Internet Engineering Task Force J. Manner
Internet-Draft M. Kojo
Expires: July, 2001 University of Helsinki
T. Suihko
VTT Information Technology
P. Eardley
D. Wisely
BT
R. Hancock
Siemens/Roke Manor Research
N. Georganopoulos
King's College London
January 12, 2001
Mobility Related Terminology
<draft-manner-seamoby-terms-00.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
There is a need for common definitions of terminology in the work to
be done around IP mobility. This memo defines terms for mobility
related terminology. It is intended as a living document for use by
the Seamoby working group, and especially for use in Seamoby drafts
and in WG discussions.
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Table of Contents
1 Introduction ................................................. 2
2 Definitions .................................................. 3
2.1 Network Components ......................................... 3
2.2 Handover Terminology ....................................... 4
2.2.1 Scope of Handover ........................................ 5
2.2.2 Technologies and Network Interfaces ...................... 5
2.2.3 Handover Control ......................................... 6
2.2.4 Simultaneous connectivity to Access Routers .............. 7
2.2.5 Performance and Functional Requirements .................. 7
2.3 Micro diversity, Macro diversity, and IP diversity ......... 7
2.4 Mobile Host States and Modes ............................... 8
2.5 User, Personal and Host Mobility ........................... 9
2.6 Macro and Micro Mobility ................................... 9
3 Acknowledgement .............................................. 10
4 References ................................................... 11
5 Author's Addresses ........................................... 11
6 Appendix A - Examples ........................................ 14
1. Introduction
This document presents a terminology to be used for documents and
discussions within the Seamoby Working Group. Other working groups
may also take advantage of this terminology in order to create a
common terminology for the area of mobility.
Some terms and their definitions that are not directly related to the
IP world are included for the purpose of harmonizing the terminology,
for example, 'Access Point' and 'base station' refer to the same
component but 'Access Router' has a very different meaning. The
presented terminology may not be adequate to cover mobile ad-hoc
networks.
The proposed terminology is not meant to 'push' new terminology.
Rather the authors would welcome discussion on more exact
definitions, better, missing and unnecessary terms. This work is a
collaborative enterprise between people from many different
engineering backgrounds and so already presents a first step in
harmonizing the terminology.
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2. Definitions
2.1. Network Components
Note: The fundamental new concept to be introduced is that of the
Access Network (AN) which supports enhanced mobility. It is a working
assumption that to support routing and QoS for mobile devices, we
need specialized routing functions (i.e. not OSPF or other standard
IGPs) which are used to maintain forwarding information for these
devices as they move physically, and these functions are implemented
in IP routers with this additional capability. We can distinguish
three types of these: Access Routers (AR) which handle the last hop
to the mobile; Access Network Gateways (ANG) which form the boundary
on the fixed network side and shield the fixed network from the
specialized routing protocols; and (optionally) internal Access
Network Routers which may also be needed in some cases to support the
protocols. The Access Network consists of the equipment needed to
support this specialized routing, i.e. A/ANG/ANR.
Mobile Node (MN)
An IP node capable of changing its point of attachment to the
network. The Mobile Node may have routing functionality.
Mobile Host (MH)
An IP node capable of changing its point of attachment to the
network. The Mobile Host only refers to an end-host without routing
support.
Access Link (AL)
A facility or medium over which a Mobile Host and an Access Point
can communicate at the link layer, i.e., the layer immediately
below IP.
Access Point (AP)
An Access Point is a layer 2 device which is connected to an Access
Router and offers the wireless link connection to the Mobile Host.
Access Points are sometimes called 'base stations' or 'access point
transceivers'. An Access Point may be a separate entity or co-
located with an Access Router.
Access Network Router (ANR)
An IP router in the Access Network. An Access Network Router may
include Access Network specific functionalities, for example, on
mobility and/or QoS. This is to distinguish between ordinary
routers and routers that have Access Network-related special
functionality.
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Access Router (AR)
An Access Network Router residing on the edge of an Access Network
and connected to one or more access points. An Access Router offers
IP connectivity to MHs. The Access Router may include intelligence
beyond a simple forwarding service offered by ordinary IP routers.
Access Network Gateway (ANG)
An Access Network Gateway that separates an Access Network from
other IP-networks. An Access Router and an Access Network Gateway
may be the same physical node. The Access Network Gateway looks to
the fixed network like a standard IP router.
Access Network (AN)
An IP network which includes one or more Access Network Routers.
The terms Access Network and (administrative) domain are often used
interchangeably (e.g., "intra-AN" is "intra-domain") since often an
Access Network has its own administration.
Serving Access Router (SAR)
The Access Router currently offering the connectivity to the Mobile
Host. This is usually the point of departure for the Mobile Host as
it makes its way towards a new Access Router (then Serving Access
Router takes the role of the Old Access Router). There may be
several Serving Access Routers serving the Mobile Host at the same
time.
Old Access Router (OAR)
An Access Router that offered connectivity to the Mobile Host prior
to a handover. This is the Serving Access Router that will cease or
has ceased to offer connectivity to the Mobile Host.
New Access Router (NAR)
The Access Router that offers connectivity to the Mobile Host after
a handover.
Candidate Access Router (CAR)
An Access Router to which the Mobile Host may move next. A handover
scheme may support several Candidate Access Routers.
2.2. Handover Terminology
These terms refer to different approaches to supporting different
aspects of mobility.
- Roaming refers to a particular aspect of user mobility. Roaming is
an operator-based term involving formal agreements between operators
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that allows a mobile to get connectivity from a foreign network.
Roaming includes, for example, the functionality by which users can
communicate their identity to the local AN so that inter-AN
agreements can be activated and service and applications in the MH's
home network can be made available to the user locally.
- Handover (also known as handoff) is the process involved when an
active MH (in the Active State, see section 2.4) changes its point of
attachment to the network, or when such a change is attempted. The
access network may provide particular capabilities to minimize the
interruption to sessions in progress.
There are different types of handover classified according to
different aspects involved in the handover. Some of this terminology
follows the description of [5].
2.2.1. Scope of Handover
- Layer 2 Handover: When a MH changes APs (or some other aspect of
the radio channel) connected to the same AR's interface then a layer
2 handover occurs. This type of handover is transparent to the
routing at the IP layer (or it appears simply as a link layer
reconfiguration without any mobility implications).
- Intra-AR Handover: This is a handover which changes the AR's IP-
layer's interface to the mobile. This causes routing changes internal
to the AR. The IP-address by which the MH is reachable does not
change.
- Intra-AN Handover: When the MH changes ARs inside the same AN then
this handover occurs. Such a handover is not necessarily visible
outside the AN. In case the ANG serving the MH changes, this handover
is seen outside the AN due to a change in the routing paths. The IP-
address by which the MH is reachable does not change. Note that the
ANG may change for only some of the MH's data flows.
- Inter-AN Handover: When the MH moves to a new AN then this handover
occurs. This requires some sort of host mobility across ANs, which
has to be provided by the external IP core. Note that this would have
to involve the assignment of a new IP address to the MH.
2.2.2. Technologies and Network Interfaces
- Intra-technology Handover: A handover between equipment of the same
technology. Layer 2 handovers are necessarily intra-technology.
- Inter-technology Handover: A handover between equipment of
different technologies.
- Horizontal Handover: from the IP point of view a horizontal
handover happens if the MH communicates with the OAR and NAR via the
same network interface. Horizontal handover is typically also an
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intra-technology handover but it can be an inter-technology handover
if the MH can do a handover between two different technologies
without changing the network interface seen by the IP layer.
- Vertical Handover: in a vertical handover the MH's network
interface to the Access Network changes. Vertical handover is
typically an inter-technology handover but it may also be an intra-
technology handover if the MH has several interfaces of the same
type.
The different handover types defined in this section and in section
2.2.1 have no direct relationship. In particular, a MH can do an
intra-AN handover of any of types defined above.
2.2.3. Handover Control
A handover must be one of the following two types (a):
- Mobile-initiated Handover: the MH is the one that makes the initial
decision to initiate the handover.
- Network-initiated Handover: the network makes the initial decision
to initiate the handover.
A handover is also one of the following two types (b):
- Mobile-controlled Handover (MCHO): the MH has the primary control
over the handover process.
- Network-controlled Handover (NCHO): the network has the primary
control over the handover process.
A handover may also be either of these two types (c):
- Mobile-assisted handover: information and measurement from the MH
are used to decide on the execution of a handover.
- Network-assisted handover: a handover where the AN collects
information that can be used in a handover decision.
A handover is also one of the following two types (d):
- Backward handover: a handover either initiated by the OAR, or where
the MH initiates a handover via the OAR.
- Forward handover: a handover either initiated by the NAR, or where
the MH initiates a handover via the NAR.
The handover is also either proactive or reactive (e):
- Planned handover: a proactive (expected) handover where some
signalling can be done in advance of the MH getting connected to the
new AR, e.g. building a temporary tunnel from the old AR to the new
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AR. Generally this is a result of a backward handover.
- Unplanned handover: a reactive (unexpected) handover, where no
signalling is done in advance of the MH's move of the OAR to the new
AR. Generally this results from a forward handover.
The five handover types (a-e) are orthogonal. Type 'c' may be present
in a handover, the other types are always present.
2.2.4. Simultaneous connectivity to Access Routers
- Make-before-break handover (MBB): During a MBB handover the MH can
communicate simultaneously with the old and new AR. This should not
be confused with "soft handover" which relies on macro diversity.
- Break-before-make handover (BBM): During a BBM handover the MH does
not communicate simultaneously with the old and the new AR.
2.2.5. Performance and Functional Requirements
- Handover Latency: Handover latency is the time difference between
when a MH is last able to send and/or receive an IP packet by way of
the OAR, until when the MH is able to send and/or receive an IP
packet through the NAR. Adapted from [5]
- Smooth handover: A handover that aims primarily to minimize packet
loss, with no explicit concern for additional delays in packet
forwarding.
- Fast handover: A handover that aims primarily to minimize delay,
with no explicit interest in packet loss.
- Seamless handover: A handover that is both smooth and fast, thus
provides fast lossless handover between two ARs.
- Context-aware Handover: A handover that is governed by a certain
specific requirement to be fulfilled while handing the connection
between two ARs.
2.3. Micro diversity, Macro diversity, and IP diversity
Certain air interfaces (e.g. UTRAN FDD mode) require or at least
support the concepts of macro diversity combining. Essentially, this
refers to the fact that a single MH is able to send and receive over
two independent radio channels ('diversity branches') at the same
time; the information received over different branches is compared
and that from the better branch passed to the upper layers. This can
be used both to improve overall performance, and to provide a
seamless type of handover at layer 2, since a new branch can be added
before the old is deleted. See also [4].
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It is necessary to differentiate between combining/diversity that
occurs layer 1/2 (physical and radio link layers) where the relevant
unit of data is the radio frame, and that which occurs at layer 3,
the network layer, where what is considered is the IP packet itself.
In the following definitions micro- and macro diversity refer to
L1/L2 and IP diversity refers to L3.
- Micro diversity is the term used for the case where, for example,
two antennas on the same transmitter send the same signal to a
receiver over a slightly different path to overcome fading.
- Macro diversity takes place when the duplicating / combining
actions take place over multiple APs, possibly attached to different
ARs. This may require support from the network layer to move the
radio frames between the base stations and a central combining point.
- IP diversity means the splitting and combining of packets at the IP
level.
2.4. Mobile Host States and Modes
Mobile systems may employ the use of MH states in order to operate
more efficiently without degrading the performance of the system. The
term 'mode' is also common and means the same as 'state'.
A MH is always in one of the following three states:
- Active State is when the AN knows the MH's SAR and the MH can send
and receive IP packets. The AL may not be active, but the radio layer
is able to establish one without assistance from the network layer.
The MH has an IP address assigned.
- Idle State is when the AN knows the MH's Paging Area, but the MH
has no SAR and so packets cannot be delivered to the MH without the
AN initiating paging.
- Detached State is when the MH is in neither the Active nor Idle
State. The MH does not have an IP address from the AN.
- Paging is a procedure initiated by the Access Network to move an
Idle MH into the Active State. As a result of paging, the MH
establishes a SAR and the IP routes are set up.
- Location updating is a procedure initiated by the MH, by which it
informs the AN that it has moved into a new paging area.
- A Paging Area is a part of the Access Network, typically containing
a number of ARs/APs, which corresponds to some geographical area. The
AN keeps and updates a list of all the Idle MHs present in the area.
If the MH is within the radio coverage of the area it will be able to
receive paging messages sent within that Paging Area.
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Note: in fact, as well as the MH being in one of these three states,
the AN also stores which state it believes the MH is in. Normally
these are consistent; the definitions above assume so.
2.5. User, Personal and Host Mobility
Different sorts of mobility management may be required of a mobile
system. We can differentiate between user, personal and host
mobility.
- User mobility: refers to the ability of a user to access services
from different physical hosts. This usually means, the user has an
account on these different hosts or that a host does not restrict
users from using the host to access services.
- Personal mobility: complements user mobility with the ability to
track the user's location and provide the users current location to
allow sessions to be initiated by and towards the user by anyone on
any other network. Personal mobility is also concerned with enabling
associated security, billing and service subscription authorization
made between administrative domains.
- Host mobility: refers to the function of allowing a mobile host to
change its point of attachment to the network, without interrupting
IP packet delivery to/from that host. There may be different sub-
functions depending on what the current level of service is being
provided; in particular, support for host mobility usually implies
active and idle modes of operation, depending on whether the host has
any current sessions or not. Access Network procedures are required
to keep track of the current point of attachment of all the MHs or
establish it at will. Accurate location and routing procedures are
required in order to maintain the integrity of the communication.
Host mobility is often called 'terminal mobility'.
2.6. Macro and Micro Mobility
Macro and micro mobility refer to host mobility in wide and local
geographical area. Correspondingly, macro- and micro-mobility
management refer to the scope of protocol operations in mobility
management.
- Macro mobility refers literally to 'mobility over a large area'.
This includes mobility support and associated address registration
procedures that are needed when a mobile host moves between IP
domains. Inter-AN handovers typically involve macro-mobility
protocols. Mobile-IP can be seen as a means to provide macro
mobility.
- Micro mobility refers to 'mobility over a small area'. Usually
this means mobility within an IP domain with an emphasis on support
for active mode using handover, although it may include idle mode
procedures also. Micro-mobility protocols exploit the locality of
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movement by confining movement related changes and signalling to the
access network.
3. Acknowledgement
This work has been performed in the framework of the IST project
IST-1999-10050 BRAIN, which is partly funded by the European Union.
The authors would like to acknowledge the contributions of their
colleagues from Siemens AG, British Telecommunications PLC, Agora
Systems S.A., Ericsson Radio Systems AB, France Tlcom R&D, INRIA,
King's College London, Nokia Corporation, NTT DoCoMo, Sony
International (Europe) GmbH, and T-Nova Deutsche Telekom Innovations-
gesellschaft GmbH.
Some definitions of terminology have been adapted from [1], [2], [3],
[5], [7] and [8].
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4. References
[1] Blair, D., Tweedly, A., Thomas, M., Trostle, J., Ramalho, M.,
"Realtime Mobile IPv6 Framework". Internet Draft (work in
progress), November 2000
(draft-blair-rt-mobileipv6-seamoby-00.txt).
[2] Deering, S., Hinden, R., "Internet Protocol, Version 6 (IPv6)
Specification". Internet Engineering Task Force, Request for
Comments (RFC) 2460, December 1998.
[3] Gustafsson, E., Jonsson, A., Perkins, C., "Mobile IP Regional
Registration". Internet Draft (work in progress), July 2000
(draft-ietf-mobileip-reg-tunnel-03.txt).
[4] Kempf, J., McCann, P., Roberts, P., "IP Mobility and the CDMA
Radio Access Network: Applicability Statement for Soft Handoff",
Internet Draft (work in progress), July 2000
(draft-kempf-cdma-appl-00.txt).
[5] MIPv6 Handover Design Team, "Fast Handovers for Mobile
IPv6". Internet Draft (work in progress), November 2000
(draft-perkins-mobileip-handover-00.txt).
[6] Pandya, R., "Emerging mobile and personal communication
systems," IEEE Communications Magazine , vol. 33, pp. 44--52,
June 1995.
[7] Perkins, C., "IP Mobility Support". Internet Engineering Task
Force, Request for Comments (RFC) 2002, October 1996.
[8] Ramjee, R., La Porta, T., Thuel, S., Varadhan, K., Salgarelli,
L., "IP micro-mobility support using HAWAII". Internet Draft
(work in progress), July 2000
(draft-ietf-mobileip-hawaii-01.txt).
5. Author's Addresses
Questions about this document may be directed to:
Jukka Manner
Department of Computer Science
University of Helsinki
P.O. Box 26 (Teollisuuskatu 23)
FIN-00014 HELSINKI
Finland
Voice: +358-9-191-44210
Fax: +358-9-191-44441
E-Mail: jmanner@cs.helsinki.fi
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Markku Kojo
Department of Computer Science
University of Helsinki
P.O. Box 26 (Teollisuuskatu 23)
FIN-00014 HELSINKI
Finland
Voice: +358-9-191-44179
Fax: +358-9-191-44441
E-Mail: kojo@cs.helsinki.fi
Tapio Suihko
VTT Information Technology
P.O. Box 1203
FIN-02044 VTT
Finland
Voice: +358-9-456-6078
Fax: +358-9-456-7028
E-Mail: tapio.suihko@vtt.fi
Phil Eardley
BTexaCT
Adastral Park
Martlesham
Ipswich IP5 3RE
United Kingdom
Voice: +44-1473-645938
Fax: +44-1473-646885
E-Mail: philip.eardley@bt.com
Dave Wisely
BTexaCT
Adastral Park
Martlesham
Ipswich IP5 3RE
United Kingdom
Voice: +44-1473-643848
Fax: +44-1473-646885
E-Mail: dave.wisely@bt.com
Robert Hancock
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Voice: +44-1794-833601
Fax: +44-1794-833434
E-Mail: robert.hancock@roke.co.uk
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Nikos Georganopoulos
King's College London
Strand
London WC2R 2LS
United Kingdom
Voice: +44-20-78482889
Fax: +44-20-78482664
E-Mail: nikolaos.georganopoulos@kcl.ac.uk)
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6. Appendix A - Examples
This appendix provides examples for the terminology presented.
A.1 Mobility
Host mobility is logically independent of user mobility, although in
real networks, at least the address management functions are often
required to attach the host to the network in the first place. In
addition, if the network wishes to determine whether access is
authorized (and if so, who to charge for it), then this may be tied
to the identity of the user of the terminal.
An example of user mobility would be a campus network, where a
student can log into the campus network from several workstations and
still get his/her files, emails, etc. services automatically.
Personal mobility support typically amounts to the maintenance and
update of some sort of address mapping database, such as a SIP server
or DNS server; it is also possible for the personal mobility support
function to take a part in forwarding control messages between end
user and correspondent rather than simply acting as a database. SIP
is a protocol for session initiation in IP networks. It includes
registration procedures which partially support personal mobility
(namely, the ability for the network to route a session towards a
user at a local IP address).
Personal mobility has been defined in [6] as "the ability of end
users to originate and receive calls and access subscribed
telecommunication services on any terminal in any location, and the
ability of the network to identify end users as they move. Personal
mobility is based on the use of a unique personal identity (i.e.,
personal number)."
Roaming, in its original (GSM) sense, is the ability of a user to
connect to the networks owned by operators other than the one he has
a direct formal relationship with. More recently (e.g. in data
networks and UMTS) it also refers to the fact that the 'foreign'
network may still be able to provide user-customized services, e.g.
QoS profiles for specific applications.
HAWAII, Cellular IP, Regional Registration and EMA are examples of
micro mobility schemes, with the assumption that Mobile IP is used
for macro mobility.
WLAN technologies such as IEEE 802.11 typically support aspects of
user and host mobility in a minimal way. User mobility procedures
(for access control and so on) are defined only over the air
interface (and the way these are handled within the network is not
further defined).
PLMNs (GSM/UMTS) typically have extensive support for both user and
host mobility. Complete sets of protocols (both over the air and on
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the network side) are provided for user mobility, including
customized service provision. Handover for host mobility is also
supported, both within access networks, and also within the GSM/UMTS
core network for mobility between access networks of the same
operator.
A.2 Handovers
A hard handover is required where a MH is not able to receive or send
traffic to two APs simultaneously. In order to move the traffic
channel from the old to the new access point the MH abruptly changes
the frequency/timeslot/code on which it is transmitting and listening
to new values associated with a new access point.
A good example of hard handover is GSM where the mobile listens for
new base stations, reports back to the network the signal strength
and identity of the new base station(s) heard. When the old base
station decides that a handover is required it instructs the new base
station to set up resources and, when confirmed, instructs the mobile
to switch to a new frequency and time slot. This sort of hand over
is called hard, mobile assisted, network initiated and backward
(meaning that the old base station is responsible for handling the
change-over).
In a TDMA system, such as GSM, the hard hand over is delayed until
the mobile has moved well within the coverage of the new base
station. If the handover threshold was set to the point where the new
base station signal exceeded the old then there would be a very large
number of handovers as the mobile moved through the region between
the cells and radio signals fluctuated, this would create a large
signalling traffic. To avoid this a large hysteresis is set, i.e. the
new base station must be (say) 10dB stronger for handover to occur.
If the same was done in W-CDMA then the mobile would be transmitting
a powerful signal to the old base station and creating interference
for other users, since in CDMA everyone else's transmissions are seen
as noise, thus reducing capacity. To avoid this soft handover is
used, giving an estimated doubling in capacity.
Support for soft handover (in a single mode terminal) is
characteristic of radio interfaces which also require macro diversity
(bi-casting) for interference limitation but the two concepts are
logically independent.
A good example of soft handover is the UTRAN FDD mode. W-CDMA is
particularly suited to soft handover because of the design of the
receivers and transmitters: typically a rake receiver will be used to
overcome the multi-path fading of the wide-band channel. Rake
receivers have a number of so-called fingers, each effectively
separate detectors, that are tuned to the same signal (e.g. spreading
code) but delayed by different times. When the delay times are
correctly adjusted and the various components properly combined (this
is micro diversity combining) the effect of multi-path fading is
removed. The rake receiver can also be used to detect signals from
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different transmitters by tuning the fingers to different spreading
codes. Soft handover is used in UTRAN FDD mode to also increase
capacity.
Every handover can be seen as Context-aware Handovers. In PLMNs the
context to be fulfilled is that the new AP can accommodate the new
mobile, for example, the new GSM cell can serve the incoming phone.
Lately, the notion of Context-aware Handovers has been enlarged by,
for example, QoS-aware handovers, meaning that the handover is
governed by the need to support the QoS-context of the moving mobile
in order to keep the service level assured to the user of the MH.
A.3 Diversity combining
In the case of UMTS it is radio frames that are duplicated at some
point in the network (the serving RNC) and sent to a number of Node
Bs and, possibly via other (drift) RNCs. The combining that takes
place at the serving RNC in the uplink direction is typically based
on some simple quality comparison of the various received frames,
which implies that the various copies of these frames must contain
identical upper layer information. The serving RNC also has to do
buffering to take account of the differing time of flight from each
Node B to the RNC.
A.4 Miscellaneous
In a GPRS/UMTS system the Access Network Gateway node would be the
GGSN component. The ANG can provide support for mobility of hosts,
admission control, policy enforcement, and Foreign Agent
functionality.
When presenting a mobile network topology, APs and ARs are usually
pictured as separate components. This is the case with GSM/GPRS/UMTS
presentations, for example. From the IP point of view APs are not
directly visible. An AP should only be seen from the MH's or AR's IP-
layer as a link (interface) connecting MHs to the AR.
When the mobile moves through the network, depending on the mobility
mechanism, the OAR will forward packets destined to the old MHs
address to the SAR which currently serves the MH. At the same time
the handover mechanism may be studying CARs to find the best NAR
where the MH will be handed next.
Note that when a network includes IP-over-IP tunnels, we need to be
very careful about which IP routing and IP address we are discussing.
Manner et al Expires July 2001 [Page 16]
Internet-Draft Mobility Related Terminology January 2001
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Manner et al Expires July 2001 [Page 17]