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INTERNET DRAFT                                                James Kempf
Category: Informational                             Sun Microsystems, Inc.
Title: draft-kempf-cdma-appl-02.txt                           Peter McCann
Date: September 2001                                   Lucent Technologies
                                                            Philip Roberts
                                                             Motorola, Inc.

            IP Mobility and the CDMA Radio Access Network:
                Applicability Statement for Soft Handoff

Status of this Memo

   This document is an individual contribution for consideration by the
   Mobile IP Working Group of the Internet Engineering Task Force.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at:


   The list of Internet-Draft Shadow Directories can be accessed at:


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

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   Recently, there have been a variety of proposals submitted to the
   Mobile IP Working Group and to other IETF working groups for IP
   mobility solutions that seek to enhance or replace mobile IP. These
   proposals, often characterized as micromobility or fast handoff, are
   addressed primarily at the perceived need of multimedia sessions such
   as video or voice over IP for faster handoff between radio base
   stations, and are primarily directed at real time multimedia traffic
   in 3rd generation cellular access networks. In this paper, we discuss
   the design of CDMA radio access networks (RANs) and the applicability
   of IP mobility to soft handoff in a CDMA RAN. We attempt to show that
   given current IP routing algorithms and the constraints on a CDMA
   RAN, IP mobility solutions have little, if any, role to play in
   handoff within the RAN. In contrast, an IP mobility solution is
   likely to play a big role in fast handoff between RANs, also called
   hard handoff. While future developments in IP networking may change
   this situation, IP mobility in CDMA networks currently seems to apply
   only when the mobile node roams between disconnected RANs rather than
   between base stations within a RAN or between connected RANs.

Table of Contents

   1.0  Introduction
   2.0  Terminology
   3.0  RAN Architecture and Characteristics
   4.0  Applicability of IP Mobility to Soft Handoff
   5.0  Applicability of IP Mobility to Hard Handoff
   6.0  Future Prospects for IP Mobility in the CDMA RAN
   7.0  Summary
   8.0  References
   9.0  Authors' Addresses
   10.0 Full Copyright Statement

1.0  Introduction

   Mobile IP [1] allows IP hosts that change their point of attachment
   to the network to keep their IP address as they change from their
   home subnet to other subnets. Recently, there have been a variety of
   proposals advanced for augmenting or replacing mobile IP in access
   networks for cellular telephony systems. These proposals are often
   characterized as supporting micromobility or fast handoff, and are
   directed towards real time multimedia streams in 3rd generation
   cellular networks (see [2] [3] [4] [5] and [7]).

   While these proposals may have some applicability if handoff between
   disconnected RANs is very frequent, their utility is lessened in the

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   presence of RAN mobility like that offered by today's TDMA and CDMA
   systems.  In fact if the RAN protocols offer transparent mobility
   throughout a given domain of interconnected RANs, these proposals do
   not contribute anything since they are essentially intra-domain
   mobility management protocols.  As cellular networks evolve towards
   more pervasive IP technologies, host mobility within these networks
   must accommodate some level of movement of IP traffic.

   In this paper, we discuss CDMA radio access networks and why current
   IP mobility solutions (including mobile IP) are not applicable to
   soft handoff in a CDMA RAN. In effect, the CDMA RAN network with soft
   handoff is a mobility mechanism transparent to L3 and above on the
   mobile terminal and its corresponding host, similar to but not
   identical with the mechanisms used for interaccess point mobility in
   wireless LAN technologies such as IEEE 802.11 [8]. It is therefore
   not an appropriate candidate for moving into the network layer given
   current IP routing algorithms. In contrast, IP mobility solutions
   that are directed at improving handoff performance within the core
   network, often called hard handoff, are likely to play an important
   role in enhancing the performance of IP networking for CDMA (see [6]
   for an example).

2.0  Terminology

   mobile terminal
     A mobile IP host. In mobile IP terminology, this is called the
     mobile node.

   base station
     A fixed, land-based radio transmitter and receiver, used to provide
     cellular telephony radio coverage in a limited geographic area.  A
     mobile terminal may be in contact with one or more base stations at
     a time in CDMA networks. Also called the Base Transceiver Station
     (BTS) or Node B.

     The radio access network. This is a wired network that sits between
     a collection of base stations and the core, wired telephone
     network.  The RAN in CDMA systems is involved in real-time
     distribution and collection of physical layer radio frames to and
     from base stations, a topic that is discussed in the next section.

   soft handoff
     The process by which a moving CDMA mobile terminal is transferred
     between one base station or set of base stations to another within
     the radio access network. Soft handoff is typically very fast (on
     the order of 20 ms) and has a low probability of dropping ongoing
     real time connections.

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   hard handoff
     The process by which a moving mobile terminal is transferred
     between one cellular service provider's network and another or
     between two RANs that do not share a direct connection within a
     provider's network.  Hard handoffs have a higher probability of
     connection droppage, and are slower (usually 100 ms or more).

     A term used to describe the fact that within the CDMA RAN, there is
     no single octet stream corresponding to the data that arrives at or
     is sent by the mobile terminal. Macrodiversity results because the
     mobile terminal can be in contact with more than one base station
     at a time.

   frame selector
     A combination software/hardware unit at the gateway to the RAN that
     combines the multiple octet streams from multiple base stations in
     contact with a single mobile terminal into a single octet stream. A
     similar process happens at the mobile terminal. Also called the
     macrodiversity combiner or Selection and Distribution Unit (SDU).

   RAN gateway
     A functional unit positioned between the RAN and the core network.
     The RAN gateway includes the frame selector, in addition to
     functional units that perform soft handoff and radio frame
     processing.  Also called the Base Station Controller (BSC) or Radio
     Network Controller (RNC).

   radio frame
     A short (usually 20 millisecond) unit of transmission at the
     physical radio layer used to transmit data over the air to and from
     the mobile terminal.  The frames do not usually contain complete IP
     packets as sent or received by applications on the mobile terminal,
     but rather contain small sections of octet stream data that must be
     framed by a higher layer protocol (such as PPP) to form IP packets.
     On a basic fundamental data rate channel one radio frame contains
     about 20 octets.  Radio frames may be retransmitted a small number
     of times to increase the reliability of the octet stream transport.
     This is performed by a negative- acknowledgement protocol known as
     the Radio Link Protocol (RLP).

3.0  RAN Architecture and Characteristics

   A RAN consists of a RAN gateway connected to one or more base
   stations.  In the network to mobile terminal (forward) direction, the
   RAN gateway performs the following functions:

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     1) Receive packets from the core network destined to the mobile

     2) Process those packets into radio frames,

     3) Replicate the radio frames and transmit copies to base stations
     that are currently in contact with the mobile terminal in a way
     such that the frames arrive at each base station in a timely

     4) Manage the retransmission of individual radio frames when
     negative acknowledgements are received.

   In the mobile terminal to network (reverse) direction, the RAN
   gateway performs the following functions:

     1) Collect copies of the radio frames fowarded by base stations
     that are currently in contact with the mobile terminal,

     2) Combine these (possibly errorful) copies into one (hopefully
     error-free) radio frame using some algorithm,

     3) From the resulting radio frame stream, synthesize an outgoing
     octet stream of packets for the core network.

   In both directions, the RAN gateway performs the following function:

     1) Manage the power with which mobile nodes and base stations are
     transmitting, so as to maintain a low error rate while at the same
     time minimizing the transmitted power and therefore interference
     among different transmitters (and drain on the mobile terminal's

     2) In concert with the mobile terminal, manage the set of base
     stations with which a mobile terminal is in contact such that the
     quality of the radio signal is maintained as the mobile terminal

   The use of multiple base stations to transmit and receive the same
   radio frames to and from the mobile node at the same time is known as
   "macrodiversity" and can help to improve the reliability of the
   wireless link.  Note that some of the base stations may be owned by a
   neighboring RAN and this necessitates a RAN-to-RAN interface to carry

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   the radio frames.

   The following figure illustrates the architecture and how the CDMA
   RAN network works:

                         Core Network

                            |   incoming/outgoing octet stream
                            |    from/to corresponding node
                            |        (e.g.
   | RAN Gateway                                         |
   |                                                     |
   |    +-------------+  +-----------+  +----------+     |
   |    |             |  |           |  |          |     |
   |    | Frame       |  | Frame     |  | Soft     |     |----------->
   |    | Selection   |  | Splitting |  | Handoff  |     |       to another
   |    |             |  |           |  | Control  |     |       RAN Gateway
   |    +-------------+  +-----------+  +----------+     |       in neighboring
   |                                                     |       RAN
   |    +---------------+                                |
   |    |               |                                |
   |    | Radio Frame   |                                |
   |    | Processing    |                                |
   |    |               |                                |
   |    +---------------+                                |
   |                                                     |
            |             |              |            |    *
            |             |              |            |      *
            |             |     ...      |            |       * radio
            |             |              |            |       * frames
            |             |              |            |      *
            |             |              |            |    *
       +---------+   +---------+   +---------+   +---------+
       | Base    |   | Base    |   | Base    |   | Base    |
       | Station |   | Station |   | Station |   | Station |
       |   B1    |   |   B2    |   |  B42    |   |   B43   |
       +---------+   +---------+   +---------+   +---------+
            \              |
             \             V
                        /     \
                    -----       ----- 

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                   | mobile terminal|(  --->
                     ( )        ( )

   The links between the RAN gateway and the base stations are typically
   point to point links today, though provisions exist in the 3rd
   generation standards for switched networks.

   In the above figure, a mobile terminal with IP address
   is corresponding with a host having the IP address,
   through a CDMA cellular network. The mobile terminal is in contact
   with two base stations, B1 and B2. The RAN gateway takes incoming
   packets from and splits them into two streams that it
   sends to base stations B1 and B2. A mobile terminal can be in
   communication with up to 3 and, in some CDMA systems, up to 6 base
   stations at a time. When the multiple octet streams are received at
   the mobile terminal, the mobile terminal performs a sophisticated
   combination of the multiple packet streams at L1 to deliver the end
   packet to the application.

   Packets flowing in the other direction, from to, are put into an octet stream which is then divided
   into radio frames.  Each radio frame is received by B1 and B2 and
   delivered to the frame selector in the RAN gateway. The frame
   selector performs signal processing on the incoming radio frames and
   produces a single frame that is then sent to the re-sequencing buffer
   where the octet stream is re-created.  The IP packets are formed from
   the octet stream and transmitted into the core IP network.

   An important point to note about the RAN is that, even if the RAN
   itself is running IP, routing in the RAN does not use the mobile's IP
   address. In fact, the IP packets sent by the mobile terminal are not
   tunneled through the RAN nor do they necessarily appear in a form
   that would be recognizable using a packet sniffer. The packets in the
   RAN contain radio frames that have been highly processed into a form
   that is extremely efficient for the base stations to handle and that
   efficiently uses radio spectrum, including compensations for the
   inherently lossy nature of the radio medium.

   On the forward leg to the mobile terminal, because the delay and
   jitter constraints between multiple base stations transmitting to the
   same mobile terminal are so tight (5ms to 80ms), the RAN gateway
   essentially puts out streams of radio frames that the base station
   can quickly pull off the wire and transmit over the air. On the
   reverse leg from the mobile terminal, the base station simply pulls
   the radio frames off the air and puts them on the wire without any
   further processing. The RAN gateway's radio frame processor is

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   responsible for making sure that the jitter and delay constraints are
   met, and for processing packets from the core network into radio

   When the mobile terminal begins to move out of range of stations B1
   and B2, the RAN gateway adds and deletes base stations from the set
   currently serving the mobile node.  This process is called soft
   handoff.  Note that new base stations may belong to a connected
   neighboring RAN which requires closely-coupled RAN-to-RAN
   interaction. The real time constraints on soft handoff are extremely
   tight. All base stations involved in soft handoff must transmit the
   command for the mobile to move at the same time. North American
   cellular networks use the Global Positioning System as a time source
   to assure that these timing constraints are met.

   As shown in the figure, two RAN gateways can be connected together
   through a direct link. This link allows radio frames containing data
   and RAN control protocol to flow between two RANs. As a result, two
   RANs can perform soft handoff between them, increasing the quality
   and reliability of the connection when a user moves between coverage
   areas. A RAN gateway and its collection of base stations can only
   cover a limited geographic area, so RAN interconnection is very
   important in cellular networks for maintaining good connection
   quality over large geographic areas.

4.0 Applicability of IP Mobility to Soft Handoff

   Most of the proposals for IP mobility in the RAN assume the

     1) An end-to-end IP routing model for routing through the RAN.

     2) A one-to-one mapping between the mobile terminal and the base
     station with which it is in contact.

     3) The IP packets coming from the mobile terminal are transported
     directly on L2 in the RAN.

   As the above discussion has attempted to show, the extremely tight
   real time constraints on traffic in the RAN require that RAN traffic
   be highly processed as radio frames for efficient delivery into and
   from the radio medium by the base stations. This precludes routing IP
   packets from the mobile directly over the RAN. Furthermore, because
   there are multiple octet streams flowing over the RAN that correspond
   to one logical octet stream going to and from the mobile terminal,
   there is no one-to-one mapping between the mobile terminal and a base

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   Taking mobile IP as an example, using mobile IP in the RAN would
   require that a mobile IP foreign agent (FA) be present at each base
   station. However, because the mobile terminal can be in contact with
   up to 6 base stations at once, there is no unique care-of address for
   the mobile (unless the mobile uses a co-located care-of address).
   Therefore, the home agent (HA) would need to forward packets to
   multiple FAs.  Furthermore, on the reverse leg, a frame selector is
   still necessary to generate a single packet for the corresponding

   In effect, the RAN controller is an application level transport and
   mobility mechanism specialized to the CDMA radio medium. An
   application's packets to/from the mobile effectively run over a
   "stratified stack" in the RAN, in which the bottom stratum is the RAN
   protocol stack, and the upper stratum is the IP stack on the mobile
   terminal and corresponding node. The RAN controller is therefore not
   a good candidate for replacing with current network level mobility
   mechanisms.  Because of the hard real time constraints involved in
   soft handoff, the RAN controller's soft handoff function is also not
   a good candidate for replacing with more general application level
   mechanisms, such as SIP [4].

5.0 Applicability of IP Mobility to Hard Handoff

   Note that the above considerations do not apply to hard handoff,
   which occurs outside of the RAN. When a mobile terminal moves between
   two RANs that are not interconnected, the RAN controller defers
   handoff to the core network. Some mechanism is necessary in the core
   network to move the mobile terminal's point of attachement at the
   network level. IP mobility solutions for fast handoff are applicable

   Proposals such as [6] that apply after frame selection do not involve
   soft handoff and therefore are appropriate for implementing fast,
   hard handoff.

6.0 Future Prospects for IP Mobility in the CDMA RAN

   What would it take to enable IP mobility in the RAN? The question is
   worth examining because the end-to-end model of networking which
   would be required to make IP mobility work in the RAN has attractions
   from the point of view of simplicity of network management and

   Certainly, routing the mobile's packets directly in the RAN would be
   a major contributor. For that to happen, IP over the air interface

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   would be necessary. The major impediment to IP over the air is
   currently header size, but new work in header compression may
   eliminate this objection. Given that a spectrally efficient
   representation of IP on the radio medium is possible, IP packets can
   be sent out over the air by the mobile terminal, and the base
   stations can handle the packets precisely as they currently do with
   the specialized radio frames.  The result would be that IP packets
   from the radio would appear directly on L2 in the RAN, rather than in
   a stratified stack.

   However, there is still a problem with the multipath nature of
   macrodiversity. On the forward leg, the routing from a single source
   at the RAN gateway to multiple base stations looks like multicast,
   but the real time constraints are extremely tight. On the the reverse
   leg, however, traffic still needs to be combined in the wired network
   behind the base stations. In order to accommodate macrodiversity, the
   RAN would need to be a routing domain in which a multipath routing
   protocol that performed frame selection in the border routers and
   featured real time routing table convergence (for soft handoff) was
   in use. These characteristics involve a considerable change from
   existing IP routing algorithms (which do not require real time
   convergence and do not involve multipath nor selection of particular
   packets based on some algorithm).

   While the prospects for moving IP into the RAN for transport of RAN
   protocols and data/voice traffic from the mobile are good, it seems
   unlikely that IP mobility will play any role in replacing CDMA soft
   handoff in the immediate future. The stratified RAN stack supporting
   CDMA soft handoff is a specialized, and therefore not a good
   candidate for replacement by more general network or application
   level mechanisms. In addition, even if IP is used for transport in
   the RAN, if voice over IP is to completely replace the current radio
   voice protocols, voice packets may need to be processed at the RAN
   gateway for maximum efficiency and robustness over the radio medium.

7.0 Summary

   Most proposals for IP mobility require a one-to-one mapping between
   the mobile terminal and a base station with which it is in contact,
   and assume end-to-end connectivity and consequently routing in the
   radio access network based on the mobile's IP address. In CDMA
   networks, macrodiversity and the need for extremely low delay and
   jitter invalidate these assumptions. Consequently, IP mobility
   solutions are not applicable to CDMA soft handoff. These
   considerations do not, however, apply to hard handoff which occurs in
   the core network after frame selection.

8.0  References

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  [1] Perkins, C. (ed.), IP Mobility Support for IPv4, revised, draft-
        ietf-mobileip-rfc2002-bis.txt (work in progress), January, 2000.

  [2] Vakil, Faramak, et. al., Host Mobility Management Protocol:
        Extending SIP to 3G-IP Networks, draft-itsumo-hmmp-00.txt (work
        in progress), October, 1999.

  [3] E. Wedlund and H. Schulzrinne. Mobility Support Using SIP.  Second
        ACM/IEEE International Conference on Wireless and Mobile
        Multimedia (WoWMoM'99). Seattle, Washington. Aug. 1999.

  [4] O'Neill, A. and Corson, S., Edge Mobility Architecture, draft-
        oneill-ema-00.txt (work in progress), October, 1999.

  [5] Campbell, A., et. al., Cellular IP, draft-ietf-mobileip-
        cellularip-00.txt, January, 2000.

  [6] Kempf, J. and Calhoun, P., Foreign Agent Assisted Hand-off,
        draft-calhoun-mobileip-proactive-fa-00.txt (work in progress),
        January, 2000.

  [7]  Ramjee, R., et. al., IP micro-mobility support using HAWAII,
        Internet Draft (work in progress), June 1999.

  [8] IEEE, "Wireless LAN Medium Access Control and Physical Layer
        Specifications", IEEE Std 802.11-1997, November 1997.

9.0  Authors' Addresses

   Questions about this memo can be directed to:

      James Kempf
      Network and Security Research Center, Sun Labs
      Sun Microsystems, Inc.
      901 San Antonio Rd., UMPK15-214
      Palo Alto, CA, 94303

       Phone: +1 650 786 5890
         Fax: +1 650 786 6445
      E-Mail: james.kempf@sun.com

      Peter McCann
      Bell Laboratories
      263 Shuman Boulevard
      Room 2Z-305

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      P.O. Box 3050
      Naperville, IL, 60566

       Phone: +1 630 713 9359
         Fax: +1 630 713 4982
      E-Mail: mccap@research.bell-labs.com

      Philip Roberts
      Motorola, Inc.
      1501 W. Shure Dr.
      Arlington Heights, IL, 60015

      Phone:  +1 847 632 3148
      E-Mail: qa3445@email.mot.com

9.0  Full Copyright Statement

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

   This document and translations of it may be copied  and  furnished
   to others,  and  derivative works that comment on or otherwise
   explain it or assist in its implementation may be prepared, copied,
   published and distributed,  in  whole  or  in part, without
   restriction of any kind, provided that the  above  copyright  notice
   and  this  paragraph  are included on all such copies and derivative
   works.  However, this docu- ment itself may not be modified in any
   way, such as  by  removing  the copyright notice or references to the
   Internet Society or other Inter- net organizations, except as needed
   for  the  purpose  of  developing Internet standards in which case
   the procedures for copyrights defined in the Internet Standards
   process must be followed, or as required  to translate it into
   languages other than   English.  The limited permis- sions granted
   above are perpetual and  will  not  be  revoked  by  the Internet
   Society or its successors or assigns.  This document and the
   information contained herein is provided on an "AS IS" basis  and

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