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Improving TCP Performance over Wireless Networks


Sharjeel Shahid
[email protected]






Abstract
Due to tremendous growth of Wireless internet users there is
a need of efficient mechanisms and protocols which
integrate the wireless networks with wired internet in such
way that existing infrastructure of wired internet and same
quality of services can be available to wireless connected
users.
A reliable data transfer protocol, TCP (Transmission
Control Protocol) was developed by keeping in view all
characteristics of wired network. Wireless network has very
different characteristics as compare to wired network which
causes the TCP inefficient performance over heterogeneous
environment of wired and wireless network.
So in this paper we will discuss the proposed solutions and
extract the future research directions to facilitate the
efficient reliable data transfer protocol for wireless
networks.

1. Introduction
Most of the internet applications use TCP as its underlying
transport layer protocol for reliable data transfer. It is a very
urgent need of a Reliable data transfer protocol for wireless
network as wireless users growth is increasing
tremendously. Unfortunately TCP which complete design
only focuses on the properties of wired network is
performing very poorly in wireless networks for very
obvious reasons as mentioned above.
There are many proposed solutions which have their own
advantages and disadvantages. These solutions are based on
proxy scheme, link layer modifications etc.
In following sections first of all there is a discussion on
wireless architecture and its characteristics. In section 3,
TCP and its shortcomings in wireless network is discussed.
Section 4 summarizes the challenges which need to
overcome for the solution of efficient reliable data transfer
protocol. Section 5 discusses the existing research work.
And after review the proposed research works we will
extract the fundamental design issues and then in section 6,
we try to summarize the future research directions. Last
section will describe the conclusion.

2. Wireless Network
Wireless Network has very distinct characteristics as
compare to wired network. In wired network the reason of
packet loss is only to be assumed due to congestion on the

network but in wireless network there are many other
attributes which causes the packet loss. So the
implementation of those protocols like TCP which are
developed by only focusing wired network are giving low
throughput in wireless environment.
As shown in figure 1; Wireless host, also called Mobile
Host (MH), are connected through satellite or wireless
Access point to Base Stations (BS) and these Base stations
are connected to internet through wired medium for
communication with different servers, named as fixed hosts
(FH), like web, ftp etc. We can call BS as a boundary of
wired and wireless networks. So network environment of
FH to BS (wired network) is very different from network
environment of BS to MH (wireless network).



























In wireless networks high bit-error rate and random losses
are frequent as compare to Wired Network in which these
kinds of errors are very rare.
Second important property of wireless network is Channel
Access Asymmetry which affects the performance of TCP.
Channel Access Asymmetry discussed in [6] means that the




Figure 1: Wireless Network

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bandwidth and latency characteristics of a links which
forward the data flow towards client is different from that
link which flow data from client. TCP performance
significantly dependent on acknowledgement time period
and Asymmetry of channel cause the high variation in
response time. More detail of TCP is discussed in section 3.
Third important property of wireless network is
disconnection due to handoffs and interferences for very
short time period. This disconnection impact on TCP
throughput significantly. The term handoff refers to the
process of redirecting the connection of MH from one BS to
other BS. This process requires when MH moving away
from currently connected BS’s area and moving to that area
which is covered by other BS. So this handoff cause little
interruption for very short time but it is necessary to avoid
termination of the connection.

3. Transmission Control Protocol
Transmission Control Protocol (TCP) is widely used
protocol for reliable data transfer in many applications. It
manages and controls the connection, data flow and the
most important congestion.
Two main reasons of poor performance of TCP studied in
[1] are firstly poor error detection mechanism of TCP which
is unable to distinguish the nature of the error in
heterogeneous environment of wired and wireless networks
and secondly the error correction strategy which treat all
type of errors like random errors, interruption due to
handoff etc. in wireless environment in a same manners. In
short TCP only able to detect that the packet is lost but
unable to detect reason of the packet loss and assume that
the reason of packet loss is congestion on the network which
is not always true in wireless networks.
All TCP implementations which include TCP Tahoe, TCP
Reno, TCP Vegas discussed in [1] & [5] perform “slow
start” if no acknowledgment of packet transfer receive with
in timeout interval. So when no acknowledgement receives
then TCP assume that network is too congested and it sets
its transfer rate to 1 segment which is called “slow start”. In
Wireless Network due to other properties, Packet or its
acknowledgement’s loss can be due to transient random
error but the way TCP back off in such scenarios when
bandwidth is available but due to wrong assumption of
congestion it set its transfer rate to 1 segment which causes
the significant deterioration in throughput [4].
Mostly internet traffic consists of short flows. As TCP take
slow start to send the data so until it reaches to utilize the
complete available bandwidth the transfer data gets finished.
So in spite of available bandwidth TCP not utilize it
efficiently. This issue exists for wired network as well.

4. Challenges
Now after reviewing the Wireless architecture and reasons
of poor performance of TCP in wireless environment, there

is a need of such robust solution which can be able to detect
and recover from following nature of errors which exist in
wireless environment…

High Bit-Error Rate
Low Bandwidth
Handoff/Disconnections
Large RTT and RTT Variation
High rate of Packet Losses

And second important challenge is that it should be easy
to implement and also easy to deploy.

5. Proposed Solutions
There are many different proposed solutions to overcome
and to improve the TCP performance for wireless networks.
These proposed schemes can be classified mainly in three
sections 1) Proxy Based, 2) End-to-End and 3) Link layer.
One Discussion and summary of these proposed solutions is
as follow…

Indirect-TCP (I-TCP) [7]
It splits the transport link at the wired-wireless boundary; it
makes two connections one from FH to BS and second
connection from BS to MH. Both these connections can use
different transport layer protocols. In this mechanism BS
buffer the packets received from FH and then forward it on
second connection which is established between BS to MH.
In that way this mechanism attempts to hide the effect of
wireless losses from TCP. This mechanism loses the TCP’s
end-to-end semantics. It also causes the high overhead of
two connections at boundary of wired-wireless. BS can run
out of buffer and during handoff BS needs to transfer all
cached packets to new BS

MTCP [2]
It is very similar to I-TCP and split connections into two.
But it maintains the end-to-end semantics by acknowledge
the last byte of data to the FH to the source only when it is
received by the MH. So in case when MH is handoff state it
send packet by sending available window size as zero
(ZWA) so that source can freeze all the transmission.

WTCP
It also splits connection from FH to BS and from BS to MH.
But BS only send acknowledgement to FH when it receive
acknowledgement from BH. So in that way it maintains the
End-to-end semantics.

Snoop [2]
It is implemented at Data Link Layer. Base station,
boundary of wired-wireless network, monitor packets in
both directions. It cache the packets and if detect the
duplicate acknowledgements then it retransmit the packet
from cache to mobile host. Sack is used when traffic is send
from mobile host to fixed server. Based on this mechanism
there is no change required in fixed host. End-to-End

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mechanisms will also be preserved. But the Link layer needs
to aware that that the segment is of TCP packet. It can also
not be implemented for encrypted headers.

TCP-Probing [1]
This scheme exchanges a structured packet between source
and destination when ever packets get lost or timeout events
occur to check the network situation. And until “prob”
packet acknowledgment, all transmission gets suspended. In
that way this mechanism does not reduce the congestion
window size or increase the RTT. So if the error is due to
transient random loss then Congestion window will not be
reduced. There are chances that “probe” packet
acknowledgement received successfully but when sending
the actual data it again get drop due to any packet loss
pattern. On packet drop as its stops complete transmission
and only initiate a “prob” cycle which is again decrease the
throughput if the error is of transient nature.

Explicit Bad State Notification (EBSN)
It is implemented at link layer level. According to EBSN
scheme; in case if wireless link is in erroneous state then BS
sends EBSN message to source. And after receiving the
EBSN message source reset its timer so that time out even
can not be happen and which will avoid the TCP to apply
back off strategy. To implement this scheme there are
changes required at source side.


TCP-SACK
In this scheme receiver send the information of all packets
which are successfully received instead of cumulative
acknowledgement. In that way this scheme does not enforce
Sender to show aggressive behavior against packet loss.
There is a hearer size limitation in which receiver will send
the detail of all successfully received packets.

Explicit Control Protocol (XCP) [3]
In this scheme network explicitly tells the sender the state of
congestion and how to react to it. In that way if the network
is too congested then sender will decrease its sending rate
very quickly and if the congestion is not too high then
sender decreasing rate will be low. It will avoid a aggressive
back off as in TCP. It also decouples the concept of
congestion control and fairness control. For congestion
control this scheme uses MIMD policy (Multiplicative-
Increase Multiplicative-Decrease) and for Fairness control it
uses Additive-Increase Multiplicative Decrease policy
(AIMD). It also coexists with TCP. But it requires changes
at Network layer as well as in Transport layer.

6. Design Issues
After reviewing the Wireless Network Architecture, TCP
limitations, issues in proposed solutions, following is the
summary of fundamental design related questions for the
appropriate solution….

First question is that which part of the network to implement
the solution. It can be following…

Sender
Receiver
Wired part of the Network
Wireless part of the network
Hybrid Approach


Second fundamental design related question is that which
layer is the best place to implement the solution. Following
are the possible options…

Transport Layer
Network Layer
MAC Layer


And third fundamental question is that which scheme will
be the most appropriate…

End-To-End Schemes
Split-Connection Schemes



7. Future Research Directions
Transport layer seems to be most appropriate place for Error
Control so main research should be focus for the
implementation at transport layer. But there may also
include lower layers for optimization based on more precise
information that could be gathered from lower layers.
Optimization at link layer must be able to provide fine grain
information which can facilitate the transport layer and
design of error recovery techniques based on different
nature of error.
Existing solution must have backward compatibility so that
it can be deployed in an incremental fashion.
While establishing the connection two end sources may
communicate their different network properties so that error
detection can be judge in a better way.

8. Conclusion
TCP is not able to perform efficiently in Wireless network
due to wireless network distinct properties as compare to
Wired Network. After reviewing different proposed
solutions it seems that improvement in reliable transfer
protocol for wireless network really need enhancements in
Transport layer and as well as at Link layer so that better
detection or may be prevention can be achieve. Backward
compatibility should also be hold for viable deployment.

References:
[1] Open Issues on TCP for Mobile Computing by Vassilis

Tsaoussidis and Ibrahim Matta
[2] Improving Reliable Transport and Handoff

Performance in Cellular Wireless Networks by Hari
Balakrishnan, Srinivasan Seshan, and Randy H. Katz

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