 4, and 8 TCP sessions)
• Regular positioning to remove the effect from
  node positions

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Three TCP sessions
802.11
MAC-SCC give fairness to node 5
MAC-SCC
X-axis is nodeID, Y-axis is system
throughput Node 2, 3 are one hop session Node 5
is two hop seesion
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Four TCP sessions
802.11
MAC-SCC
Node 2, 6 are one hop session Node 3, 8 are two
hop seesion
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Eight TCP Sessions
802.11
MAC-SCC
Node 2, 6, 11, 13 are one hop session Node
3,8,13,15 is two hop seesion
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Bandwidth Partitioning in VBR traffic

• We deployed VBR in application layers to study
  bandwidth partitioning in non-Poisson
  distribution with different traffic loads
• D is between 8 and 12

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System throughput in high traffic load

• Scalar is used to control traffic load
• UDP is used for constructing high traffic load

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Conclusion

• We have proposed a novel Medium Access Control
  protocol with a Separate Control Channel
  (MAC-SCC).
• To reduce hardware complexity, the station
  transmits or receives on one channel only at any
  given time.
• The performance of MAC-SCC is quantified via
  extensive simulations in both a stand-alone
  simulator developed by using PARSEC and a
  comprehensive network simulator called QualNet
  with whole protocol stack.
• Our results show that MAC-SCC can effectively
  reduce the link failure probability, reduce
  control packet overhead and transmission power,
  achieve fair medium access when running multiple
  TCP sessions, and yield a throughput gain up to
  60 under high traffic load, when compared with
  the basic RTS/CTS scheme.