Subject: [NEXTEL-1] Enhancing Wireless Internet Performance

From: [email-masked]
Date: Sun, 20 Jul 2003 21:37:27 -0400

Enhancing Wireless Internet Performance

Fei Hu, Rochester Institute of technology
Neeraj K. Sharma, Clarkson University

The convergence of wireless access and the Internet will be the next wave in
the information industry [1]. Currently, network providers try to offer services
overwireless Internet infrastructures such as GPRS [2]. However, numerous studies
remain to be carried out on the enhancement of wireless Internet performance. Some

major obstacles, such as the high error rate of wireless links, frequent handoff,
limited power, long delay, and limited wireless bandwidth, could inhibit its
widespread
use. Although some papers have introduced some enhancement schemes, they did
not cover the multiple network protocol layers and focused on a specific aspect of

wireless Internet, such as Transport Control Protocol (TCP) modifications for
wireless communications. This article will survey various proposed approaches to
mitigate the effect of the abovementioned obstacles. Our discussion of wireless
Internet enhancements is based on the classification as shown in Fig. 1. Broadly
speaking, we can classify the enhancements into seven categories (refer to the tan
shaded rectangles in Fig. 1):
Saving Power: In the past, energy efficiency research centered around the physical
layer, only because the power consumption in a mobile computer was a direct result
of the hardware. Now people have realized that the goal of saving power should be
implemented in multiple wireless Internet protocol layers instead of the physical
layer only.
Improving Mobile Performance: In a wireless Local Area Network (LAN), mobility
performance is not a major concern since a mobile host could visit the Internet
through a wireless outlet and their mobility is limited to the local area.
However, in GSM or other cellular networks, the handoff could occur frequently
between the neighboring cells. Therefore, we need enhancement strategies for
guaranteeing the smoothness of the handoff communications.  Guaranteeing
Quality-of-Service (QoS): QoS requirements such as end-to-end delay and
bandwidth-per-flow should be met in multimedia wireless networks. Networking
experts proposed some enhancement mechanisms such as priority-queue and
differentiated services for guaranteeing QoS.  Application-Layer Performance: The
performance of the Internet applications such as FTP could be optimized to
overcome the obstacles of wireless links. For example, local caching could save a
large amount of wireless bandwidth in the Web applications.
TCP-Layer Performance: Although TCP has been greatly improved in its ability to
adapt to high-speed links, many versions of TCP over wireless links still cannot
keep the comparative throughput as can TCP in wired networks [3]. The main
disadvantage is that traditional TCP assumes that all packet losses are due to
network congestion. This assumption needs significant modification in wireless
Internet applications because most packet losses are due to wireless link errors
[4].
This article will survey typical TCP enhancements for wireless environments.
Data-Link Layer Performance: Wireless Internet could produce many symbol
interferences, multi-path fading, and bit errors in the data-link layer. Forward
error correction (FEC) and retransmission can be used to overcome the link errors.

Non-TCP Enhancements: This basically refers to the adoption of non-TCP
protocols in the transport layer of the Internet in order to optimize the specific

networking performance, such as in wireless LANs. However, it should provide a
seamless interface for interoperation with the regular TCP standard.  It should be
noted that these categories could be further classified. For example,
application-layer enhancements could be based on a mediator (sometimes called
proxy) or not. Some data-link-layer enhancements need to interact with the TCP
layer, while others do not.
The remainder of this article will summarize the main mechanisms proposed in
recent literature for improving wireless Internet performance. Each of the next
seven sections will be devoted to each of the abovementioned categories. For each
category, the main technical characteristics are outlined, the architectural
aspects are discussed, and the advantages and disadvantages are analyzed. We later
list the proper scenario for each enhancement technology. Finally, the conclusions
are draw.
Application-Layer Enhancement (ALE)
In this section we discuss some typical enhancing technologies for adapting
Internet applications to the wireless environment. We define each of these
techniques individually, and we identify application environments where these
techniques have been applied, and comment on the impact they have on performance.
Finally, we summarize our findings in a table.
Technique 1: Protocol Reduction
In the wireless Internet Application Layer, traditional HTTP has inefficiencies
such as large connection overhead, redundant transmission capabilities, and
verbose protocol setup. To save the limited wireless bandwidth, we can enhance the
traditional HTTP through the following two approaches:
Simplify the protocol headers. HTTP request headers contain lists of MIME
content-types that can be hundreds of bytes in length. The server and the client
could negotiate with each other on the reduction of the HTTP Request Headers.
Reduce the transmission rounds. If Internet users are requesting the same content
as the last request, the cached local information could be used.
Technique 2: Caching
Caching strategy is based on the Least Recently Used (LRU) algorithm and can
be adopted on both the client and the server side. The caching objects in wireless

environments should be consistent across each browser session and may have a
digital signature for security. Caching could efficiently save wireless bandwidth.

New caches have to be reconstructed on the client side or the server side when
the handoff occurs across the cell boundary. In [5], a cache relocation solution
was
proposed to enhance the roaming performance of the mobile hosts. Cache relocation
assumes the existence of user profiles stored in specialized nodes within the
user's
home network (HN). When the mobile host moves into a new sub-network different
from its HN, the registered profile database is queried and the relevant
information is
forwarded to the new sub-network. A path prediction algorithm is used to determine

which cell the mobile user is likely to move to.
Technique 3: Dynamic URL
Current Web infrastructure cannot accommodate mobile clients because it transmits
the dynamic information explicitly through form-based interfaces that require the
users' input in the client machine. In other words, it cannot support
automatically updated documents.
To address this problem, dynamic uniform resource locators (D-URLs) are proposed
in [6] to allow a single URL to return different documents corresponding to
different mobile locations. For example, the following D-URL could be used to
return the HTML document that describes the mobile user's current location (Fig.
2): http://ownserver/places/$(Location).html.
In Fig. 2, the actual document URL name is assigned to the D-URL variable [6].
Technique 4: Differencing
The principle of differencing is to take advantage of the fact that different
replies from the same program are usually very similar. For example, successive
replies from a stock-quote server contain significant redundancy, such as graphic
art. The differencing operation is typically implemented by inserting two
components into the data path between the client and the server: client side
intercept (CSI) and server side intercept (SSI). The SSI performs the differencing
operation, the result being a difference stream, which represents the difference
between the received report and the initially established CGI objects. Then, the
difference stream is transmitted between CSI and SSI. Differencing techniques are
implemented in some applications, such as IBM WebExpress [7].
Technique 5: Split Proxy
In the split proxy scheme, two peer mediators are introduced, i.e., the agent on
the client side and the proxy on the server side. The goal of using split proxy is

to
hide the influences of high-error-rate wireless links from the visible Internet
applications. Some enhancing techniques can be adopted on the tunnel between
those two peer mediators. For example, caching can be carried out in the proxy
tunnel. For each cached object, we can define a lease as a time period after which

the server will assume that the client has terminated the connection unless the
client renews the lease. The mediator keeps the server alive by periodically
renewing the leases [8]. In addition, differencing and data compression can also
be performed in this tunnel in a transparent way.
Technique 6: Dynamic Documents
The dynamic documents scheme adopts a type of program written in the portable Tcl
scripting language, which is then transmitted to the browser and interpreted
there. The client may apply different presentation policies on a per-document
basis. For example, if the network bandwidth is not sufficient, the system can
delay the fetching of large multimedia resources or just execute the Tcl script in
the background. The dynamic documents scheme needs help from the interface
processing policies (IPP) [9]. A typical example of IPP is the processing of
fill-out form contents in many wireless applications. This operation can be
restricted to the client only instead of relaying the request to the server.
Communication with the sever too frequently could largely consume wireless
bandwidth.
Technique 7: Pipelining
Pipelining means the fetching of several files simultaneously over a single
Internet connection. The benefit of adopting pipelining is to reduce the
transmission latency and to allow the file transfer to take advantage of the
available bandwidth.
HTTP 1.1 with pipelining has been standardized and is now in use. A non-HTTP
protocol based on pipelining technology was proposed in [10]. A useful extension
made in pipelining is the ability to resume file transfers when a connection is
broken.
Technique 8: GUI Enhancement
Due to the small size of handheld devices, developing interactive graphical user
interface (GUI) Web applications for wireless transmission requires thoughtful
planning. Here are some good hints for optimizing enterprise-wide HTML
applications for small mobile display devices [11]:
Choose only the most essential contents to display.
Organize information effectively.
Try to avoid graphics that need large amounts of transmission bandwidth.

In Table 1 we list the main techniques for enhancing wireless Internet application
performance and the corresponding examples.
Transport-Layer Enhancement (TLE)
Wireless TCP, which is defined as the TCP version in a network scenario with
wireless sections, can present many challenging issues, such as failing to respond
to burst errors and entailing wasteful effort of retransmissions [14]. Current
mechanisms to enhance Wireless TCP performance are based on the modification of
the regular TCP algorithm that was originally designed for wired links.
Recommendations From IETF on Enhancing TCP
performance Over Wireless Links

Recently the Performance Implications of Link Characteristics (PILC) group in IETF
suggested some valuable TCP modifications for enhancing the end-to-end performance
of wireless links (please refer to RFC 3155 [15] and Internet Draft [16]).
The main conclusion from PILC was that avoiding congestion in wireless scenarios
has to take precedence over quickly repairing transmission errors, since
congestion
affects all traffic while transmission loss affects only the specific traffic that

encounters errors. Thus, a good TCP scheme for wireless scenarios should decrease
the amount of time spent unnecessarily in the congestion avoidance phase.
TCP Enhancement for Satellite Networks
The characteristics of satellite channels, such as large latencies, path
asymmetries, and occasionally high error rates, provide Satellite TCP operation
with a challenging environment [17]. Satellite TCP performance enhancement is
required mainly due to two problems:
The regular TCP version needs a long time (generally more than three seconds) to
reach the full recommended window size in satellite links, which can waste
satellite bandwidth in the slow-start phase of TCP evolution [18] The steady-state
behavior of regular TCP cannot fully utilize the bandwidth provided by T1
satellite channels [19]
Satellite Transport Protocol (STP) — The key to Satellite Transport Protocol (STP)

[17] is saving the reverse-satellite-channel bandwidth to overcome the asymmetry
of satellite networks. First, it suggests the use of a "split-TCP" scheme, i.e.,
the TCP connection is split at the gateway between the satellite network and the
Internet.
In addition, a new transport protocol is adopted over the satellite portion. The
idea of split-TCP could shield high-latency satellite links from Internet
applications.
Second, STP adopts a different type of data acknowledgement compared to regular
TCP: the transmitter periodically requests the receiver to acknowledge only
successfully received packets, which is different from regular TCP, in which the
receiver typically sends an ACK for every received TCP segment. Thus STP could
save the reverse-channel bandwidth, which is beneficial for overcoming the
asymmetry of satellite networks.
TCP-Peach — The key concept of TCP-Peach [20] is Sudden Slow-Start, an
enhanced TCP algorithm for fully utilizing the wide satellite channel resource in
the
forward direction (i.e, from the Internet server to the client). TCP-Peach uses
the
dummy segments to probe the availability of network resources. The dummy segment
does not carry valid data information to the receiver. Besides the traditional TCP

algorithms, i.e., Congestion Avoidance and Fast Retransmit, TCP-Peach introduced
two new algorithms: Sudden Start (SS) and Rapid Recovery (RR). The basic idea of
SS is that the sender sets the congestion window size to 1 in the beginning of a
connection, but after the first data segment it will suddenly transmit (MaxWin-1)
segments in one round trip time (RTT) [20]. Thus after one RTT, the congestion
window size increases much more quickly than the regular TCP Slow-Start algorithm.

RR is used to replace the Fast Recovery algorithm in the regular TCP version. As
shown in Fig. 3, after the detection of a packet loss, the dummy segments can tell

the sender the type of loss, i.e., whether it is a congestion loss or a link-error
loss.
If it is a wireless link-error loss, TCP-Peach will quickly recover its congestion

window
size to the threshold value, but a regular TCP version, such as TCP-Reno, can only

increase one segment per RTT. Thus, TCP-Peach improves the satellite TCP
throughput.
BA-TCP — The key technique of Bandwidth Aware TCP (BA-TCP) [21] is to
guarantee fair bandwidth sharing among several competing connections with round
trip times that may differ by more than two orders of magnitude. In BA-TCP, the
IPv6 optional fields are used to convey delay and available bandwidth measurements

to the TCP receivers in order to compute a proper advertised window size. In
BA-TCP, a satellite connection can fairly share a bottleneck link with wired
connections.
Indirect TCP (I-TCP)
The key concept of Indirect TCP (or I-TCP) is TCP-Split.  I-TCP splits an
end-to-end TCP connection between a fixed host and a mobile host into two separate
connections:
A wired TCP connection between the fixed host and the mobility support router
(MSR) currently serving the mobile host.
A wireless TCP connection between the MSR and the mobile host [22].
A wireless protocol is built for communicating between the mobile host and the
MSR. If the MH switches cell areas during the lifetime of an I-TCP connection, a
new MSR will be assigned to become the bridge between the wired connection and the
wireless connection. The fixed host (FH) is completely unaware of the indirection
and will not change its communication mode even when the MH switches cell areas.
M-TCP
The key technique of M-TCP [23] is to split TCP while guaranteeing the TCP
end-to-end semantics. It was proposed to work well in the presence of frequent
disconnection events and in low bit rate wireless links subject to dynamically
changing bandwidth. When the M-TCP side at the mobile host (MH) is notified that
the connection has been lost, it freezes all M-TCP timers (Fig. 4). This
essentially ensures that disconnections do not cause the MH's M-TCP to invoke the
congestion control algorithm.
TCP Decoupling
The key concept of TCP decoupling is to decouple TCP congestion control from
TCP error control, and allow them to be performed separately and independently
[24]. As shown in Fig. 5, the guaranteed minimum bandwidth (GMB) sender sends
data packets to the TCP circuit. At the same time, a control TCP connection is set

up to probe for the available bandwidth beyond the allocated GMB. By this
decoupling scheme, data packets are transmitted as independent packets from header
packets, and the data packet stream does not suffer from TCP's congestion control
mechanism that is applied only to the header packets stream.  A big advantage of
TCP decoupling is that a corrupted or lost data packet will not trigger the TCP
congestion control algorithm, which could reduce the sending rate upon packet
corruption. Because the error probability for these tiny header packets is much
smaller than that of full-size packets carrying data payload, the probability of
mistakenly triggering the TCP congestion control algorithm is significantly
reduced. Thus it provides a reliable and high-throughput data transfer over the
wireless network links while using TCP congestion control to avoid network
congestion.
Fast Start
The key technique of Fast Start [25] is to optimize the slow-start behavior of
short TCP connections. A wealth of evidence suggests that most TCP connections
today are short-sized, usually under 10 kb [26]. This means that the common
assumption about TCP evolution, i.e., slow-start phase plus congestion avoidance
phase, may not be valid [25]. As shown in Fig. 6, a short TCP connection often
cannot enter congestion avoidance phase. It consists of unfinished slow-start1
sections and a series of timeouts.
The slow-start performance for short TCP connections needs to be improved
since regular slow-start behavior starts from window size 1 instead of a larger
value
(>1), which can waste the wireless resource. A typical proposal is stated in [27],

called Fast-Start, which uses a double-queue model to derive the optimized initial

TCP parameters such as threshold window size (ssthresh) and the starting window
size of slow-start phase (cwnd). They deduced that the choice of cwnd is based on
the file transfer size and the bandwidth-delay product of the wireless link. A
technology called shift enhancement is used in [27] to adapt to the variable
bandwidth-delay product of the wireless link. This idea is useful since actual
wireless characteristics could vary frequently due to multi-path fading and
symbol-interference.
Link Error Aware (LEA)
The idea of link error aware (LEA) is based on the observation that usually losses
in the wired section are due to networking congestion, while losses in the
wireless section are related to link errors. LEA can distinguish and handle
efficiently these two types of losses, i.e., congestion loss and error loss [28].
In LEA, the base-station is responsible for generating and transmitting a special
ICMP (Internet Control Message Protocol) message, called ICMP-DEFER, to the
sender, when the first attempt at transmitting the packet on the wireless side is
unsuccessful.
This
policy ensures that, within one round trip time, the TCP sender will receive
either an
acknowledgment for the packet or an ICMP message. A lack of both indicates a
networking congestion loss instead of a wireless link error loss. A LEA scheme can

ensure that end-to-end retransmissions do not start while link-layer
retransmissions are going on.
Wireless Wide-Area-Network TCP (WTCP)
The main goal of Wireless Wide-Area-Network TCP (WTCP) is to address the
problem of rate control and reliability over commercial wide-area wireless
networks
[29]. WTCP performs rate control at the receiver, and uses inter-packet delays as
the primary metric for rate control. WTCP uses the average per-packet separation
to
distinguish networking congestion losses from wireless error losses. This is
similar to
the approach in [30]. If it is a wireless error loss, the sender will not decrease

its
sending rate. Because the value of round-trip-time is large in the wireless wide
area
networks and many data transmissions are short-sized, WTCP attempts to compute
the appropriate transmission rate for a connection immediately upon startup rather

than going through the slow-start phase. Thus, WTCP can save wireless bandwidth.
TCP for Asymmetric Wireless Environments
In some wireless networks such as cable modem Internet, a faster downstream
(>10Mb/s) and a slower upstream (<56kb/s) typically characterize a bandwidth
asymmetry. Other asymmetric wireless cases include:
Latency asymmetry (such as in satellite networks).
Media access asymmetry (such as in IEEE 802.11-based wireless LAN).  Packet error
asymmetry (as in some cellular networks, the power from the base station to MH may
be much larger than the reverse link, resulting in the uplink and downlink
channels exhibiting different packet error rates) [31].
Good solutions to mitigate the effect of the asymmetry on TCP performance
include TCP/IP header compression, ACK congestion control, ACK filtering [32], ACK

prioritization [33], and other methods. Generally, the performance of TCP in the
high
bandwidth direction can be severely reduced due to the delay of ACK packets in the

reverse link.
In order to overcome bandwidth asymmetry, a good solution called ACE
(acknowledgment based on cwnd2 estimation) was proposed in [34]. The idea of ACE
is to let the number of packets per acknowledgment vary according to the sender's
cwnd (congestion window-size). When cwnd is small, the number of packets per
acknowledgment will be small. Thus, we can help speed up initial transfer and
build up the cwnd. When cwnd is larger, the number of packets per acknowledgment
is larger, and thus the number of acknowledgments sent on the narrow bandwidth
link is reduced without much impact to the sender.
Delayed Duplicate Acknowledgments (DDA)
The key concept of delayed duplicate acknowledgment (DDA) [35] is to inactivate
the TCP congestion avoidance algorithm by delaying transmission of duplicate
acknowledgments. DDA is an extension of the SNOOP scheme proposed in [36].
However, unlike SNOOP, DDA does not need to look at the TCP headers, which makes
DDA efficient for encryption. Like SNOOP, DDA also uses link-level retransmission.
But DDA attempts to reduce the interference between TCP-layer retransmissions and
link-level retransmissions by delaying the third and subsequent duplicate packets
for an interval of d.3 Specifically, when out-of-order packets are received, the
TCP receiver responds to the first two consecutive out-of-order packets by sending
duplicate packets immediately. By setting the proper value of d, DDA can imitate
SNOOP in case of wireless error loss.
Comparison of Different TLEs
Within the above discussion, we did not mention the performance of different TCP
versions such as TCP-SACK, TCP-Reno and TCP-Tahoe because these TCP versions are
not designed for enhancing wireless performance. Interested readers may refer to
[37], which provides a good analysis of the performance of these TCP versions in
wireless LAN.
In Table 2 we compare different wireless TCP proposals from the following nine
factors:
Adopting a proxy to split TCP into two parts: the wired part and the wireless
part.
Maintaining the end-to-end TCP semantics.
Distinguishing error loss from congestion loss.
Handling handoff disconnections.
Optimizing the RTT (round trip time) delay to avoid long-period time-outs.  Saving
wireless bandwidth (e.g., fully utilizing wireless bandwidth through probing).
Needing an extra type of packets for probing available bandwidth or identifying
wireless loss.
Modifying the existing TCP algorithm.
Enhancing the performance of short-TCP connections.
Based on Table 2, we can see that only Indirect-TCP does not maintain the
end-to-end semantics. Other methods, such as M-TCP, Mobile-TCP, and DDA, can all
overcome this drawback, although they also adopt the split-TCP approach.
Basically, they achieve this by hiding wireless error loss from the TCP sender and
not blocking the regular TCP ACK flows.
Only M-TCP and Mobile-TCP take into consideration the serious effect from frequent
handoff disconnections through the relocation of TCP connection information.
Only TCP-Peach and Fast Start consider the improvement of short-TCP performance.
Generally, short-TCP may not reach the congestion avoidance phase and stay in the
slow-start phase only. TCP-Peach attempts to probe the wireless bandwidth within
one RTT, and suddenly adjusts to the maximum window size (called rapid recovery).
In Table 2, modifying regular TCP could include the following approaches:
Changing initial slow-start window size (such as in Fast Start).
Adding new phases (such as rapid recovery in TCP-Peach).  Adopting rate-based flow
control instead of window-based flow control (such as in TCP-decoupling).
Adding new TCP operations (such as identifying ICMP message in LEA).
It is very useful to differentiate error loss from congestion loss since TCP does
not need to decrease its window size when responding to error loss. Table 3 lists
some common methods adopted by four wireless TCP schemes to distinguish those two
types of losses.
When choosing proper TCP enhancements for practical wireless Internet design,
drawbacks of each approach should be considered. We list the disadvantages of some
wireless TCP schemes in Table 4.
Data-link-Layer Enhancement (DLE)
Unlike TCP approaches, which focus on end-to-end networking behaviors and ignore
the details of immediate nodes, data-link layer enhancement attempts to modify
each immediate node to hide the wireless errors from the TCP layer. Thus, the
sender will trigger its congestion avoidance algorithm under the assumption that
each lost packet is due to networking congestion instead of the wireless link
error.
The PILC group provides suggestions on adopting performance enhancing proxies
(PEPs) to mitigate the link-related degradations (please refer to RFC 3135 [38]).
However, it does not advocate the use of PEPs in general cases since PEPs could
interfere with the end-to-end usage of the IP mechanism.
Technique 1: Utilizing the Coarse RTT Timeout
The key technique used in the Transport Unaware Link Improvement Protocol
(TULIP) [39] is utilizing the coarse RTT timeout. Some papers such as [40]
conclude
that introducing reliability at the data-link layer introduces unnecessary and
redundant retransmissions because of the competing retransmission strategies
between the transport layer and the data-link layer. However, TULIP disagrees with

this opinion. When wireless error loss occurs, TULIP carries out necessary
retransmission in the data-link layer within the long timeout period whose value
can be calculated by the TCP algorithm (usually multiple times of 500ms).
Technique 2: Using FEC to Reduce Wireless Error Loss
This technique in effect decreases the wireless error rate in the data-link layer
by using forward error correction (FEC). Because each erroneous packet will be
discarded by the receiver and considered a loss that will trigger the TCP
congestion avoidance algorithm, this technique could efficiently overcome packet
errors and avoid the activation of the TCP congestion avoidance algorithm.
A typical example that uses this technique is AIRMAIL (Asymmetric Reliable
Mobile Access In Link-layer) [41], which is an asymmetric link-layer protocol for
reducing the processing load at the mobile host that has limited power compared to

the base station, and establishes the reliability by a combination of automatic
repeat request (ARQ) and FEC. The key ideas of AIRMAIL include:
Placing most intelligence in the base station side instead of placing it evenly on

both base station and mobile hosts. This can result in as much as a one-third
reduction of compiled code.
Requiring the mobile terminal to combine several ACKs into a single ACK to
conserve power.
Modifying regular FEC to incorporate three levels of channel coding and combing
the FEC and link-layer retransmission to obtain better performance in terms of the
end-to-end throughput and latency.
Technique 3: Invoking Multiple Flows for Each TCP
Connection

This technique of invoking multiple flows for each TCP connection is adopted in
the multi-service link layer (MSLL) scheme that can provide multiple simultaneous
services at the link layer [42]. In the MSLL scheme, an internal packet classifier

distributes incoming packets to appropriate services. The allocation measurement
tracts the fraction of data allocated to each service by the classifier. A
self-clocked fair queueing (SCFQ) scheduler strictly enforces the desired
bandwidth allocation.
The goal of the link-layer scheduler is not the provision of end-to-end QoS but
the preservation of higher layer scheduling decisions. The scheduler decides which
packet to send next based on the measured bandwidth allocations, labels it with a
service number, and hands it to the MAC layer for transmission.
Technique 4: Link Shaping
The key idea of link shaping [43] is to use a suitable link-layer error recovery
mechanism to hide the fluctuations of the wireless medium from the TCP layer. TCP
throughput can deteriorate significantly if the loss rate is above a threshold
that
scales as the inverse square of the bandwidth-delay product [43]. Link shaping can

control the wireless link loss below a threshold, and thus prevent the degradation
of TCP throughput. Link shaping suggests that each TCP packet that enters the
buffer in the wired/wireless interface is ultimately delivered to the destination
of the wireless link without invoking TCP error-recovery mechanism, and the only
losses seen by TCP are those due to buffer overflow at the wireless link. Given
the packet-loss model in [43], link shaping chooses a proper size of the buffer
(B) at the wired-cum-wireless interface so that the buffer overflow probability is
less than a warning threshold.
Summary of Data-Link-layer Enhancements
Based on the above discussions of data-link-layer enhancements, the following
factors should be considered for the practical choice of a proper technique:
For current wireless Internet that uses coarse timeouts (generally 500ms), through
proper retransmissions of lost packets in the wireless hop (between base station
and mobile host), data-link-layer enhancements can avoid the frequent trigger of
TCP congestion control due to error loss.  The choice of the buffer size value in
the wired-cum-wireless interface is crucial since the buffer needs to deal with
the wireless channel's time variations, i.e., the variations of the
bandwidth-delay product.  The combination of FEC and auto retransmission quest
(ARQ) could be adopted in the wireless hop. FEC facilitates the correction of
packets with errors before they are delivered to the TCP layer. ARQ can utilize
the remaining wireless bandwidth to carry out retransmission. However, the
retransmissions should not interfere with the delay-sensitive traffic types such
as CBR (constant bit rate).4 Data-link-layer enhancements can carry out error
handling and dispatch differentiated-served frames through priority-based queues.
Mobile Performance Enhancement (MPE)
Before discussing enhancement techniques for mobility performance, we need to
point out that there are major differences between wireless access and mobility
[44]. Wireless access does not mean that mobility should be supported. For
example, a fixed-location user might use a wireless interface via a LAN to visit
the Internet while sitting in his office. On the other hand, mobile users do not
necessarily use wireless interfaces. For instance, a mobile user can simply
connect to a fixed network using a wired interface as he moves to a new place. In
wireless Internet, mobility performance enhancement is often an important factor.
In many cases people attempt to modify existing Internet protocols to improve
mobility performance. Most modifications are based on the concept of Mobile IP
[45].
However, mobile performance enhancement (MPE) need not be implemented in the IP
layer only. That is, there is a need to implement it in multiple protocol layers
to complement each other. This section will introduce several typical MPEs that
exhibit good mobility performance in wireless Internet.
Dynamic Host Configuration (DHC) from Microsoft
Research

The goal of dynamic host configuration (DHC), which has recently been
implemented in the Microsoft wireless Internet called the CHOICE network [46], is
to
make the mobile host auto-configurable when it moves between cell areas instead of

relying on the base station's location management. The crucial protocol that
enables the features of the CHOICE network is the Protocol for Authorization and
Negotiation of Services (PANS) [47], which supports the global authentication of
mobile users and device auto-configuration. DHC addresses the mobility problems by
configuring the mobile host to migrate between public and private networks, while
regular Mobile IP is primarily concerned with locating the mobile host and
re-routing packets to the mobile host's current destination.
Enhanced Mobile IP (EMIP)
Enhanced Mobile IP (EMIP) [48] is the extension of Mobile IP. EMIP performs
routing-enhancement in an attempt to implement seamless handoff.5 Compared to
Mobile Ipv6, EMIP can be performed faster, and can reduce the number of packets
that are lost during the handoff. EMIP is more efficient than Mobile Ipv6 since it
does not require any additional network bandwidth during the handoff.
Freeze-TCP (F-TCP)
The key concept of Freeze-TCP (F-TCP) is to change the TCP algorithm in the mobile
host so that the base station can be prevented from sending packets during
handoff. We outline the F-TCP scheme here because the main goal of F-TCP is to
overcome the handoff disconnection instead of wireless link errors.
The idea of F-TCP is to move the onus of signaling an impending disconnection
to the mobile host side instead of the base station side [49], because a mobile
host
can easily monitor signal strengths and detect an impending handoff, and even
predict a temporary disconnection. If a handoff occurs, the mobile host advertises

a
zero window size to force the sender into frozen mode and to prevent it from
dropping its congestion window size. This "cheating" behavior is implemented based

on one of TCP's characteristics, i.e., the sender will not change its window size
if one of the zero window probes is lost.
Cellular IP (CIP)
The key goal of Cellular IP is to implement global mobility. Cellular IP is a new
approach to support local mobility, and it can also inter-work with Mobile IP to
provide wide-area mobility support [50]. Two crucial components of CIP are:
Paging caches, which maintain a distributed cache for location management and can
quickly pinpoint idle mobile hosts that wish to engage in active communications.
Paging caches can accommodate a large number of mobile hosts without overloading
the location-management system.  Routing caches, which maintain the positions of
active mobile hosts in the service area and dynamically refresh the routing state
in response to the handoff of active mobile hosts.
Mobile Application Support Environment (MASE)
Mobile Application Support Environment (MASE) [51] is implemented in the mobile
host as well as in the mobility gateway that is a node between the mobile host and

the fixed host. MASE attempts to make the mobile operations transparent by masking

the link outages and disconnections due to handoff from the applications. The key
component of MASE is the communication manager. The session manager embedded in
the communication manager could perform typical session-management functions such
as initiating, maintaining, terminating, and recovery of TCP connections.
Fast-Routing (FR)
The key concept of Fast-Routing (FR) [52] is routing domain, which is defined as
follows:
All route changes within the routing domain are accomplished with the help of
route table changes.
All route changes between the routing domains are accomplished by Mobile IP.
A routing-domain can be any management-specified domain. In Fig. 7, subnet A and
subnet B can form a routing-domain, while subnet C forms another routing domain. A
mobile host has a home agent (HA) that is in the same subnet as the sender. The
foreign agent (FA) acts as a gateway to a particular routing domain.  Two routers
in Fig. 7, i.e., R1 and R2, are FAs. The left section of Fig. 7 shows that the
mobile host is in the vicinity of subnet A. Routing from HA to the FA (i.e., R1)
is carried out through Mobile IP. All mobility within subnets A and B is taken
care of by local route changes. When the MH moves into the region of subnet C
(another routing-domain), the HA will choose R2 as the FA.
We can see that the routing domain is a useful idea since local mobility can be
achieved by changing the local routing table when a mobile host does not move out
of the same routing domain. This local routing change can be fast since no global
routing change is involved.
Fast-Retransmit
The key concept of Fast Retransmit [53] is to make the base station avoid going
through the state of time-out during handoff disconnection since time-out can
greatly sacrifice TCP throughput. During the long period of a handoff process, a
whole window worth of data may be lost. As shown in the left side of Fig. 8, ACKs
cannot be received by the sender after the handoff is complete. Thus, the sender
will time-out and retransmit the lost packets. During the retransmission of lost
packets, another time-out could occur if handoff is still not complete.  Fast
Retransmit suggests that the mobile host can immediately send back three duplicate
ACKs to the sender when it finishes a handoff (right side of Fig. 8).  This will
trigger the sender to enter fast retransmit instead of waiting for the occurrence
of a time-out. Thus, TCP throughput can be improved during handoff disconnection
since the time-out event is avoided.
Multimedia-QoS Enhancement
This century will see more and more multimedia applications in wireless
environments [54]. The most important issue for wireless multimedia communications

is to maintain mobile-QoS requirements, such as handoff delay, disconnection loss
rate, and desired bandwidth [55]. The main policies of multimedia-QoS enhancement
include intelligent resource ReSerVation Protocol (RSVP), adaptive resource
management, and Class-Based Queuing (CBQ).
Recently, the Next Steps In Signaling (NSIS) working group discussed the
requirements for QoS signaling protocols in an Internet Draft [56]. One important
point from NSIS is that, in wireless environments, it is essential to define
interoperability between multimedia traffic, mobile behavior, and QoS management
[56].
Distributed Fair Scheduling (DFS) Scheme from
Microsoft Research

The Microsoft Communications Projects Research Group proposed the concept of
Distributed Fair Scheduling (DFS) to allocate wireless resources in proportion to
the priorities of the packet flows sharing the wireless channels [57]. DFS happens
in the media access control (MAC) layer of wireless networks. A key technique
adopted by DFS is the Exponential Mapping Scheme, which calculates the value of
the backoff interval6 based on an exponential function instead of a simple linear
function that is currently adopted by the IEEE 802.11 standard [57]. DFS overcomes
the shortcomings of the IEEE 802.11 standard that could not perform fair resource
allocation among different multimedia flows on short time scales.
Modified RSVP
The key idea of Modified RSVP is to modify the regular RSVP to adapt to the
wireless multi-user sharing environment. Regular RSVP is a signaling mechanism for

carrying the QoS parameters from the sender to the receiver, and also for making
reservations along the communication path.
Because the wireless link is shared among multiple mobile hosts in uplink and
downlink directions, it is necessary to adopt a link-sharing mechanism such as
Class-Based Queuing (CBQ) that can meet the QoS requirements of the mobile hosts.
Modified RSVP is thus based on the combination of regular RSVP and the CBQ scheme
[52].
MobiWeb
The key idea of MobiWeb is to adopt a proxy-based architecture to enhance
multimedia performance [58]. It includes a priority scheme that preserves media
smoothness despite short-term fluctuations of the wireless link, as well as an
adaptation mechanism applied to streams to respond to long-term link changes and
handoffs.
The adaptation mechanism works as follows: MobiWeb adopts a set of timers to
increase the utilization of the resources when the resources are abundant, but
forces enhanced streams to back off to more moderate resource usage when resources
are scare.
The priority scheme consists of two aspects:
An admission control scheme is used to reserve resources for each stream in order
to guarantee at least the minimum acceptable level of quality in the stream's
performance.
A dynamic prioritization scheme is used to protect the higher-priority streams
from losing their shares of resources to streams with lower priority.
IP-QoS
The key idea of IP-QoS is to integrate the existing QoS approaches, such as RSVP
over IP layer, with the protocols supporting mobility [59]. It optimizes the
existing differentiated services architecture standard based on three levels of
protocol implementations:
Session Layer Negotiation. The QoS-aware applications are able to specify their
own traffic and QoS requirements, and a QoS API is then used to map the
application service profile to an understandable form for the underlying resource
manager (RM).
End-to-End QoS Resource Reservation. RM performs the resource allocation and
admission control for the core network.
Local Inquiry. It is used for local resource inquiry and can communicate with the
global network RM.
Selective Packet Prioritization (SPP)
The main goal of Selective Packet Prioritization (SPP) [60] is to enhance the
performance of Wireless Voice IP (VoIP) service, which is a typical wireless
multimedia application and is gaining great attention today. The SPP architecture
assumes a hot-spot network scenario, such as an IEEE 802.11 wireless LAN where the
interface above the MAC layer is indistinguishable from the IP interface on a
fixed network.
SPP attempts to develop a QoS-support scheme that selectively marks the packets to
a higher network priority at the sender based on the properties of the speech
signal. Differentiated services packet priorities are mapped to the QoS control
mechanism of the lower protocol layers. Thus, prioritized packets are protected
against other flows using the shared medium and also against channel errors.
Alternative Non-TCP Enhancements
For specific wireless scenarios, such as last-hop-wireless or some high-speed
campus wireless LAN, more efficient wireless protocol suites may be used to
optimize the local wireless performance.
A typical example is Wireless Application Protocol (WAP), which defines an
industry-wide specification for wireless networks [61]. A key aspect of WAP is
that all connections between MH and FH will pass through a proxy, i.e., WAP
gateway.  Thus, WAP is a split-connection proposal.
If the last hop is a wireless link, another efficient non-TCP protocol, called
Mobile-End Transport Protocol (METP), replaces TCP/IP in wireless Internet by a
simpler protocol with smaller headers [62].
SWAN (Seamless Wireless ATM Network) [63] is one of the earliest systems to
realize the concept of the mobile ATM network. It uses room-sized pico cells and
mobile multimedia end-points.
Power-Consumption Enhancement (PCE)
One of the hurdles to the extension of wireless Internet is the high power
consumption. This obstacle has not been overcome efficiently today. The value of
power consumption depends on parameters such as the amount of avoidable extra
traffic and the total duration of the TCP connection. The key to saving power in
wireless Internet lies within the higher levels of the wireless protocol stack
instead of only the physical hardware.
Table 5 summarizes the typical schemes in the protocol layers of wireless Internet
for saving power.
Discussions
Methodology for the Choice of Proper Enhancements
The choice of enhancements for wireless Internet should be based on different
networking architectures and performance requirements. For example, since wireless

LAN usually forms an independent Intranet, ATM-based or other non-TCP/IP
architectures could be adopted. When one is choosing a proposal, trade-offs should

be made between performance efficiency and implementation cost. For instance,
because TCP is an end-to-end protocol, the requirements of modifying all immediate

routers may not be practical in global mobile networks. Table 6 lists the
appropriate environments of some enhancements.
Other W-Internet Layer Enhancements
Apart from the abovementioned enhancements, there could be other enhancements that
can be applied to other wireless Internet protocol layers, including the session
layer, the MAC layer, and the physical layer.  A hot research area for the session
layer of the cellular Internet (one type of wireless Internet) is the Connection
Admission Control (CAC), which focuses on allocating wireless bandwidth to handoff
calls and new calls efficiently [65]. We have proposed a novel CAC algorithm for
multi-class wireless applications in [66]. For MAC layer performance enhancement,
[67] lists some typical MAC proposals. One of the main goals on the MAC layer is
saving power. On physical-layer enhancements, coding and retransmission are the
main strategies. An efficient error-control architecture for wireless ATM was
recommended in [68] (Fig. 9). In addition, a high-rate extension of the IEEE
802.11 physical-layer protocol is proposed in [69]
for
enabling 22 Mb/s wireless transmission.

Conclusions
This article reviewed the modern approaches for improving wireless Internet
performance from the following seven aspects:
Application-layer
TCP-layer
Data-Link-layer
Multimedia-QoS
Mobility performance
Power consumption
Other non-TCP/IP schemes

We discussed the essential features for each enhancement technology, and compared
the advantages and disadvantages of these approaches. It was pointed out that the
selection of different enhancement schemes should be based on specific
characteristics of the wireless networks and implementation costs. In order to
obtain the optimized networking performance, the enhancements had better be
carried out in multiple protocol layers, including the physical layer, the MAC
layer, the CAC layer, the data-link layer, the TCP layer, the Mobile IP panel, and
the application layer.
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7,
no. 5, Oct. 1999, pp. 605–15.

Biographies
Fei Hu is currently working as an assistant professor in the Department of
Computer
Engineering, Rochester Institute of Technology in Rochester, New York. He obtained

his Ph.D. degree in 2002 in electrical and computer engineering at Clarkson
University, Potsdam, New York. He received a Master's degree in telecommunication
engineering from Shanghai Tiedao University of China in 1996. His research
interests
include high-speed computer networks, wireless and mobile computing, Internet, and

ATM.
Neeraj K. Sharma is currently a system engineer at Intel Corporation. From 1999 to

2000 he was an associate professor with the Department of Electrical and Computer
Engineering, Clarkson University, Potsdam, NY. From 1993 to 1998 he was on the
faculty of the Department of Computer Science and Computer Engineering, La Trobe
University, Melbourne, Australia. He received his Ph.D. and MSEE degrees from the
University of Akron, Akron, Ohio, and a BSEE degree from the University of South
Alabama, Mobile, Alabama, all in electrical engineering. His research interests
include
fault-tolerant system design, and performance and reliability analysis of computer

systems and networks.

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