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Versions: 00 01                                                         
Internet Draft        Cellular RAN Resource Issues             June 2001


Internet Engineering Task Force                          D. Partain (ed)
INTERNET-DRAFT                                            G. Karagiannis
Expires December 2001                                       P. Wallentin
                                                             L. Westberg
                                                                Ericsson
                                                               June 2001

     Resource Reservation Issues in Cellular Radio Access Networks
               draft-westberg-rmd-cellular-issues-00.txt





   Status of this Memo

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

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.
















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1.  Abstract


   The rapidly growing popularity of IP and its flexibility make it a
   good candidate to be used for transmission in cellular networks.

   Using IP-based transport on the wired transmission links in the
   cellular networks gives operators an opportunity to upgrade their
   transport network to a packet-based one. When compared with a
   traditional STM-based system, the gain is seen in the statistical
   aggregation of traffic that can be done.  This results in increased
   transmission efficiency and reduced leasing cost for the operator.

   A radio access network (RAN) provides the radio access (e.g., GSM,
   CDMA, or WCDMA) to mobile stations in a cellular network.  To
   accomplish this, radio frames are transported on the wired links
   between different cellular-specific nodes in the RAN. The majority of
   the traffic (up to 100%) is delay-sensitive traffic.

   The cellular user is unaware of the IP-based transport network
   underneath, and the service must work the same way as the user has
   come to expect the cellular services to work in an STM-based
   transport network. In addition to this requirement, the situation is
   further complicated by the fact that the RAN is large in terms of its
   geographic size, the number of inter-connected nodes, and the
   proportion of real-time traffic.

   To satisfy the above requirements, it is absolutely critical that we
   have a simple and scalable bandwidth resource management scheme for
   real-time traffic in this type of network.

   In order for real-time services to function satisfactorily in an IP-
   based RAN, we need to ensure that there are adequate transport
   resources on the links available in the RAN to handle this particular
   instance of the service (e.g., a phone call).  Note that, in rest of
   this draft, whenever the term "resources" is used, it refers to
   bandwidth on the links.

   If the RAN is bandwidth-limited and does not use any mechanism to
   limit the usage of the network resources, congestion might occur and
   degrade the network performance.  For example, speech quality might
   degrade due to packet losses.  Note that this memo is a successor of
   the draft "draft-partain-wireless-issues-00.txt".







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2.  Terminology

The following terminology is used in this memo:

 * BSC:  Base Station Controller

 * RNC:  Radio Network Controller

 * MSC:  Mobile Services Switching Center

 * GGSN: Gateway GPRS Support Node

 * SGSN: Serving GPRS Support Node

 * GPRS: General Packet Radio Service, the packet-switched access
         scheme and service provided in GSM.

 * GSM:  Global System for Mobile Communications

 * UMTS: Universal Mobile Telecommunications System, the third
         generation (3G) mobile system based on WCDMA and GSM,
         specified by 3GPP (third generation partnership project).

 * radio frame: a short data segment coded/decoded and
         transmitted/received by the radio base station.
         It originates from a mobile station or the BSC/RNC.

 * WCDMA: Wideband Code Division Multiple Access, the
        radio transmission technology used in UMTS


3.  Background and motivation


   The context of the issues described in this document is the cellular
   radio access network (RAN).  This section briefly discusses two
   examples of radio access networks (for GSM - Global System for Mobile
   Communication - and for WCDMA - Wideband Code Division Multiple
   Access) and then outlines the motivation for this memo. Note that
   this memo is a successor of the draft "draft-partain-wireless-
   issues-00.txt".









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3.1.  IP transport in radio access networks

   This section introduces the radio access network and its use of IP
   transport. A radio access network (RAN) provides the radio access
   (e.g., GSM, CDMA, or WCDMA) to mobile stations for a cellular system.
   The boundaries for a RAN are the radio transmission and reception
   access points (terminated by base stations) at one end and, at the
   other end, the interfaces to the gateways (e.g., MSC and SGSN/GGSN),
   which in turn provide connections to the fixed public network.

   The radio access network consists of a number of nodes as shown in
   the Figure 1 below.

                             IP              IP
                            <--->           <--->

                 |---------|     |---------|     |---------|
   Mobile     v  |  Base   |     |         |     |  MSC/   |    Fixed
   stations   |--| station |-----| BSC/RNC |-----|  SGSN/  |--- public
            ^    |         |  ^  |         |  ^  |  GGSN   |    network
            |    |---------|  |  |---------|  |  |---------|
            |                 |               |
        Wireless            Wired           Wired
        interface         interface       interface

                   Radio access network
              <------------------------------->

        Figure 1:  Typical radio access network and its boundaries

   The base station provides the radio channel coding/decoding and
   transmission/reception function to and from mobile stations in its
   coverage area, which is called a cell.

   The BSC/RNC controls a number of base stations including the radio
   channels and the connections to mobile stations.  For a WCDMA radio
   access network, the BSC/RNC provides soft handover combining and
   splitting between streams from different base stations belonging to
   the same mobile station.  Furthermore, the BSC/RNC is also
   responsible for the allocation of transport resources within the
   radio access network.  The transport is either between the base
   station and the BSC/RNC, between multiple BSC/RNCs, or between the
   BSC/RNC and the MSC/SGSN.







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   The MSC provides, among others things, support for circuit-switched
   services towards mobile stations including mobility management,
   access control and call control as well as interworking with external
   circuit-switched networks such as the public switched telephony
   network (PSTN).  The SGSN/GGSN provide, amongst other things, support
   for packet switched services towards mobile stations, including
   mobility management, access control and control of packet data
   protocol contexts. In addition, the GGSN provides interworking with
   external packet-switched networks such as the public Internet.

   The radio access network consists of potentially thousands of base
   stations and a significant number of BSCs/RNCs.  The traffic volume,
   in terms of voice-traffic, generated by these nodes can vary from a
   few up to fifty voice calls per base station, and up to several
   thousand simultaneous calls (Erlang) per BSC/RNC site.  Therefore, a
   router in the network has to handle many thousands of simultaneous
   flows.

   The transmission between base stations and the BSC/RNC is usually on
   leased lines, and this part (due to the wide area coverage of the
   cellular network) is usually extremely expensive when compared to the
   cost of transmission in the backbone.  Due the number of base
   stations, the cost for these leased lines can be quite significant.
   Dimensioning using over-provisioning might therefore be prohibitively
   expensive, and it is unlikely that the network will be dimensioned
   without using the statistical properties of traffic aggregation
   (e.g., Erlang trunking).























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    |--------|
    | Upper  |<--------------------------------------------> towards the
    | layers |                                               MSC/SGSN/GGSN
    |--------|                                     |---------|--------|
    |  Radio |                                     |  Radio  |        |
    |  Link  |<----------------------------------->|  Link   |        |
    |  Layer |                                     |  Layer  |        |
    |--------|                                     |---------|        |
    |        |                                     |Radio    |        |<-
    |        |                                     |Physical | Frame  |
    |        |                                     |Layer    | transp.|
    |        |   |-------|--------|                |---------| layer  |
    |        |   |       | Frame  |<-------------->| Frame   |        |
    |        |   |       | transp.|                | transp. |        |
    |Radio   |   | Radio |--------|                |---------|--------|
    |Physical|   | Phys. |  UDP   |<-------------->|  UDP    |  UDP   |
    |Layer   |<->| Layer |--------|   |--------|   |---------|--------|
    |        |   |       |   IP   |<->|   IP   |<->|   IP    |   IP   |
    |        |   |       |--------|   |--------|   |---------|--------|
    |        |   |       | Link   |   | Link   |<->|  Link   |  Link  |
    |        |   |       | Layer  |<->| Layer  |   |  Layer  |  Layer |
    |        |   |       |--------|   |--------|   |---------|--------|
    |        |   |       |Physical|   |Physical|   |Physical |Physical|
    |        |   |       | Layer  |<->| Layer  |<->| Layer   | Layer  |
    |--------|   |-------|--------|   |--------|   |------------------|
     Mobile          Base station       Router            BSC/RNC
     Station   ^                    ^            ^
               |                    |            |
           Wireless               Wired        Wired
           interface            interface     interface

                                 Radio access network
                 <----------------------------------------------------->

         Figure 2:  Example of a protocol stack in the
               radio access network (simplified)

   Figure 2 shows a simplified example of protocol layering when using
   IP transport in the radio access network.

   The radio physical layer performs radio transmission and reception
   functions, including soft handover splitting and combining in case of
   WCDMA.

   The frame transport layer is used to transmit radio frames between





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   the base station and the BSC/RNC.  A radio frame is a short data
   segment coded/decoded and transmitted/received by the radio base
   station at a given point in time. The radio frames must be delivered
   in a timely fashion with limited delay.  Otherwise, the frames are
   discarded by the base station or RNC/BSC. The traffic is therefore
   very sensitive to delays.

   The radio link layer performs segmentation/re-assembly,
   retransmission and multiplexing/scheduling functions as well as radio
   resource control.  The adaptation of user data performed by the radio
   link layer depends on the type of radio channel and the type of
   service. In one case, that very small radio frames might be
   transferred, while in other cases the packets are significantly
   larger.

   Introducing IP in the radio access network implies that an IP QoS-
   capable domain, e.g. a Differentiated Services domain, will have to
   be introduced and managed in the radio access network. This domain
   consists of edge and interior nodes, where the edge nodes are the
   nodes located at the boundary of the domain.  All the nodes which are
   part of this QoS-capable domain and are not edge nodes are defined as
   interior nodes.

   An edge node can be defined as an ingress node, or a node that
   handles the traffic as it enters the QoS-capable domain.
   Alternatively, an edge node might be an egress node, or a node that
   handles the traffic as it leaves the QoS-capable domain.  In this
   memo, an edge node (ingress or egress) is denoted as the first hop
   router that the base station or BSC/RNC is connected to. The first
   hop router might be a part of the base station or BSC/RNC.

   Furthermore, the base station and BSC/RNC must be able to handle
   algorithms used for purposes other than edge node functionality that
   are many times more complex than the algorithms required for handling
   the edge node functionality. Therefore, the edge node functionality
   will only have minimal impact on the complexity of the base station
   or BSC/RNC.


3.2.  Motivation for this memo



   The issues described in this document concern only the cellular radio
   access network (RAN).





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   The architecture of the RAN and the nature of the transported data
   mean that the RAN has different characteristics when compared with
   other IP-based networks.  However, those differences, which are the
   motivation for this memo, are limited to the domain of the RAN and do
   not extend into the backbone of the IP network.

   In order for the transport within the RAN to function satisfactorily,
   even if the transport network is IP-based, we need to ensure that
   there are adequate resources in the transport network to meet the
   needs of the data flows between the nodes within the RAN.

   Based upon the characteristics of the RAN (described in Section 4
   below), the current strategies for resource management do not meet
   the requirements for an appropriate resource management strategy
   within a RAN.  This document seeks to initiate a dialog on how to
   correct that situation.


4.  Network characteristics of cellular access networks today


   Cellular RANs today have a unique set of characteristics compared to
   other kinds of IP networks.  These characteristics result in a set of
   requirements on any resource reservation scheme that might be used in
   the RAN.


4.1.  General aspects of the network structure

   The network structure for cellular radio access networks can be
   described as having the following characteristics:

    * Operator relationship

      The RAN is typically controlled by a single cellular operator
      with full control over the network.  The IP network used to
      transport radio frames might be leased from another operator
      or be built by the same cellular operator.  This network
      could be thought of as an "intranet".

    * Size of network

      RANs can be very large routed networks.  Networks including
      thousands of nodes are certainly within reason.






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    * Traffic volume

      The traffic from a large number of radio base stations
      needs to be supported by the same transport network. Even
      if a single radio base station generates a modest volume of
      traffic, the total number of flows for radio frame transport
      in the radio access network is very large.

    * Transmission sharing

      The network between the BSC/RNC and the base stations is
      built to support transmission sharing between different
      nodes even if they are geographically distributed.
      One transmission link (possibly with redundancy) can
      support more than one base station. In other words, one
      piece of hardware can serve more than one base station and
      therefore can support more cells in one location, such as
      a three sector site. This means that the cells that are
      located at the same location will have to compete for the
      same transmission resources.

    * Unicast transport of radio frames

      The transport of radio frames in the radio access network is
      point-to-point transport. Even if the soft handover splitting
      in the BSC/RNC is multicast of radio frames in some sense,
      this is handled above the IP layer by the frame transport
      protocol. For each radio channel in each base station the
      frame transport protocol needs a separate flow. Therefore,
      the frame transport protocol requires unicast transmission
      from the IP layer.


4.2.  High cost for transmission

   The transmission between base stations and the BSC/RNC is usually on
   leased lines, and this part (due to the wide area coverage of the
   cellular network) is usually extremely expensive when compared to the
   cost of transmission in the backbone.  Due to the number of base
   stations, the cost for these leased lines can be quite significant.
   Even if the cost for the leased line decreases over the years, the
   "last mile" to the base station is likely to be expensive due to the
   location of the base station.

   Cellular RANs are built over a very wide geographic area. There are,





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   of course, many different networks that cover a wide geographic area
   (e.g., across USA, and the world), but the dispersion of nodes over
   the area in the RAN case is different. Due to the fact that the base
   stations are positioned based on a radio network perspective, i.e.,
   radio coverage, and not based on a transmission perspective, a large
   proportion of nodes are distributed throughout rural and urban areas,
   not close to installed high-capacity transmission hubs.  Even worse,
   the the base stations could be positioned far out in the countryside.

   The peak bitrate of the multirate radio channels is selected on-
   demand.  To utilize the bandwidth of the expensive transmission links
   used for radio frame transport efficiently, dimensioning using over-
   provisioning might therefore be prohibitively expensive, and it is
   unlikely that the network will be dimensioned for peak allocation.
   Dynamic allocation and optimization to reduce the cost are therefore
   a fundamental requirement.  Resource reservations make it possible to
   have high utilization of the network for real-time sensitive traffic
   as well as avoiding congestion in the network.


4.3.  Transportation of radio frames

   The traffic in cellular access networks is dramatically different
   from the Internet in general.  The Internet primarily supports best-
   effort traffic today, while the traffic on a RAN is (at least today)
   largely real-time traffic. The network from the base station to the
   BSC/RNC is the part of the network that has the highest volume of
   real-time traffic and where delay must be minimized as much as
   possible. The reasons for the this are:

    * End-to-end delay for speech traffic consists of delays in the
      mobile stations, the RAN and in the MSC (see Figure 1 in
      Section 3.1). The major portion of delay in the RAN is caused
      by the radio-related functionality (e.g., interleaving and
      coding in the base station and adaptation in the BSC/RNC).
      Therefore, the combined delay in all parts (MSC, radio
      network, and mobile stations) must be minimized as much
      as possible to give the end user proper speech quality.

    * Handover is a major issue.  For GSM, with typically multiple
      handovers per call, excessive delay in the control, e.g.
      radio network control traffic, of the radio network will
      cause a longer handover interruption period. The majority
      of handovers are also made within the radio network.






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    * The transport of radio frames is very delay-sensitive. In
      the direction from the BSC/RNC to the base station, a radio
      frame is a short segment of data (payload) to be coded and
      transmitted on a given radio channel by the base station at a
      given point in time. In the direction from the base station
      to the BSC/RNC, a radio frame is a short segment of data
      (payload) that was received and decoded by the base station
      at a given point in time and potentially needs to be combined
      in the BSC/RNC with radio frames received by other base
      stations at the same point in time as this particular frame.
      Note that the data segment in a radio frame may contain user
      data but also control signalling information and the same
      type of synchronized frame transport is needed for almost
      all kinds of radio channels and is generally not coupled
      to the type of service. Therefore, even if an end to end
      application is best effort, the transport of the radio frames
      originating from this application might be treated as real-time
      within the radio access network.


   The real-time traffic on the RANs is today almost exclusively voice
   (up to 90% with 10% signalling), but the cellular systems are
   evolving to provide capabilities for other kinds of real-time traffic
   (e.g., video).  Nonetheless, voice continues to be one of the most
   important sources of revenue in most cellular environments today. The
   transport resources are today allocated when the call is accepted,
   and the radio frame transport over an IP network has to provide the
   same guarantees. If real-time traffic cannot be engineered to work
   correctly, the primary revenue stream will disappear.

   Some of the sources, such as video-based services and gaming, will be
   able to send data at a variable bitrate at higher rates. For radio,
   the rates of the radio channels are selected on-demand.  In reality,
   the radio network can support a wide range of partitioning of the
   radio resources among the different radio channels. A rather large
   portion of the transmission resources between the base station and
   BSC/RNC will have to be allocated for such services. To be able to
   utilize the bandwidth used for radio frame transport efficiently, the
   same flexibility is required in assignment of the transport resources
   as in the air-interface. Therefore, statically-assigned resources
   will induce a cost which is too high for the operator.









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4.4.  Mobility aspect of radio frame transport

   The mobility of the mobile stations imposes strong requirements on
   managing the transmission resources available in the RAN,
   Furthermore, this also implies that there are strong requirements on
   the RAN's internal signaling and not only on transferring of packets
   sent by the mobile station.

   Hard handover is one of the issues. For GSM, with typically multiple
   handovers per call, excessive delay in the control of the radio
   network will cause a longer handover interruption period. Typically,
   most of the handovers will be made between base stations controlled
   by the same BSC and therefore extensive delay between base station
   and BSC will degrade more than delays in the MSC and SGSN/GGSN
   network.

   Moreover, for maximal utilization of radio spectrum in WCDMA (and
   also in CDMA), fast and frequent (soft) handover operations between
   radio channels and radio base stations are required. The frequency of
   handover events is therefore typically higher in WCDMA radio access
   networks than in GSM and means even higher performance requirements
   on the transport solution. If the soft handover cannot  be performed
   fast enough, spectrum cannot be utilized efficiently, which will
   cause degradation of the radio network capacity. At each handover
   event, resource reservation is needed, and therefore resource
   reservation needs to be fast and will be used very frequently.

   The impact of mobility in the radio access network has therefore two
   major differences compared to the fixed network:
    (1) High volume of resource reservation events
    (2) Requirement on short response time for reservations


5.  Requirements on a Resource Reservation Scheme

   This section outlines what we believe are the fundamental
   requirements placed on any resource reservation scheme in a cellular
   radio access network. Later sections will outline how current schemes
   match these requirements.


5.1.  Main requirement on resource reservation scheme


   One of the primary requirements that real-time applications impose on





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   any resource reservation scheme is the provisioning of good QoS
   (delay and packet loss) guarantees. This can only be achieved if the
   network can be utilized while avoiding congestion and without having
   too high packet losses. The level of utilization depends on network
   topology, traffic mix, scheduling discipline, delay and packet loss
   requirements. The utilization is given by network dimensioning but
   should be as high as possible.


   The resource reservation scheme must be able to keep the real-time
   traffic under a certain pre-defined network utilization in order to
   bound congestion. Otherwise, it is impossible to guarantee percentile
   bounds on the QOS requirements for real-time traffic.



5.2.  IP must provide same service behavior as the transport networks
used today


   Today's commercial IP networks are mainly optimized to carry best-
   effort traffic. As explained above and also discussed in [WeLi99],
   the transport of radio frames in the radio access network puts real-
   time requirements on the underlying transport network.  All of these
   characteristics are fulfilled by the connection-oriented transport
   networks (STM and ATM) used by cellular networks today.  By, at a
   minimum, meeting these same requirements, the IP networks will be
   capable of providing the same behavior as the transport networks that
   are currently used by cellular systems while gaining the advantages
   of IP networking. It should be noted that IP networks will be able to
   meet these requirements only if the following two constraints are
   met:

   (1) that service guarantees are percentiles, see Section 5.1
   (2) strictly limited to a given operator's IP network, see
       Section 5.9.


5.3.  Efficient network utilization


   Due to the high cost of the leased transmission, we must utilize the
   network to the highest degree possible, and this must be facilitated
   by the resource reservation scheme.






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   However, in considering a resource reservation scheme, its impact
   upon the performance and scalability of the network as a whole must
   also be taken into account.  For example, the performance and
   scalability impact on the edge and internal routers is a very
   important consideration.




5.4.  Handover performance requirements on resource reservation scheme


   Whatever reservation scheme is used must be highly performant for at
   least the following reasons:

     * Handover rates

       In the GSM case, mobility usually generates an average
       of one to two handovers per call. For third generation
       networks (such as WCDMA and cdma2000), where it is
       necessary to keep radio links to several cells
       simultaneously (macro-diversity), the handover rate is
       significantly higher (see for example [KeMc00]).
       Therefore, the admission control process has to cope
       with far more admission requests than call setups alone
       would generate.

     * Fast reservations

       Handover can also cause packet losses. If the processing
       of an admission request causes a delayed handover to the
       new base station, some packets might be discarded, and
       the overall speech quality might be degraded
       significantly.

       Furthermore, a delay in handover may cause degradation
       for other users. This is especially true for radio access
       technologies using macro-diversity, such as WCDMA and CDMA,
       where a handover delay will cause interference for other
       users in the same cell. Furthermore, in the worst case
       scenario, a delay in handover may cause the connection
       to be dropped if the handover occurred due to bad radio
       link quality.

       Therefore, it is critical that an admission control





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       request for handover be carried out very quickly. Since
       the processing of an admission control request is only
       one of many tasks performed during handover, the time to
       perform admission control should be a fraction of the
       time available for handover.
       Furthermore, in the situation that the transport network
       in the RAN is over-utilised it is preferable to keep
       the reservation on already established flows while new
       requests might be blocked. Therefore, the handover
       requests for resource reservation should be treated
       with a higher priority than the new requests for
       resource reservation.




5.5.  Edge-to-edge reservations, not end-to-end



   Real-time applications require a high level of quality of service
   (QoS) from the underlying transmission network. This can only be
   achieved by accomplishing the QoS management on an end-to-end basis
   (i.e., end user to end user), from application to application,
   potentially across many domains.

   However, this does not mean that the resource reservation protocol
   must be applied end-to-end. The end-to-end QoS management
   architecture may consist of many interoperable edge-to-edge QoS
   management architectures where each of them might use a different
   edge-to-edge resource reservation protocol.  In fact, this is far
   more likely to be the case than that a global signaling structure
   will be available across all different domains in an end-to-end
   perspective.  This will increase the flexibility and the openness of
   the transmission network since various access networks that are using
   the same transmission network and different edge-to-edge QoS
   management architectures will be able to interoperate.

   It is critical that the appropriate mechanisms for providing the
   service guarantees needed in the radio access network be put in place
   independently of solving the more difficult problem of end-to-end
   QoS.

   In our case, the access network is simply an intranet in which we
   need to solve a local QoS problem.  This implies that a general





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   solution which handles the end-to-end QoS problem is unnecessarily
   complicated for solving the intranet problem in the cellular access
   network.


5.6.  Reservation functionality in edge nodes versus interior routers

   In our network, it is important that the reservation mechanism be as
   simple as possible to implement in the interior nodes since in most
   cases there might be more interior routers (<= 10 depending on
   network structure) in the path than there are edge nodes.

   Typically, in a RAN there are two edge nodes located in a
   communication path. Moreover, the average number of interior nodes in
   a communication path within a RAN depends on the chosen network
   topology by the RAN operator.  As such, the scheme must be optimized
   for the interior nodes and not for the edge nodes, thus reducing the
   requirements placed on the functionality of the interior routers.
   This means that we can have complicated mapping of traffic parameters
   at the edges and a simplified model in the interior nodes, and that
   the necessary set of parameters required for setting up reservations
   shall be based upon their effect on interior nodes and not on edge
   nodes.

   The edge routers typically have to perform per-session
   management/control, and hence complex per flow handling is not a
   significant burden.


   However, interior nodes do not need to have per flow
   responsibilities. We must therefore optimize for simple QoS
   mechanisms on these interior devices, and use more complex mechanisms
   in the edge devices.


   In our case, edge device functionality is implemented in the first
   hop router that the base station or BSC/RNC is connected to (see
   Section 3.1). In this way we optimize for simple QoS mechanisms on
   the interior devices, while the more complex mechanisms are applied
   on the edge devices, e.g., base station, BSC/RNC.

   This emphasis on simplicity is due to performance requirements listed
   above.  We need to make sure that we understand the minimal level of
   functionality required in the reservation scheme in order to
   guarantee the performance of real-time traffic.





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5.7.  On-demand and dynamic allocation of resources

   Real-time services require that a portion of network resources is
   available to them. These resources can be reserved on a static or
   dynamic basis, or potentially based on some kind of measurement of
   network load.

   In the first situation, this may result in an poorly utilized
   network. This is mainly due to the fact that the network resources
   are typically reserved for peak real-time traffic values.  Mobility
   in the network makes static configuration even less desirable as the
   resources will be used even less effectively.

   If using dynamic allocation, this problem will be avoided since the
   resources are reserved on demand.  However, the load from resource
   reservation will be much higher than if static allocation of
   resources is used.  If the dynamic allocation of the resources is
   done on a micro-flow basis, the resulting network load from resource
   reservation might be quite high.

   We might use other methods, such as measurement-based admission
   control, to simplify the reservation protocol, as long as these
   methods can fulfill the requirements (now or in the future).

   What is important is that all of these mechanisms can be used for
   solving part of the network utilization problem, and, as such, any
   reservation scheme must have the flexibility to provide both on-
   demand reservations as well as measurement-based admission control.

   As high bitrate and variable bitrate applications enter the cellular
   space, the need for on-demand reservations of resources will become
   even more acute.



5.8.  Unicast and not multicast



   The majority of the traffic in the RAN is point-to-point unicast
   transport of radio frames between the base station and the BSC/RNC.
   As such, the resource reservation scheme need not to be optimized for
   multicast.







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5.9.  A single operator in the RAN


   It is realistic to assume that end-to-end communication in IP
   networks as well as the end-to-end QoS management architectures will
   be managed by more than one operator.

   Furthermore, it also realistic to assume that an edge-to-edge
   resource reservation protocol can be managed by one single operator.
   As such, it is reasonable to limit reservation scheme to a single
   operator domain. This will ensure that each operator can optimize the
   edge-to-edge QoS management architecture for their needs. Moreover,
   this limitation (a single operator domain) means that the reservation
   scheme does not need to handle the issues inherent in a multi-
   operator domain, thus simplifying the scheme.



5.10.  Minimal impact on router performance


   The performance of each network node that is used in an end-to-end
   communication path has a significant impact on the end-to-end
   performance of this communication path. Therefore, the end-to-end
   performance of the communication path can be optimized by optimizing
   the router performance.  It is absolutely critical that the
   introduction of QoS mechanisms and signaling does not overly impact
   the performance of the infrastructure.  Obviously, you cannot
   introduce new things that need to be done by networking
   infrastructure without impacting its performance, but that impact
   must be minimized to the greatest extent possible.

   One of the factors that can contribute to this optimization is the
   minimization of the resource reservation signaling protocol load on
   each router. When the dynamic allocation of the resources is on a per
   micro-flow basis, the resource reservation signaling protocol could
   easily overload a router located in a core network, causing severe
   router performance degradation.  Furthermore, any mechanisms defined
   must be such that it is reasonable to implement them in hardware
   which will increase the scalability of the solution.










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5.11.  Scalably Manageable


   Any strategy for resource management in a RAN must be done in such a
   way that it is easily manageable in a very large network.  This
   implies as little "laying on of hands" as possible and as much
   automation as possible.  In networks made up of many thousands of
   routers, changing of even a single parameter in all routers may be
   prohibitively difficult.  Minimizing the involvement of the operator
   (or the operator's management tools) is therefore an important
   requirement.


5.12.  Bi-directional reservations


   In current RANs, the BSC/RNC is responsible for initiation of
   reservation of resources in the transport plane.  Therefore, via the
   resource reservation signaling protocol, the BSC/RNC has to support
   the initiation and management of the resource reservations for both
   directions, both to and from the base station, simultaneously. In
   this way a simpler edge-proxy resource reservation functionality will
   be implemented in the base station, decreasing its complexity.


6.  Evaluation of existing strategies


   In order to understand whether technology exists today which will
   allow us to manage the resources in cellular networks, we briefly
   look at the protocols that currently exist which address parts or all
   of these requirements.


6.1.  End-to-end per-flow resource reservation protocol


   An end-to-end per-flow resource reservation signaling protocol is
   applied in an end-to-end IP communication path, and it can be used by
   an application to make known and reserve its QoS requirements to all
   the network nodes included in this IP communication path.  This type
   of protocol is typically initiated by an application at the beginning
   of a communication session. A communication session is typically
   identified by the combination of the IP destination address,
   transport layer protocol type and the destination port number.  The





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   resources reserved by such a protocol for a certain communication
   session will be used for all packets belonging to that particular
   session.  Therefore, all resource reservation signaling packets will
   include details of the session to which they belong.

   The end-to-end per-flow resource reservation signaling protocol most
   widely used today is the Resource Reservation Protocol (RSVP) (see
   [RFC2210], [RFC2205]). The main RSVP messages are the PATH and RESV
   messages.  The PATH message is sent by a source that initiates the
   communication session. It installs states on the nodes along a data
   path.  Furthermore, it describes the capabilities of the source. The
   RESV message is issued by the receiver of the communication session,
   and it follows exactly the path that the RSVP PATH message traveled
   back to the communication session source. On its way back to the
   source, the RESV message may install QoS states at each hop. These
   states are associated with the specific QoS resource requirements of
   the destination. The RSVP reservation states are temporary states
   (soft states) that have to be updated regularly. This means that PATH
   and RESV messages will have to be retransmitted periodically. If
   these states are not refreshed then they will be removed.  The RSVP
   protocol uses additional messages either to provide information about
   the QoS state or explicitly to delete the QoS states along the
   communication session path. RSVP uses in total seven types of
   messages:

    * PATH and RESV messages

    * RESV Confirm message

    * PATH Error and RESV Error messages

    * PATH Tear and RESV Tear messages


   An overview of the functionality of the RSVP functionality includes:

    * End-to-end reservation with aggregation of path
      characteristics such as fixed delay.

    * The same type of reservation functionality in all
      routers. Only policy handling separates the edge of the
      domain from other routers.

    * Multicast and unicast reservations with receiver initiated
      reservations. RSVP makes reservations for both unicast and





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      many-to-many multicast applications, adapting dynamically
      to changing routes as well as to group membership.

    * Shared reservations for multiple flows.

    * Support for policy handling to handle multi-operator
      situations since more than one operator will be
      responsible for RSVP's operation.

    * Flexible object definitions. RSVP can transport and maintain
      traffic and policy control parameters that are opaque to
      RSVP. Each RSVP message may contain up to fourteen classes of
      attribute objects. Furthermore, each class of RSVP objects
      may contain multiple types to specify further the format
      of the encapsulated data. Moreover, the signaling load
      generated by RSVP on the routers is directly proportional
      to the flows processed simultaneously by these routers.
      Furthermore, processing of the individual flows in the
      networking infrastructure may impose a significant processing
      burden on the machines, thus hurting throughput. These
      issues make it reasonable to question the scalability and
      performance in a large cellular radio access network.

    * support for uni-directional reservations, not bi-directional.

   In the situation where a mobile moves or the connection moves from
   one base station to another, it could force the communication path to
   change its (source/dest) IP address.  The change of IP address will
   require that RSVP establish a new RSVP session through the new path
   that interconnects the two end points involved in the RSVP session
   and release the RSVP session on the old path.  During this time, the
   end-to-end data path connection is incomplete (i.e., QoS disruption)
   and it will negatively affect the user performance.

   This approach includes much more functionality and complexity than is
   required in the cellular RAN. Our problem is significantly simpler to
   solve.  The trade-off between performance and functionality is one of
   the key issues in the RAN. In our case, the majority of the
   functionality in RSVP is not required.  This is true for four
   reasons:

    * Unicast reservations are much less complex than multicast.

    * Edge-to-edge with one operator does not require policy
      handling in the interior routers.





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    * Path characteristics and flexible traffic parameters and
      QoS definitions could be solved by network dimensioning
      and edge functionality.

    * Per microflow states in intermediate routers cause severe
      scalability problems. Furthermore, receiver-initiated
      reservations impose high complexity in the states due to
      reverse-direction routing of the RESV messages.  A scheme
      based on sender oriented reservation (see e.g., [AhBe99])
      decreases the complexity of the per microflow states due
      to the fact that no reverse-direction routing is
      required.



6.2.  Integrated Services over Differentiated Services


   The IntServ over DiffServ framework addresses the problem of
   providing end-to-end QoS using the IntServ model over heterogeneous
   networks. In this scenario, DiffServ is one of these networks
   providing edge-to-edge QoS. This is similar to the underlying
   architecture for this draft, where the specific network is the
   cellular RAN, and where the end-to-end model is unspecified. As such,
   the problem addressed by IntServ over DiffServ is similar in nature
   to the problem described here, although the specific requirements
   (such as network utilization and performance) are different.

   The IntServ over DiffServ framework discusses two different possible
   deployment strategies. The first is based on statically allocated
   resources in the DiffServ domain.  In this strategy the Diffserv
   domain is statically provisioned (see Section 6.3). Furthermore, in
   this strategy the devices in the Diffserv network region are not RSVP
   aware. However, it is considered that each edge node in the customer
   network is consisting of two parts. One part of a node is a standard
   Intserv that interfaces to the customer's network region and the
   other part of the same node interfaces to the Diffserv network
   region. Any edge node in the customer network maintains a table that
   indicates the capacity provisioned per SLS (Service Level
   Specification) at each Diffserv service level. This table is used to
   perform admission control decisions on Intserv flows that cross the
   Diffserv region.  A disadvantage of this approach is that the edge
   nodes in the customer network will not be aware of the traffic load
   in the nodes located within the Diffserv domain.  Therefore, a
   congestion situation on a communication path within the Diffserv





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   domain cannot be predicted by any of these edge nodes. Due to the
   "Efficient network utilization" requirement explained in Section 5.3,
   the RAN is dimensioned such that it may have performance bottlenecks
   which are not visible to the edges. More advantages and disadvantages
   of this approach are discussed in Section 6.3.


   The second possible strategy is based on dynamically allocated
   resources in the DiffServ domain. According to [RFC2998], this can be
   done using RSVP-aware DiffServ routers.  However, this approach has
   most of the drawbacks described in Section 6.1, and per-microflow
   state information is kept in the intermediate routers.

   Alternatively, resources in the DiffServ domain can be dynamically
   allocated using Aggregated RSVP. This will be discussed in Section
   6.4.

   Other approaches related to the bandwidth broker concept are still
   very immature and will not be discussed here.


6.3.  Statically-assigned trunk reservations based on Differentiated
Services


   A significant problem in deploying an end-to-end per-flow resource
   reservation signaling scheme is its scalability. This can be solved
   by aggregating (trunking) several individual reservations into a
   common reservation trunk.  The reservation trunks can be either
   statically or dynamically configured.  When the reservation trunks
   are statically configured, no signaling protocol is required for
   performing the reservation of network resources but is likely to be a
   difficult management problem.  However, due to the different mobility
   requirements (such as handover) and QoS requirements (such as
   bandwidth) that the multi-bitrate applications impose on the RAN, it
   will be difficult to configure the trunked reservations statically
   and utilize the RAN efficiently.


6.4.  Dynamic trunk reservations with Aggregated RSVP


   The reservation trunks can be dynamically configured by using a
   signaling protocol that manages various mechanisms for dynamic
   creation of an aggregate reservation, classification of the traffic





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   to which the aggregate reservation applies, determination of the
   bandwidth needed to achieve the requirement and recovery of the
   bandwidth when the sub-reservations are no longer required.

   The first router that handles the aggregated reservations could be
   called an Aggregator, while the last router in the transit domain
   that handles the reservations could be called a Deaggregator.

   The Aggregator and Deaggregator functionality is located in the edge
   nodes. In particular, an Aggregator is located in an ingress edge
   node, while a Deaggregator is located in an egress edge node,
   relative to the traffic source.

   The aggregation region consists of a set of aggregation capable
   network nodes.  The Aggregator can use a policy that can be based on
   local configuration and local QoS management architectures to
   identify and mark the packets passing into the aggregated region.
   For example, the Aggregator may be the base station that aggregates a
   set of incoming calls and creates an aggregate reservation across the
   edge-to-edge domain up to the Deaggregator.  In this situation the
   call signaling is used to establish the end-to-end resource
   reservations. Based on policy, the Aggregator and Deaggregator will
   decide when the Aggregated states will be refreshed or updated.

   One example of a protocol that can be used to accomplish QoS dynamic
   provisioning via trunk reservations is the RSVP Aggregation signaling
   protocol specified in [BaIt00].

   With regards to aggregated RSVP, even if the reservation is based on
   aggregated traffic, the number of re-negotiations of the allocated
   resources due to mobility (handover) does not decrease and each re-
   negotiation of resources has the same performance requirements as the
   per-flow reservation procedure.

   Note that the aggregated RSVP solution may use a policy to maintain
   the amount of bandwidth required on a given aggregate reservation by
   taking account of the sum of the underlying end to end reservations,
   while endeavoring to change it infrequently. However, such solutions
   (policies) are very useful assuming that the cost of the
   overprovisioned bandwidth is not significant, since this implies the
   need for a certain "slop factor" in bandwidth needs. In a RAN, where
   overprovisioning is not preferable due to high costs of transmission
   links, a more dynamic QoS provisioning solution is needed.

   Furthermore, the aggregated RSVP scheme is receiver initiated and





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   cannot support bi-directional reservations.

   In the aggregated RSVP scheme the resource reservation states stored
   in all the RSVP aware Edge and Interior nodes represent aggregated
   RSVP sessions (i.e., trunks of RSVP sessions). Therefore, the number
   of the resource reservation states in the aggregated RSVP scheme
   compared to the (per-flow) RSVP scheme, is decreased.  However, in a
   Diffserv based RAN the number of the aggregated RSVP sessions depends
   on:

     *  the number of Aggregators/Deaggregators; Considering
        that each base station and each BSC/RNC is used as
        Aggregator/Deaggregator, the total number of
        Aggregators/Deaggregators within the RAN is
        significantly high. This is due to the fact
        that the number of BSCs/RNCs is significantly
        high and the number of base stations in a RAN is
        in the range of thousands, see Section 3.1.
     *  the network topology used; The communication between
        RNCs is performed in a meshed way, i.e., all to all
        communication. This will imply that many communication
        paths will have to be maintained by the RAN
        simultaneously.
     *  the number of Diffserv Code Points (DSCPs) used; More
        than one traffic classes will be supported
        within the RAN. Therefore, the number of the Diffserv
        Code Points (DSCPs) used within the RAN will probably
        be higher than one.

   Therefore, the number of the aggregated RSVP reservation states
   within a Diffserv based RAN will be significantly large.



7.  Conclusion



   Cellular radio access networks and coming wireless applications
   impose different requirements on reservation strategies than typical
   Internet conditions.

   Firstly, the reservation solution does not need to have the same
   level of complexity:






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     * Edge-to-edge not end-to-end: The IP traffic is generated
       in the network and is only transported as far as the
       cellular-specific nodes (such as the base station and
       BSC/RNC).

     * Single operator domain and no inter-domain transport: The
       transport is owned and managed by a single operator.

     * Only unicast not multicast: The end-to-end payload is
       transported between nodes. This transport only requires
       a unicast reservation.

   Furthermore, a cellular radio access network has much higher
   performance requirements on the reservation strategy:

     * Efficient usage of the transmission network: The transport
       between the BSC/RNC and the base station represents a significant
       cost for the cellular operator, and efficient usage of
       the transmission network is therefore critical from a cost
       point of view. The network should allow dynamic allocation
       of resources to allow efficient statistical aggregation
       of traffic without causing congestion.

     * A wide-area network with significant volume of real-time
       traffic: Real-time traffic levels up 100% must be
       supported.

     * The resource reservation process has to handle a
       significantly higher volume of requests, and the process
       has to be fast enough to avoid packet losses in the
       air-interface during handover.

     * The scheme must be optimal for interior nodes and not
       for the edge nodes. In this way the necessary set of
       parameters required for setting up reservations should be
       based upon their effect on interior nodes and not on edge
       nodes. This reduces the complexity on the interior routers.

   Given these requirements, we believe that appropriate standardization
   should take place to create the necessary protocols for edge-to-edge
   resource management for a single operator domain.









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8.  References


   [AhBe99]  Ahlard, D., Bergkvist, J., Cselenyi, I., "Boomerang
             Protocol Specification", Internet draft, Work in progress.

   [BaIt00]  Baker, F., Iturralde, C. Le Faucher, F., Davie, B.,
             "Aggregation of RSVP for Ipv4 and Ipv6 Reservations",
             Internet draft, Work in progress.

   [KeMc00]  Kempf, J., McCann, P., Roberts, P., " IP Mobility
             and the CDMA Radio Access Network: Applicability
             Statement for Soft Handoff", Internet draft, Work
             in progress.

   [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, A., Jamin, S.,
             "Resource ReSerVation Protocol (RSVP) -- Version 1
             Functional Specification", IETF RFC 2205, 1997.

   [RFC2210] Wroclawski, J., "The use of RSVP with IETF Integrated
             Services", IETF RFC 2210, 1997.

   [WeLi99]  Westberg, L., Lindqvist, M., "Realtime Traffic over
             Cellular Access Networks", Internet draft, Work in
             progress (expired).

   [RFC2998] Bernet, Y., Yavatkar, R., Ford, P., baker, F., Zhang, L.,
             Speer, M., Braden, R., Davie, B., "Felstaine, E.,
             "Framework for Integrated Services operation over
             Diffserv Networks", IETF RFC 2998, 2000.


9.  Acknowledgements


   Special thanks to Brian Carpenter, Steven Blake, Phil Chimento,
   Anders Bergsten, Marc Greis, Hamad el Allali,
   Nicola Blefari-Melazzi, Giuseppe Bianchi and Geert Heijenk for
   providing useful comments and input.











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10.  Authors' Addresses


   David Partain
   Ericsson Radio Systems AB
   P.O. Box 1248
   SE-581 12  Linkoping
   Sweden
   EMail: David.Partain@ericsson.com

   Georgios Karagiannis
   Ericsson EuroLab Netherlands B.V.
   Institutenweg 25
   P.O.Box 645
   7500 AP Enschede
   The Netherlands
   EMail: Georgios.Karagiannis@eln.ericsson.se

   Pontus Wallentin
   Ericsson Radio Systems AB
   P.O. Box 1248
   SE-581 12  Linkoping
   Sweden
   EMail: Pontus.Wallentin@era.ericsson.se

   Lars Westberg
   Ericsson Research
   Torshamnsgatan 23
   SE-164 80 Stockholm
   Sweden
   EMail: Lars.Westberg@era-t.ericsson.se


Table of Contents



1 Abstract ........................................................    2
2 Terminology .....................................................    3
3 Background and motivation .......................................    3
3.1 IP transport in radio access networks .........................    4
3.2 Motivation for this memo ......................................    7
4 Network characteristics of cellular access networks today .......    8
4.1 General aspects of the network structure ......................    8
4.2 High cost for transmission ....................................    9





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4.3 Transportation of radio frames ................................   10
4.4 Mobility aspect of radio frame transport ......................   12
5 Requirements on a Resource Reservation Scheme ...................   12
5.1 Main requirement on resource reservation scheme ...............   12
5.2 IP must provide same service behavior as the transport  net­
     works used today .............................................   13
5.3 Efficient network utilization .................................   13
5.4  Handover  performance  requirements on resource reservation
     scheme
          .........................................................   14
5.5 Edge-to-edge reservations, not end-to-end .....................   15
5.6 Reservation functionality  in  edge  nodes  versus  interior
     routers ......................................................   16
5.7 On-demand and dynamic allocation of resources .................   17
5.8 Unicast and not multicast .....................................   17
5.9 A single operator in the RAN ..................................   18
5.10 Minimal impact on router performance .........................   18
5.11 Scalably Manageable ..........................................   19
5.12 Bi-directional reservations ..................................   19
6 Evaluation of existing strategies ...............................   19
6.1 End-to-end per-flow resource reservation protocol .............   19
6.2 Integrated Services over Differentiated Services ..............   22
6.3  Statically-assigned trunk reservations based on Differenti­
     ated Services ................................................   23
6.4 Dynamic trunk reservations with Aggregated RSVP ...............   23
7 Conclusion ......................................................   25
8 References ......................................................   27
9 Acknowledgements ................................................   27
10 Authors' Addresses .............................................   28





















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