Download Automation of Batch Vacuum Pans At D.R.B.C Sugar Factory

TCP PERFORMANCE OVER AD HOC
NETWORKS
Presented by
Vishwanee Raghoonundun
Assisted by
Maheshwarnath Behary
MSc Computer Networks
Middlesex University
Introduction
Definition of Ad hoc networks
TCP/IP in Ad hoc networks
Causes of performance degradation
TCP enhancements in Ad hoc networks
Conclusion
Reference
Definition of Ad hoc network
Ad hoc network is a collection of two or more devices equipped with
wireless communication and networking capability. An Ad hoc
network is infrastructureless as it does not rely on any fixed network
entities, i.e, there is no need for any fixed radio base stations, wires or
fixed routers.
A mobile ad hoc network (MANET) is a multi-hop connection and is
suitable for application areas such as:
 battlefield communications
 disaster recovery
 inimical environment monitoring (where fixed wired
infrastructure is not available)
TCP/IP in Ad hoc networks
TCP is an end-to-end protocol
Designed to provide flow and congestion control in a network
Connection-oriented protocol
Reliable for retransmitting lost packets in wired networks
Uses Congestion Control Mechanism
TCP/IP cannot distinguish between the nature of the problems on the
network and assume network congestion in all cases.
Inefficient due to the wireless, unpredictable nature of ad hoc
networks.
Result in poor performance in ad hoc networks
Causes of performance degradation
Channel Errors
Bit errors corrupt packets in transmission leading to packet (ACK) losses.
Sender reduces congestion window to one packet and reacts with a slow
start.
Medium Contention and Collision
Hidden and exposed terminals situations prevent one node from reaching the
other, thus triggering a route failure. Route has to be recomputed and that
causes delay resulting in TCP invoking congestion control algorithms if
timeout occurs
Mobility
Handovers cause link breakage and packet loss. TCP again assumes
congestion and reacts accordingly. Furthermore routes have to be recalculated
causing delay and throughput is severely degraded.
Congestion
Congestion may give rise to buffer overflow and increased link contention,
which degrades TCP performance.
TCP Enhancements in Ad hoc networks
Feedback-based techniques
TCP-F
Upon route failure, a Route Failure Notification (RFN) is sent to
sender which enters in a ‘snooze’ state, i.e, it stops transmission until it
receives a Route Reestablishment Notification (RRN).
TCP-ELFN (Explicit Link Failure Notification)
 Upon receiving ELFN, sender disables congestion control
mechanism and enters into a “stand-by” mode until a new route is
established.
 While on stand-by, sender sends a packet to probe network to see if
a route has been established.
ATCP (Ad hoc TCP)
 If route breakage, sender enters “persist state”
 If network congestion, sender enters “congestion control” state
 If high bit error rate, sender enters “retransmit” state
TCP Enhancements in Ad hoc networks (cntd)
TCP without feedback
Uses Adaptive Congestion Window Limit (CWL) Setting, which is
dynamically adjusted according to current round-trip hop-count (RTHC) of the
path (CWL should be < than RTHC)
It reduces delay at any hop along the path
Indirect TCP
Segments a TCP connection into a fixed and a wireless part
Access Point acts as a proxy and relays data in both direction. Neither mobile
node nor fixed node knows what takes place at the other end.
In case of packet loss, AP forwards the packet without the transmitter being
aware of the problem.
Transmission errors on wireless link cannot propagate to wired link
Loss of end-to-end semantics
TCP Enhancements in Ad hoc networks (cntd)
Snoop TCP
Foreign agent buffers all packet with destination mobile host and
snoops acknowledgements in both directions
To remain transparent foreign agent must not acknowledge data to
the correspondent host
Transmission from fixed node to mobile node, sensing no ACK,
foreign agent will retransmit loss packet
Transmission from mobile node to fixed node , FA snoops packet
streams and on detecting missing packets, will send a NACK to
mobile node to activate retransmission
Preserve end-to-end semantics
Useless if encryption scheme is applied to data and require more
buffer
TCP Enhancements in Ad hoc networks (cntd)
Mobile TCP
Suitable for lengthy or frequent disconnections
The Base Station is known as Supervisory Host (SH)
No proxy/does not have a buffering system, instead require sender to
retransmit lost packet
If SH does not receive ACK, it assumes disconnection and changes the
window size of the sender to 0 - Choking
Maintain TCP end-to-end semantics
Packet loss due to error bits is propagated to sender
Freeze TCP
True end-to-end scheme and does not require the involvement of any
intermediaries (such as base stations) for flow control
Changes in TCP code are restricted to the mobile client side, making it
possible to fully inter-operate with the existing infrastructure
Conclusion
Traditional TCP is reliable in wired networks but it needed further
enhancements in order to operate efficiently on wireless networks.
TCP enhancements considered earlier on, have proved to reduce
throughput degradation considerably. However they are not fool-proof
as they carry some trade-offs. Further enhancements are required until
a fool-proof version is specifically designed for wireless and ad hoc
networks.
References
[1]
[2]
[3]
Jochen Schiller, “Mobile Communications”, Second Edition,
2003
Xiang Chen, Hongqiang Zhai, Jianfeng Wang and Yuguang
Fang, “TCP Performance over Mobile Ad Hoc Networks”,
http://www.ecel.ufl.edu/jwang/publications_files/tcp_cjece.pdf
A. Lasebae, “Mobile and Wireless Networks”.