CAN MOBILE IP BE USED AS A LINK BETWEEN IMT 2000 TECHNOLOGIES AND
OPERATORS?
M. Kraner, C.N. Yap, S. Cvetkovic, Erika Sanchez
Department of Electronic and Electrical Engineering, University of Sheffield, UK
ABSTRACT
IMT 2000 technologies provide IP based services to
cellular telephones. Network operators are consequently
offering data services comparable to Internet Service
Providers. A rising issue is to provide interoperability
between the technologies and between the different
providers. A possible candidate suggested by 3GPP and
3 GPP2 is Mobile IP.
The major decision point whether or not Mobile IP
is suitable is the performance of handoffs between
different networks. The paper therefore studies the
performance delay for TCP, UDP and suggests an
enhanced Mobile Internet Protocol since MIP proved to
be unsuitable. IIP is using strong cell switching and
avoids tunnelling. It therefore performs superior to
Mobile IP. While Mobile IP performs handoffs with an
end-to-end delay for TCP within seconds and UDP
traffic in average in about 500ms IIP allows handoff to
be performed below 100 ms.
conference with data and video support of different
streaming sources etc.
This can gradually lead to a situation in which
network operators become wireless Internet Service
Providers. On the other side it is imaginable that
Internet Service Providers will offer Cellular contracts
with different network operators. It might even be
possible that “micro cellular” network operators will
offer services around their premises to everybody
passing their cell that is linked to the Internet.
Consequently solutions are required which allow handoffs between different wireless ISPs with the capability
to be scaled down to the size of one wireless cell with a
range of less than 100m (see Figure 1)
Internetworking
Radio Network
Finally a solution is suggested to avoid handoffs.
The solution would allow Mobile IP to be deployed
even for interactive real time applications such as IP
based telephony. However the solution is in work in
progress state.
INTRODUCTION
The first IMT 2000 standards are emerging and the
first UMTS licenses are auctioned in the UK. In about
two years the mobile user will face a variety of
providers offering services based on analogue wireless
systems, GSM (900 and 1800) and UMTS. The
providers are facing increasing competition and
consequently will steadily offer more services such as
data services beyond a diversity of plain telephony and
messaging services.
Schulzrinne et al. [1] shows a scenario on how
future telephony integrates with data services using the
Internet. Telephony on packet-based networks may
offer services such as dialling to different telephones of
one user in a multicast way. The user just picks the
communication device, which is most convenient and
commences as a member of a multipoint telephone
End Host
Home Agent
Mobile Station
PDSN 1
PDSN 2
Radio Network
Mobile Station
Figure 1: Hand-off between 2 network operators
Ideally the hand-off event should not cause service
interruption and should support different technologies
and protocols. In the context of the Internet Protocol
(IP) hand-off means transgression between two different
sub-networks with the consequence that hosts are not
anymore visible in the new sub-network unless the host
will be reconfigured or the new sub-network or the host
supports a IP enhancement providing mobility support,
such as Mobile IP. But also hand-off between subnetworks results in different transmission paths with
different transmission characteristics. The different
delay or caused data-loss might be interpreted in upper
layer protocols in different ways and might lead to
transmission throughput degradation. Protocols like
TCP would assume congestion and throttle down the
traffic.
The capability to perform hand-offs without service
interruption is the decision point, which can prevent the
usage of Mobile IP as link technology between IMT
2000 technologies and network operators.
The paper reviews hand-off results of Mobile IP and
IIP and the influence on TCP and UDP protocols.
Furthermore it reports on work in progress, which aims
at seamless and interruption free hand-offs.
Foreign Agent
Foreign Agent
Subnet C
Subnet B
Mobile Node
Mobile Node
Correspondent Node
MOBILE IP
In 1996 Mobile IP was proposed [2] as an
enhancement of IP to provide support for mobility in
contrary to portability. Portability requirements are
provided with DHCP [3], which allows setting of new
parameters within a network segment during the booting
process of a host. Mobility tries to avoid the
reconfiguration of a host while moving between
different networks. Mobile IP requires new network
elements and specifies a protocol for the interaction of
the new components and the host. Very briefly the new
components and notions are Mobile Node,
Correspondent Node, Home Agent, Foreign Agent. As
depicted in Figure 1 the Mobile Station (Mobile Node)
moves between two operators with different IP
networks. Any end host (Correspondent Node)
communicating with the mobile node still addresses the
packets with the mobile node’s address. Since the
mobile node has changed the network the data packets
can’t reach the destination and thus the home agent
pretends to be the Mobile Node, intercepts the traffic
and tunnels it to the foreign agent, which “decapsulates” the packet and relays it to the Mobile Node.
Periodic advertisements between the Mobile Node and
Home and Foreign Agents and the usage of the Address
Resolution Protocol (ARP) facilitate the hand-off
process. There are different methods, which allow
detecting the movement of the Mobile Node from one
network to another one. Eager cell switching assumes a
movement as soon as the mobile node receives
advertisements of a different network. Lazy cell
switching waits until three advertisements have timed
out and changes to the new network’s advertisement
thereafter. Prefix cell switching differs in that it uses the
network prefix of the IP address to determine the
location of the Mobile Node. Another different
technology is called strong cell switching and is based
on beacons provided from Data Link Layer protocols
indicating the movement to the Transport Layer
Protocol Mobile IP.
TCP PERFORMANCE OF MOBILE IP DUE TO HANDOFFS
The measurements are based on an experimental setup using standard PC in which the mobile node is
equipped with two network interface cards. The handoff
is performed with switching the data transmission from
one network card to the other, which is connected to a
different sub-network (see Figure 2) [4].
Mobile Node
Home Agent
Subnet A
Figure 2: Test-bed for TCP performance of a MIP
handoff
Measurements of TCP performance on an early
implementation of Mobile IP demonstrate the
degradation of performance of TCP during a hand-off.
Figure n presents the results for the case of eager cell
switching with a service interruption of about 7.7s of
which 5 s are due to TCP congestion control
mechanism. UDP would therefore see an interruption
interval of about 2.7s [4]. Eager cell switching
represents the motion detection technology, which
allows the quickest hand-off and therefore represents
the best case aside from using strong cell switching
methods, which MIP currently does not support. The
analysis presented in [5] and depicted in Figure n show
that theoretically the handoff can be finished in 500ms,
which is a forth of the measured value stated before.
lost advertisments
x
A
y
z
0
B
Time
x
Figure 3: Event diagram for eager cell switching
The delay is the mean elapsed time in which the
advertisement of the new sub network B can be
expected. Therefore x/2 represents the expected mean
delay for movement detection with x denoting the
advertisement period of 1s and y representing the offset
value at which the movement takes place.
However the delay of 500 ms as best case is not
acceptable for real-time applications such as telephony,
which expect end-to-end delays below 300ms.
Additionally eager cell switching can lead to
oscillations when the mobile node frequently moves
between the same cells with the effect of small TCP
transmission windows and poor performance.
Consequently MIP is not suitable to be used for 3 rd
Generation cellular systems unless it is optimised for
these systems.
Reg (CoLA)
Ack
Periodicly this is redone
MOBILE IP USING STRONG CELL SWITCHING
Query
Add (CN)
MOVEMENT DETECTION
Ack (CoLA)
Data without tunneling
mobile node moved
Mobile Internet Protocol
42
Packet Sequence Number
36
30
24
18
12
6
0
0.8
1
1.2
1.4
tH
1.6
1.8
2
2.2
2.4
2.6
Time (seconds)
tR tP
tF
Figure 4: IIP handoff using eager cell switching
The movement detection method eager cell
switching required tH-tR=720ms, which is about 220ms
slower as the theoretical mean value. The time required
for registration request tR and registration reply tP until
the handoff was finished tF took 150ms. Consequently
Mobile IP could improve the handoff performance
drastically to 150ms if strong cell switching movement
detection were used.
IIP – ITINERANT INTERNET PROTOCOL
IIP is an enhanced version of Mobile IP optimised
for Wireless Networks using Home Location Registries
(HLR) and Strong Cell Switching movement detection
method. The design is based on the suggestion of Jain
[6]. The protocol uses as entities Mobile Nodes, Home
Location Registries and Correspondent Nodes. It
assumes that all Nodes are mobility enabled, i.e. all
nodes have an IIP compliant mobility protocol stack.
The functionality described in brief: A Correspondent
Node first queries the Home Location Registry and
obtains the IP address of the Mobile Node at its current
location. Further traffic is directly addressed to the
Mobile Node. The Mobile Node informs the
communication partner and the Home Location register
whenever it moves between networks. Movement is
detected via a beacon provided by the data link layer of
the used transport technology. Figure 5 depicts the
protocol timeline [8].
Data loss
Binding update
Reg ( New CoLA)
Ack
Periodicly this is redone
Correspondent
Node
Base hOme
ReGister
Mobile Node in
Subnet A
Mobile Node in
Subnet B
Figure 5: IIP timeline
Major differences to Mobile IP are the lack of data
encapsulation for tunnelling and the avoidance of
triangular routing. Consequently the ratio between
header and transported data is better than with MIP and
data transmission through firewalls is not prevented as it
would be with the source routing characteristics of
Mobile IP. The imposed delay for ingress routing using
the foreign agent to pass firewalls can be saved. The
performance of IIP in respect of handoffs is mainly
dependent on the DHCP service of the network
segments into which the Mobile Node migrates and
from which it obtains its new IP address.
The handoff delay was measured on an experiment
consisting of 3 Intel Pentium based PCs with Windows
2000 operation system installed. The measurement was
taken on a separate PC with a public domain version of
TCP Dump. The computers were equipped with 64
Mbytes of RAM. The set-up deliberately was based on
almost minimum requirements in order to get
conservative results, i.e. computers with higher
specifications and carefully selected software
components could improve the speed of the handoff.
The handoff event was initiated alike the experiment for
MIP as described above. The mobile node switched
from one network interface to a second one configured
with other network parameters [8]. The test data consists
of a UDP stream with a rate of 25ms and a packet size
of 200 octets.
Figure 6 depicts a typical measurement for UDP
based traffic using IIP.
Itinerant Internet Protocol
48
42
Packet Sequence Number
One optimisation is to study the influence of the
movement detection method and to identify the possible
gain if strong cell switching were used. An experiment
therefore has been set up to explore the influence of the
different type of movement detection method. In
addition the experiment should provide UDP
measurements on Mobile IP since UDP is important for
IP telephony. Figure 4 shows the measured result.
36
30
24
18
12
6
0
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
Time (Seconds)
Figure 6: IIP handoff using UDP
The handoff is completed within 69ms and is
therefore suitable for IP telephony. Analysis of this time
period revealed that the DHCP process takes about 9ms,
the binding update about 20ms and the update of the
routing table about 40ms. Therefore the remaining
budget for end-to-end delay suitable for IP telephony
would be about 231ms.
SEAMLESS HANDOFFS
Although the optimisations of Mobile IP allows the
usage for real-time applications the remaining time with
service interruption can still cause problems with
protocols such as TCP. The service interruption of
several seconds can be of an order in which the mobile
host is able to pass several cells. In this case even
session interruptions are imaginable. Consequently
either TCP needs to be adjusted to the requirements of
wireless systems or handoffs need to be avoided.
One solution to enable seamless handoffs suggests a
structural improvement, which allows movement
predictions based on multicast of data traffic to all
possible cells the Mobile Node might move to [9].
FAST HANDOFF METHOD FOR REAL-TIME TRAFFIC
OVER SCALABLE MOBILE IP NETWORKS
The proposed Fast Handoff Method suggests a
hierarchical order of Proxy Foreign Agents (PFA) and
multicasting of traffic when mobile nodes move
between cells. Mobile nodes hold multiple bindings to
Foreign Agents (FA) and receive traffic from both Base
Stations (BS) for a short time[9]. Figure 7 presents a
both Base Stations, multicast the data traffic to both
Foreign Agents 11 and 12. Due to eager cell switching
movement detection and lazy cell switching based
connection abundance the mobile node would switch to
the new cell once it receives advertisements but still
have bindings to both networks in place. In the current
base station the mobile node would receive agent
advertisements of FA12 and FA3 and would establish a
binding with both. Moving to the Base Station of
Foreign Agent 4 would represent a handoff between
networks, which requires the Top Proxy Foreign Agent
(TPFA) common to both Foreign Agents 3 and 4 to
multicast the traffic. The movement between Base
Stations attached directly to TPFA or a Foreign Agent
and both representing a network directly linked to the
Internet until now would still represent a service
interruption unless the Mobile Node would not be able
to store the whole hierarchical order of Proxy Agents in
lists and it would not be able to accept multiple
bindings. However another mechanism would still be
required. The Home Agent would need to be informed
and needed to store the information about the Mobile
Node being in the interception area of two different
networks. Consequently the Home Agent needs to
tunnel the data traffic to both entities, i.e. TPFAs or
FA6.
Further study is required to explore the performance
of notification while the Mobile Node is in the
interception area of Base Stations of several TPFAs.
Another area subject to further investigation is the
roaming stability and oscillation likelihood if multiple
Foreign Agents share one Base Station. Oscillations are
possible since the Mobile Node accepts bindings from
any Foreign Agent representing a different network. The
Mobile Node is required to eager switch between
networks and to lazy release the binding to the network,
which it is leaving. In case of a shared Base Station it
consequently would assume continuous movement
between two networks. Also open for research is the
solution for Mobile IP systems without Foreign Agent
and for IIP.
CONCLUSIONS
possible structure.
Figure 7: Fast Handoff Hierarchy
The movement of a mobile node between wireless
cells of base stations are handled by link layer
technologies, which is transparent to the IP layer.
Handoffs between cells only occur when sub-networks
are passed. If the Mobile Node passed the Base Station
of Foreign Agent 11 (FA11) to Foreign Agent 12
(FA12) and both represented a different network a
handoff would occur. In this case the Proxy Foreign
Agent 1 (PFA1) would for a short period of time, i.e.
while the mobile node is within the interception area of
Handoffs between different networks have been
identified as a knock out factor in order to investigate
the feasibility of the usage of Mobile IP for IMT 2000
based technologies. Plain Mobile IP has been studied
and the measured handoff delays showed that it is not
suitable for real time applications such as IP based
telephony services. The results obtained for eager cell
switching movement detection method was about
720ms. Even the theoretical mean value of 500 ms
would not be acceptable according to the widely
accepted threshold of 300ms. In case of TCP based
applications the short interruption could build up to a
service interruption of several seconds due to the inbuilt
congestion and flow control mechanisms. Additionally
the transmission throughput would decrease remarkably.
Consequently Mobile IP would not even be satisfactory
to users for elastic applications in respect of Quality of
Service requirements.
The main part of the delay is caused by the
movement detection, which could be improved using
strong cell switching based on Link Layer beacons.
For this purpose Mobile IP was enhanced to IIP
protocol, which proved to be suitable for real time
interactive data services since the delay below 100 ms
were measured. However it still suffers from service
interruptions, which TCP would interpret as congestion.
In order to prevent handoffs a Fast Handoff Method
was introduced which has been published as an Internet
draft and therefore represents work in progress. The
Fast Handoff Method assumes hierarchically structured
Foreign Agents and traffic multicasting while the
Mobile Node is in an interception are of base stations of
different networks. Several issues are open for future
research. Among these are roaming stability for Mobile
Nodes in shared Base Stations and performance of the
notification of the Home Agent if the handoff would
take place between two Top Level Proxy Foreign
Agents (TPFA).
REFERENCES:
[1] H. Schulzrinne, J. Rosenberg, "The IETF Internet
Telephony Architecture and Protocols", May/June 1999, IEEE
Network, pp 18 - 23
[2] C. Perkins, “IP Mobility Support (MIP)”, Request For
Comment 2002, Oct 1996, http://www.ietf.org
[3] R. Droms, “Dynamic Host Configuration Protocol”,
Request for Comments 2131, March 1997, http://www.ietf.org
[4] N. A. Fikouras, K. El Malki, S.R. Cvetkovic, C.
Smythe, “Performance of TCP and UDP during Mobile IP
Handoffs in Single-Agent Subnetworks”, Proceedings IEEE
WCNC ’99 (Wireless Communication Networks Conference,
New Orleans), September 1999.
[5] N. A. Fikouras, K. El Malki, S.R. Cvetkovic, M.
Kraner, “Performance Analysis of Mobile IP handoffs”,
Proceedings IEEE APMC ’99 (Asia Pacific Microwave
Conference, Singapore), December 1999.
[6] Jain, Ravi, Enhancing Survivability of Mobile Internet
Access Using Mobile IP with Location Register, IEEE
Transaction on Networking, Mar 1999
[7] S.R. Cvetkovic, C. N. Yap, N. A. Fikouras and M.
Kraner, “Novel and Enhanced Mobile Internet Protocol for
Third Generation Cellular Environments Compared to MIP
and MIP-LR”, 28.3.2000, IEE International Conference in 3G
Mobile Communication Technologies 2000.
[8] Chern Nam Yap, “Itinerant Internet Protocol”, Master
in Research Dissertation 1999, University of Sheffield,
Department of Computer Science, Regent Court, 211
Portobello Street, Sheffield S1 4DP, UK
[9] K. El Malki, N. A. Fikouras, S. Cvetkovic, “Fast
Handoff Methods for Real-Time Traffic over Scalable Mobile
IP Networks”, June 1999, Internet Draft – work in progress,
http://www.ietf.org
AUTHORS
Prof. Srba Cvetkovic was born
in Yugoslavia. He is currently
the professor and head of
Mobile
Communications
Research
Group
at
the
Departments
of
Electrical
Engineering
/
Computer
Science, University of Sheffield.
He gained his BSc (1983, City University, London) and PhD
(1987, University College London) degrees in Electronic
Engineering. He held academic posts at Surrey (1987) and
Brunel (1992) Universities, before joining the University of
Sheffield in 1995. His research interests are mobile
communications. He is an Honorary Member of Eta-KappaNu Association in the USA and a Member of the Institution of
Electrical Engineers in the UK. He can be contacted at:
s.cvetkovic@sheffield.ac.uk (100 words)
Matthias Kraner born in Germany
gained his Dipl.- Ing.
University in Electrical Engineering
and
Information
Technology.
(Technical University of Munich,
Germany, 1992), and an MSc. In
Data Communications (Brunel University, London, UK in
1995). After running a consultancy firm and a company for
soft and hardware solution for 5 years Mr. Kraner joined the
Department of Computer Science at the University of
Sheffield in 1998. His research interests are network
architectures, QoS for mobile communication systems, virtual
environments for healthcare and education. Mr Kraner is
Member of the Chamber of Engineers in Baden- Württemberg,
Germany.
He
can
be
contacted
at
Matthias_Kraner@credo.shef.ac.uk (98 words)
Erika Sánchez was born in Mexico.
In 1996 she obtained a First Degree
in Computer Systems Engineering
(ITESM CEM, Mexico), later she
gained a Masters Degree in
Computer Science in 1998 (ITESM
CEM, Mexico). While studying she held a position as an
academic in the ITESM CEM. In 1999 she began her PhD
studies in the Mobile Communications Research Group at the
Department of Computer Science in the University of
Sheffield. Her research interests are network security and
programming languages. She can be contacted at
E.Sanchez@dcs.shef.ac.uk
Chern Nam Yap was born in
Singapore. In 1994 he obtained a
Degree in Mechatronics and a BSc in
Computing and Information Systems.
From 1997 and 1998 he was working
with EDNOVATION on VRML on
Education. In 1999 he finished his MSc(Res) working on MIP
and IIP and is reading for his PhD since then. Currently he is
researching into mobility performance issues of concurrent
mobile systems. He can be contacted at CNY@dcs.shef.ac.uk