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
                                                           R. Hancock
                                          Siemens/Roke Manor Research
                                                    N. Georganopoulos
                                                King's College London
                                                     January 12, 2001

                      Mobility Related Terminology

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire in July, 2001.

   Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.


   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

   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

   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

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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

   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

   - 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

   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

   - 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

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

   - 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

   - 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

   - 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 T‰l‰com 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

   [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

   [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

   [5] MIPv6 Handover Design Team, "Fast Handovers for Mobile
       IPv6". Internet Draft (work in progress), November 2000

   [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

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)

   Voice:  +358-9-191-44210
   Fax:    +358-9-191-44441

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   Markku Kojo
   Department of Computer Science
   University of Helsinki
   P.O. Box 26 (Teollisuuskatu 23)

   Voice:  +358-9-191-44179
   Fax:    +358-9-191-44441

   Tapio Suihko
   VTT Information Technology
   P.O. Box 1203
   FIN-02044 VTT

   Voice:  +358-9-456-6078
   Fax:    +358-9-456-7028

   Phil Eardley
   Adastral Park
   Ipswich IP5 3RE
   United Kingdom

   Voice:  +44-1473-645938
   Fax:    +44-1473-646885

   Dave Wisely
   Adastral Park
   Ipswich IP5 3RE
   United Kingdom

   Voice:  +44-1473-643848
   Fax:    +44-1473-646885

   Robert Hancock
   Roke Manor Research Ltd
   Romsey, Hants, SO51 0ZN
   United Kingdom

   Voice:  +44-1794-833601
   Fax:    +44-1794-833434

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   Nikos Georganopoulos
   King's College London
   London WC2R 2LS
   United Kingdom

   Voice:  +44-20-78482889
   Fax:    +44-20-78482664

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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

   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

   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

   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

   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.

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Internet-Draft        Mobility Related Terminology          January 2001

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