Subject: [SIP] Applicability of IP Mobility to CDMA Soft Handoff

From: James Kempf <[email-masked]>
Date: Wed, 3 May 2000 08:19:33 -0700 (PDT)

There has been some discussion on the mobile IP and SIP lists about
replacing CDMA soft handoff with IP mobility mechanisms. The attached draft, 
authored by Pete McCann, Phil Roberts, and myself, attempts to explain how
a CDMA RAN works and why soft handoff is not a promising candidate
for replacing with IP mobility mechanisms in the near term. In contrast,
the analysis is not applicable to hard handoff, which is a potential
candidate for replacing with IP mobility. We have submitted this to
the drafts alias as an individual contribution 

Comments are welcome.

		jak

INTERNET DRAFT                                                James Kempf
Category: Informational                             Sun Microsystems, Inc.
Title: draft-kempf-cdma-appl-00.txt                           Peter McCann
Date: April 2000                                       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
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   material or to cite them other than as "work in progress."

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      http://www.ietf.org/ietf/1id-abstracts.txt

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

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Abstract

   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 RANs rather than between base
   stations within a RAN.

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
   RANs is very frequent, their utility is lessened in the presence of

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

   In this paper, we discuss CDMA radio access networks and why IP
   mobility solutions (including mobile IP) are not applicable to soft
   handoff in a CDMA radio access network. In effect, the CDMA RAN
   network with soft handoff is an application layer transport and
   mobility mechanism, and therefore not an appropriate candidate for
   moving into the network layer. 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.

   RAN
     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.

   hard handoff
     The process by which a moving mobile terminal is transferred

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     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).

   macrodiversity
     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 contain complete IP
     packets, 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:

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

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     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
     fashion,

     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,

     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
     battery).

     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
     moves.

   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
   the radio frames.

   The following figure illustrates the architecture and how the CDMA

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   RAN network works:

                         Core Network

                            ^
                            |   incoming/outgoing octet stream
                            |    from/to corresponding node (129.142.68.79)
                            |
                            V
   +-----------------------------------------------------+
   | RAN Gateway                                         |
   |                                                     |
   |    +-------------+  +-----------+  +----------+     |
   |    |             |  |           |  |          |     |
   |    | Frame       |  | Frame     |  | Soft     |     |-----------
   |    | Selection   |  | Splitting |  | Handoff  |     |       to another
   |    |             |  |           |  | Control  |     |       RAN Gateway
   |    +-------------+  +-----------+  +----------+     |
   |                                                     |
   |    +---------------+                                |
   |    |               |                                |
   |    | Radio Frame   |                                |
   |    | Processing    |                                |
   |    |               |                                |
   |    +---------------+                                |
   |                                                     |
   +-----------------------------------------------------+
            |             |              |            |    *
            |             |              |            |      *
            |             |     ...      |            |       * radio
            |             |              |            |       * frames
            |             |              |            |      *
            |             |              |            |    *
       +---------+   +---------+   +---------+   +---------+
       | Base    |   | Base    |   | Base    |   | Base    |
       | Station |   | Station |   | Station |   | Station |
       |   B1    |   |   B2    |   |  B42    |   |   B43   |
       +---------+   +---------+   +---------+   +---------+
            \              |
             \             V
              \--------->
                           |
                         -----
                        /     \
                    -----       -----           162.42.42.42
                   | mobile terminal|(  --->
                   ------------------
                     ( )        ( )

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   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 162.42.42.42
   is corresponding with a host having the IP address 129.142.68.79,
   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 129.142.68.79 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 162.42.42.42 to
   129.142.68.79, 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
   responsible for making sure that the jitter and delay constraints are
   met, and for processing packets from the core network into radio
   frames.

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   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 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
   following:

     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
   station.

   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

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   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
   node.

   In effect, the RAN controller is an application level transport and
   mobility mechanism specialized to the CDMA radio medium. It is
   therefore not a good candidate for replacing with 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
   here.

   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
   transparency.

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

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   However, there is still a problem with 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. A multicast routing algorithm with routing tree
   modifications that converge in real time might contribute to solving
   the problem. On the other hand, the reverse leg traffic would still
   need to be combined in the wired network behind the base stations.

   While the prospects for moving IP into the RAN are good, it seems
   unlikely that IP mobility will play any role in replacing CDMA soft
   handoff in the immediate future. CDMA soft handoff appears to be a
   very specialized form of transport-level or link layer mobility, 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

  [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-

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        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.

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
      USA

       Phone: +1 650 786 5890
         Fax: +1 650 786 6445
      E-Mail: [email-masked]

      Peter McCann
      Bell Laboratories
      263 Shuman Boulevard
      Room 2Z-305
      P.O. Box 3050
      Naperville, IL, 60566
      USA

       Phone: +1 630 713 9359
         Fax: +1 630 713 4982
      E-Mail: [email-masked]

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

      Phone:  +1 847 632 3148
      E-Mail: [email-masked]

9.0  Full Copyright Statement

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

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---

From: "Ashutosh Dutta" <[email-masked]>
Date: Wed, 3 May 2000 13:52:15 -0400

HI James & co,  I have few questions after my first reading of the draft
Page 7:
             When you talk about RAN-RAN interconnection (closley coupled way),
what is your assumption? Does each neighboring  RAN (RAN-GW) belong to a
different IP subnet? or these RAN-GWs belong to the same subnet but are just
distributed along.

Page 6:

               Client listening to multiple base stations (3 or 6) at the same
time: Do you assume multiple interfaces on the client or just one?
In cases where the neighboring BSs for a mobile client are part of RANs
belonging to different subnets ( and controlled by different RAN-GW), how does
the RAN-GWs interact with each other to take care of soft-handoff, probably some
sort of IP-mobility solution like IP multicast can be helpful here.

Thanks
Ashutosh

_______________________________________________
SIP mailing list
[email-masked]
http://lists.bell-labs.com/mailman/listinfo/sip

---

From: James Kempf <[email-masked]>
Date: Thu, 4 May 2000 08:22:07 -0700 (PDT)

>different IP subnet? or these RAN-GWs belong to the same subnet but are just

Whether or not they belong to a different subnet is not important from the
point of view of a moving terminal, since the terminal's IP packets
are not routed through the RAN based on the terminal's IP address.
The termimal's packets are processed into radio frames, in which
the IP packets are encoded. If the RAN is IP based (it may not be and, in fact, 
in current 2G and 3G designs is not), then the links are typically point to 
point or at most one hop, due to the stringent timing constraints. The
radio frames in an IP-based RAN would run on top of IP (maybe UDP) directly, and
the terminal's IP packets would not be identifiable in the RAN as 
such, except by performing the same processing the terminal's CDMA
hardware performs to decode the radio frames.

>sort of IP-mobility solution like IP multicast can be helpful here.

The client has only one "interface" in the IP sense. A mobile CDMA
phone uses a signal processing technique to combine the incoming
signals to produce a single packet. This process proceeds at
the physical layer, i.e. L1, and therefore the upper layers never
see more than a single packet as was incoming to the frame selector/
distributor from the corresponding node. So the phone can have only a single IP 
address for the CDMA connection, even though at the physical layer there are 
multiple signals coming in with the same (modulo errors) octet stream.

This characteristic is the single largest deterrent to using IP
mobility in CDMA RANs.

		jak