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. D. Partain, ed. Expires December 2001 [Page 1] Internet Draft Cellular RAN Resource Issues June 2001 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". D. Partain, ed. Expires December 2001 [Page 2] Internet Draft Cellular RAN Resource Issues June 2001 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". D. Partain, ed. Expires December 2001 [Page 3] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 4] Internet Draft Cellular RAN Resource Issues June 2001 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). D. Partain, ed. Expires December 2001 [Page 5] Internet Draft Cellular RAN Resource Issues June 2001 |--------| | 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 D. Partain, ed. Expires December 2001 [Page 6] Internet Draft Cellular RAN Resource Issues June 2001 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). D. Partain, ed. Expires December 2001 [Page 7] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 8] Internet Draft Cellular RAN Resource Issues June 2001 * 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, D. Partain, ed. Expires December 2001 [Page 9] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 10] Internet Draft Cellular RAN Resource Issues June 2001 * 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. D. Partain, ed. Expires December 2001 [Page 11] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 12] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 13] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 14] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 15] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 16] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 17] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 18] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 19] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 20] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 21] Internet Draft Cellular RAN Resource Issues June 2001 * 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 D. Partain, ed. Expires December 2001 [Page 22] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 23] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 24] Internet Draft Cellular RAN Resource Issues June 2001 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: D. Partain, ed. Expires December 2001 [Page 25] Internet Draft Cellular RAN Resource Issues June 2001 * 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. D. Partain, ed. Expires December 2001 [Page 26] Internet Draft Cellular RAN Resource Issues June 2001 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. D. Partain, ed. Expires December 2001 [Page 27] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 28] Internet Draft Cellular RAN Resource Issues June 2001 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 D. Partain, ed. Expires December 2001 [Page 29]