1 WIRELESS COMMUNICATIONS AND MOBILE COMPUTING Wirel. Commun. Mob. Comput. 23; 3: (DOI: 1.12/wcm.96) Radio resource management schemes for combined GSM/GPRS mobile systems C. N. Konstantinopoulou, K. A. Koutsopoulos, P. P. Demestichas and M. E. Theologou*, National Technical University of Athens (NTUA) Department of Electrical and Computer Engineering 9 Heroon Polytechniou Street GR Zographou Athens Greece G. L. Lyberopoulos COSMOTE Mobile Telecommunications S.A Department of Switching and Network Management 9 Marinou Antypa GR N. Iraklion Athens Greece Summary Data services like Web browsing, and file transfer are becoming more and more popular in cellular systems. In contemporary systems like Global System for Mobile communications (GSM), data transfer has been circuit-switched, that is, physical resources are allocated to a user for the entire call/session duration. However, this is inefficient in case of bursty traffic, where bursts are separated by long intervals of inactivity. This has been the main reason for the introduction of General Packet Radio Service (GPRS), which on the one hand acts as a mobile access network to the Internet, while on the other hand it enables the operator to offer a wide variety of value-added services [Wireless Access Protocol (WAP) over GPRS, e/m-banking, e/m-commerce, push services, etc.] efficiently. However, in contemporary commercial implementations of GPRS the radio resource allocation algorithm does not take into account the Quality of Service (QoS)-related service characteristics although such information is exchanged between the terminal and the network and consequently all service requests are treated the same way ( best effort ). In this paper, we propose and evaluate via a simulation platform various Radio Resource Management (RRM) schemes capable of differentiating the handling of service requests (in uplink and downlink), taking into account the GPRS-related QoS parameters (precedence, reliability, delay, mean and peak throughput). The evaluation is performed for a range of voice (circuit-switched) traffic loads, number of Transmit Receive exchange (TRXs), offered data (packet-switched) services characteristics, number of dedicated Packet Data Channels (PDCHs), and so on, taking into account the respective QoS requirements for both service types (circuit- and packet-switched). Copyright 23 John Wiley & Sons, Ltd. Ł Correspondence to: M. E. Theologou, National Technical University of Athens (NTUA), Department of Electrical and Computer Engineering, 9, Heroon Polytechniou Street, , Zographou, Athens, Greece. Copyright 23 John Wiley & Sons, Ltd.
2 358 C. N. KONSTANTINOPOULOU ET AL. KEY WORDS GPRS traffic source models interactive services background services QoS parameters GPRS QoS profile radio resource management (RRM) priority-based RRM schemes 1. Introduction The impressive growth of cellular mobile telephony, as well as the number of Internet users, promises an exciting potential for a market that combines both innovations, that is the cellular wireless data services. It is foreseen that within the next few years there will be an extensive demand for wireless data services. In particular, it is expected that data usage drive up traffic per user as much as 3 to 5 per cent and may also act as a stimulator of additional voice calls, as already witnessed with voice mail. To cope with the inefficiencies, which are inherent to contemporary GSM technologies mainly related to inefficient use of radio resources the GPRS is conceived as the first vital step toward the provision of higher speed data rates. The GPRS, by enabling much more effective data use of the radio spectrum, will allow operators to develop new applications with richer content (e.g. location-based information in conjunction with geographical data) and therefore to further enhance their business role. GPRS may increase the data transfer speed (beyond 1 kb/s), offering session establishment times below one second. We shall stress that the actual transfer rate of a single session depends on a variety of factors that are relevant both to the terminal [multislot capability, Coding Scheme (CS)] and the network (number of active terminals, CSs supported, the utilized slot assignment scheme, the currently available bandwidth for GPRS services, etc.). In addition, GPRS may offer a user-friendlier billing compared to current charging models based on call/session duration since it allows charging based on the volume of transferred data. The advantage for the user is that he/she can be always connected and that charging may be based on the transferred data volume. However, the realization of the GPRS system requires upgrades to the existing GSM network elements (BTS, BSC, MSC and HLR ) as well as the implementation of an IP-based backbone network (Figure 1) consisting of new GPRS-related network elements (SGSN, GGSN, BG, CG, DNS, DHCP/RADIUS, FW ). The GSM/GPRS air-interface will be capable of supporting efficiently circuit-switched and packetswitched (GPRS) traffic demands, taking into account the application/service related QoS parameters. In addition, it is envisaged that owing to the extra GPRS traffic/signaling both the signaling traffic and the overall interference level will increase. To sum up, GPRS improves the utilization of the radio resources, may offer volume-based billing, higher transfer rates, shorter access times, and simplifies the access to packet data networks (e.g. internet) [1 3]. As far as radio resource allocation is concerned, European Telecommunications Standards Institute (ETSI) proposes the fixed and on-demand channel allocation mechanisms . P. Lin and Y. Lin have proposed and evaluated the following four resource allocation mechanisms : (i) Fixed Resource Allocation (FRA), (ii) Dynamic Resource Allocation (DRA), (iii) Fixed Resource Allocation with Queue capability (FRAQ) and (iv) Dynamic Resource Base Transmitter Station (BTS), Base Station Controller (BSC), Mobile Switching Center (MSC), Home Location Register (HLR). Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN), Border Gateway (BG), Charging Gateway (CG), Domain Name Server (DNS), Dynamic Host Configuration Protocol (DHCP), Remote Access Dial- In User Service (RADIUS), Firewall (FW).
3 RADIO RESOURCE MANAGEMENT SCHEME 359 PSTN BSC MSC Internet Intranet Um RADIUS/ DHCP SGSN BG GPRS GPRS Backbone backbone SS7 CG GGSNs Billing System FW HLR AuC EIR Router Corporate LAN Server Inter-PLMN DNSs To OMC Internet To other PLMNs Intranet Fig. 1. GPRS architecture. Allocation with Queue capability (DRAQ). However, none of these algorithms considers (circuit-switched) voice priority, neither takes into account the QoS characteristics of data sessions. Fixed and on-demand channel allocation have also been investigated by Stuckman and Muller [6,7]. In Reference , no voice traffic has been considered in the system, while in Reference , circuit-switched voice calls are assigned higher priority than data services, but no distinction between data requests has been implemented. In Reference  Stuckman and Muller studied the issue of QoS management of GPRS resources. The data rate and the one-way delay have been considered as QoS parameters, while the traffic source models described in Reference  have been adopted. The aim of this paper is to investigate and evaluate (via a simulation platform) various RRM schemes capable of differentiating the handling of service requests (in uplink and downlink) taking into account GPRS-specific QoS parameters. The evaluation will be performed for a range of voice (circuit-switched) traffic loads, number of TRXs, offered services, number of dedicated PDCHs, and so on, taking into account the respective QoS requirements for both service types (circuit- and packet-switched). The simulation platform through which the evaluation of the proposed RRM schemes has been performed exhibits the following capabilities : ž Handling of any number of TRXs. ž Adoption of the frame structure as specified in Reference  for GSM/GPRS. ž Uplink and downlink transmissions are performed according to Reference . More specifically, up to seven transmissions (from different terminals jj ) can be assigned to an uplink PDCH, while up to nine transmissions ŁŁ can be assigned to a downlink PDCH. ž Possibility to define dedicated PDCHs for GPRS use only. ž Possibility to assign higher priority to circuitswitched voice calls, by preempting active data sessions. ž Consideration of the GPRS terminal multislot capability in slot assignment procedure. ž Support of all CSs (CS-1, CS-2, CS-3 and CS-4). A dedicated PDCH is a slot that can be used exclusively by GPRS connections. jj DiscriminatedbytheUplink State Flag (USF) value. ŁŁ Discriminated by the Temporary Flow Identity (TFI) value.
4 36 C. N. KONSTANTINOPOULOU ET AL. ž Adoption of realistic traffic source models for a variety of services. ž Possibility to prioritize services requests, both uplink and downlink, based on the applied RRM scheme (see Section 5). The material included in this paper is organized as follows. In Section 2, the QoS profiles specified for the GPRS system are presented. Section 3 assesses some open issues regarding the radio resource allocation in contemporary GPRS systems. The adopted traffic source models for the various data services and the relative parameters are described in Section 4. In Section 5, we propose priority-based RRM schemes, while in Section 6 evaluation results are presented. Finally, in Section 7 some concluding results have been drawn. 2. QoS Issues In GPRS, the Mobile Station (MS), before any information exchange with the system has to perform the so-called Packet Data Protocol (PDP) context activation procedure in order: (i) to be assigned a (public or private) IP address and (ii) to agree upon a QoS profile. A QoS profile comprises the parameters shown in Table I. The combinations of these parameters result in 1 26 different QoS profiles . In GPRS, we may discriminate four QoS profile types (Figure 2): MS SGSN QoS Requested 1 QoS Negotiated Accept 2 Reject 3 Check QoS Subscribed QoS Negotiated (1) At PDP Context Activation request (2) if (QoS Negotiated) > (QoS Minimum) (3) if (QoS Negotiated) < (QoS Minimum) HLR QoS Negotiated Fig. 2. QoS profile information exchange. GGSN 1. The QoS Subscribed profile that is defined at subscription time and is stored at the HLR. 2. The QoS Requested profile that is sent during the PDP Context Activation. 3. The QoS Negotiated profile containing the QoS parameter values that the system can ensure depending on the current loading conditions and the SGSN/GGSN capabilities. 4. The QoS Minimum profile that contains the minimum acceptable values for each QoS parameter for the specific GPRS service. Even if contemporary commercial implementations of GPRS consider QoS profile information exchange between the MS, the SGSN and the GGSN, they never exploit such information. More specifically, the Table I. QoS parameters. QoS parameters Definition # Values Minimum value Maximum value Precedence class Delay class Reliability class Peak throughput class Mean throughput class Indicates the relative importance of maintaining the service commitments under abnormal conditions (e.g. limited resources, network congestion) Defines the maximum values for mean (end-to-end) and 95-percentile delay to be incurred by the transfer of data through the GPRS network(s) Indicates the transmission characteristics that are required by an application Specifies the maximum rate at which data is expected to be transferred across the network for an individual PDP context Specifies the average rate at which data is expected to be transferred across the GPRS network during the remaining lifetime of an activated PDP context 3 1 (High) 4 1 (Predictive) 5 1 (Nonreal time traffic) 9 1 (8 kb/s) 19 1 (¾.22 b/s) 3 (Low) 4 (Best effort) 5 (Real-time traffic) 9 (248 kb/s) 31 (Best effort)
5 RADIO RESOURCE MANAGEMENT SCHEME 361 resource management algorithm that is applied does not take into account the specific GPRS QoS parameters of the services requests, that is, all requests are treated the same way ( best effort ) no priority in handling higher priority requests is employed. In addition, the standards have not proposed resource management algorithms that take into account services QoS requirements. This task has been left to the GPRS vendors, and as such the performance of the GPRS air-interface is strongly dependent on the specific resource management scheme applied by the vendor. 3. Radio Resource Management: Open Issues A nonexhaustive list of the RRM schemes open issues is given below: 1. Currently, the RRM scheme does not take into account QoS parameters. Currently, the commercial versions of the GPRS offer the best effort option only. The relation between user/service QoS parameters and network-related ones has to be investigated. 2. Session Admission Control (SAC) algorithm. In addition, the current versions of the GPRS do not employ a SAC algorithm that takes into account the loading conditions of the radio airinterface. 3. Allocation of the GSM and GPRS slots/pdchs onto the TRXs. One of the main RRM issues is the allocation of timeslots/pdchs to circuit- and packet-switched traffic. In contemporary GSM, slot allocation is based on the low interference criterion, that is, an incoming circuit-switched call is assigned to the slot exhibiting the lowest interference level. For the combined GSM/GPRS case, the following two alternatives can be envisaged: a. Apply the low interference criterion for GPRS traffic too. The GPRS data sessions reserve slots/pdchs as GSM calls, based on low interference. However, in this case, the scheme shall take into account that a GPRS session may need Apart from the resource management scheme used, the performance of the GPRS air-interface depends on service behavior (session arrival rate, session duration), user behavior (user tolerance) and network behavior (number of GPRS capable TRXs, cell reselection criteria, delays, packet interarrival times) parameters. to reserve more than one PDCHs (in the same TRX). Unavailability of PDCHs in a single carrier may lead to performance degradation due to assignment of lower number of PDCHs than requested/needed for a single session although there may be available resources in the system. b. Consecutive slot allocation for both GSM and GPRS. According to this scheme, the system assigns consecutive slots to GSM and GPRS requests starting from different carriers (Figure 3). The major advantage of this scheme is that the GPRS performance is not compromised. On the contrary, it may lead to increase in intracell handovers (under high loading conditions). 4. PDCH selection to be preempted by incoming circuit-switched call during congestion. Criteria shall be defined for the selection of the proper PDCH to be preempted, such as the number of assigned USFs per PDCH. 5. The mechanism that selects the proper PDCHs during the activity periods of a data session so as to satisfy the QoS Negotiated parameters. An efficient RRM scheme will be capable of prioritizing service requests and assigning the proper number of PDCHs as well as those that can provide the maximum throughput (if possible) so as to respect the QoS Negotiated parameter values. 4. Traffic Source Models Source modeling is one of the most important issues that has to be dealt with in this paper, since it is one of the critical parameters that determines the overall performance of the GPRS. We shall stress though that it is impossible to define a generic source model representing all possible service types, for example, conversational, streaming, interactive and background . However, independently of the service type, the traffic source model shall take into account the following: Interactive services are typical instant response-request messaging services. Example services are Web browsing, database retrieval, server access, polling for measurement records and automatic database enquiries (telemachines). In this service type, the end user (typically a computer) sends and receives data files in the background. Examples are background deliveries of s, database downloads, reception of measurement records, and so on.
6 362 C. N. KONSTANTINOPOULOU ET AL. TRX # C C C C Signaling TRX #2 TRX #3 C C C C C C C C C C C C C Circuit-swithced slots Slots occupied by circuit-swithced voice GPRS PDCHs TRX #4 C C C Dedicated GPRS PDCHs GSM/GPRS border GSM/GPRS border Fig. 3. Consecutive slot allocation. ž A data session consists of a sequence of packet calls, each composed of a (bursty) sequence of datagrams. ž Session parameters such as number of packet calls per session, number of datagrams/packet call, packets interarrival time, packet call size. ž User behavior parameters (e.g., user tolerance). Since this paper focuses on GPRS, we only consider interactive and background service classes; more specifically Web-browsing, ftp upload, ftp download. The source models for the above-mentioned service types are described in the following subsections Interactive Services A typical example of a Web-browsing data session is depicted in Figure 4, while its detailed traffic source model is shown in Figure 5. As shown, four different states can be identified: ž Active state (A active, B inactive). The user initiates a request. As shown, the user (A) is active, while User request Uplink Downlink Waiting state A packet call A data session Reading state Waiting state Datagrams Fig. 4. Example of a Web-browsing data session. t t the host (B), which for example is the Web server for Web-browsing, is inactive. ž Waiting state (A inactive, B inactive). This state represents the time interval between the completion of transfer of user request for uplink and the beginning of downlink transmission. This state includes delays due to GPRS network itself as well as external networks (e.g. Internet). ž Receiving information state (A inactive, B active). This state represents the download information transfer phase. ž Reading state (A inactive, B inactive). It represents the time that the user needs to read the downloaded information before proceeding in a new request. The active state can be decomposed in several phases. First, the MS sends a packet channel request and enters the contention phase. On collision, the MS sends again its request after back-off time. If no collision occurred, but there are no available uplink PDCHs, the request is queued. When the system finds resources, it sends the uplink Packet Assignment Message (PAM) to the MS, containing a list of assigned PDCHs, the USF(s) per PDCH(s), the TFI, as well as timing advance and power control information, if available. The MS monitors the PDCHs in the downlink channel, in order to identify its USF(s). If the MS detects its USF(s), it transmits a radioblock to the corresponding uplink PDCH(s). After the radioblock transmission, if the MS has more information to send, it waits until it listens to its USF(s) again [3,4]. In the downlink, the system after a certain number of MS radioblocks (e.g. 1 to 15) sends positive acknowledgments (ACKs) to verify The radioblock is the basic transmission unit in GPRS and it is formed by four slots in consecutive TDMA frames.
7 RADIO RESOURCE MANAGEMENT SCHEME 363 Session setup Active state Packet channel request Contention ti Session release (*) List of uplink PDCHs, USF per PDCH, TFI, timing advance, power control (*) ) Wait for packet uplink assignment Packet uplink assignment Check assigned PDCHs for its own USFs Uplink transmission of a radioblock per assigned PDCH (**) List of Downlink PDCHs, TFI, Timing Advance, Power Control Wait for Packet Downlink Assignment (**) Waiting state Fig. 5. Detailed source traffic model for Web-browsing. Reading state Check assigned downlink PDCHs for its own TFI Receiving info state that information has been received successfully, or negative acknowledgments (NACK) indicating the erroneous radioblocks. If there is information to be sent to the MS, the system sends a downlink PAM, containing a list of downlink PDCHs, the TFI, as well as timing advance and power control information, if available. The MS monitors the assigned downlink PDCHs, in order to identify the packets addressed to it through its TFI. Downlink packets are kept in a queue until downlink resources become available and each time a single radioblock is transmitted in each assigned PDCH, until the downlink information is successfully completed [3,4]. After this phase, the user shall read the downloaded information, and then proceeds in requesting a new packet call (in the same session), thus returning to the contention phase. As before, ACKs/NACKs are sent periodically in the uplink to verify the correct transmission of data or to indicate the erroneous radioblocks and ask for retransmission. We shall stress that the user may initiate a session release: 1. In the active state, if he waits for a long time in the uplink queue until the system finds available resources. 2. When the downlink response is too delayed. 3. After the reading state where the user can decide to initiate a request for a new session, releasing at the same time the existing one Background Services Upload background services A detailed traffic source model for a background upload service (suitable for , FTP upload, etc.) is depicted in Figure 6(a). As it is shown, the model is similar to the one described for interactive services, as far as uplink transmission concerns. The difference in this case is that there is only one contention phase. After the assignment of required PDCHs to the MS, the uplink transmission is performed in the specific resources, whenever the MS identifies its USF in the downlink, until the completion of the data upload. In the downlink channel, the system sends ACKs periodically to verify that information has been received successfully or NACKs indicating the erroneous radioblocks. Normally, session release occurs when the user has successfully transmitted the entire information. Abnormal session release may be initiated either by the system or the user himself whenever the QoS falls below certain limits for example, so far offered mean bit rate falls below a certain percentage
8 364 C. N. KONSTANTINOPOULOU ET AL. (a) Session setup Packet channel request Contention Session release Wait for packet uplink assignment (*) List of uplink PDCHs, USF per PDCH, TFI, timing advance, power control (*) Packet uplink assignment Check assigned PDCHs for its own USFs Uplink transmission of a radioblock per assigned PDCH (b) Session setup Active state Packet channel request Contention Session release Wait for packet uplink assignment (*) List of uplink PDCHs, USF per PDCH, TFI, timing advance, power control (* ) Packet uplink assignment Check assigned PDCHs for its own USFs Uplink transmission of a radioblock per assigned PDCH (**) List of downlink PDCHs, TFI, timing advance, power control Wait for packet downlink assignment (**) Waiting state Check assigned downlink PDCHs for its own TFI Fig. 6. (a) Traffic source model for background upload services and (b) detailed traffic source model for background download services. (e.g. 6% to 7%) and remains there for a certain time period (e.g. 3 min) Download background services In the case of a download background service the detailed traffic source model is depicted in Figure 6(b). As it is shown, the session set-up procedure and the transmission of the uplink request are performed as previously. There is only one contention phase, and after the assignment of resources and the successful transmission of the uplink request, the system sends a downlink PAM and the user starts
9 RADIO RESOURCE MANAGEMENT SCHEME 365 monitoring the assigned downlink PDCHs to identify the packets addressed to him through his TFI. Both downlink and uplink transmissions are performed in radioblocks, as described for interactive services. As before, ACKs/NACKs are sent periodically in the uplink to verify the correct transmission of data or to indicate the erroneous radioblocks and ask for retransmission. After the completion of download phase, a normal session release follows. Abnormal session release may be initiated either by the system or the user himself whenever the QoS falls below certain limits, for example, so far offered mean bit rate falls below a certain percentage (e.g. 6% to 7%) and remains there for a certain time period (e.g. 3 min) Source Models Parameters The source models parameters for interactive and background services are listed below : ž Service APN. This is a descriptive name, the Access Point Name (APN), sent during the PDP Context Activation (e.g. wap, internet ). It can be utilized as an extra priority level for services having the same precedence class (e.g. health-care applications and Web-browsing of the same precedence class). ž QoS parameters. Precedence, Delay, Reliability, Peak Throughput and Mean Throughput class (). ž Service parameters (Table II), which characterize the service behavior; these are as follows: Number of packet calls per session, N pc.this is a geometrically distributed random variable with a mean µ Npc (packet calls). Packet call size, S pc. The size of the packet call (bytes). This follows Pareto distribution with cut-off. Datagram Size, S d. Uniformly distributed between an upper and a lower bound (bytes). Table II. Interactive and background traffic source model parameters. Service characteristics Interactive Download background Upload background p p p N pc p D pc (sec) p N/A p N/A D rr (sec) p p N/A p N d p p p D d (sec) p p p S d (bytes) User behavior p N ur p N/A N/A T ut (sec) N/A N/A Interarrival time between datagrams within a packet call, D d. This is an exponentially distributed random variable with a mean µ Dd (model time steps). Reading time between packet calls, D pc.this is an exponentially distributed random variable with a mean µ Dpc (model time steps). Note that the reading time starts when the last datagram of the packet call is successfully received by the user and ends normally when the user makes a new request. GPRS and external network(s) delay, D GI.This is an exponentially distributed random variable with a mean µ Drr. This parameter represents the time interval between the successful transmission of the uplink packet call and the transmission of the first datagram in the downlink. ž User behavior parameters (Table II), which characterize the user behavior; these are as follows: User tolerance time, T ut. This is a constant variable, which mostly models user behavior. This time starts after the user uplink packet channel request. If this time expires before the user has received the expected information, the user makes a retry for the same request. Maximum number of user retries, N ur.thisis a constant variable representing the maximum number of user retries performed in the case that the user tolerance time has expired before the complete reception of the expected information. 5. Proposed RRM Schemes In our study, we propose and evaluate a number of RRM schemes, which enable services prioritization, based on precedence, service APN and mean throughput QoS parameters. As such, the following schemes have been implemented: PST: Prioritized by Precedence Service APN mean Throughput PTS: Prioritized by Precedence class mean Throughput Service APN TSP: Prioritized by mean Throughput Service APN Precedence class TPS: Prioritized by mean Throughput Precedence class Service APN STP: Prioritized by Service APN mean Throughput Precedence class SPT: Prioritized by Service APN Precedence class mean Throughput
10 366 C. N. KONSTANTINOPOULOU ET AL. Note: For the sake of comparison, the classical nonpriority ( best effort ) scheme, the so-called Round Robin (RR) scheme has been implemented Priority-based RRM Schemes Call/session admission control When a new voice call arrives, the system searches for a free slot in both uplink and downlink. If not found and if there are PDCHs allocated to GPRS (not dedicated), it will preempt a PDCH (uplink and downlink) and allocate it to the incoming voice call. The selection of the proper GPRS PDCH will be based on the following: ž The number of packet calls that each GPRS PDCH serves. ž The number of packet calls of services with the highest priority that each GPRS PDCH serves. Otherwise, the voice call is blocked. Note: Voice call dropping has not been considered because (a) intracell handovers are not performed because of radio quality violation but for respecting the consecutive slot implementation (Figure 3) and (b) intercells handovers have not been assumed. All incoming data sessions enter the system session blocking is not performed. An active data session is dropped when the QoS parameters are violated as described in Sections 4.1 and Slot allocation to GSM/GPRS domain In our study, we have adopted the consecutive slot allocation scheme (Figure 3) that operates as follows: the assignment of slots to circuit-switched connections starts from the beginning of the first TRX, while for the GPRS connections the allocation of PDCHs starts from the end of the last TRX. Every slot may be used either by GSM or GPRS connections, unless a slot has been characterized as dedicated PDCH. If dedicated PDCHs have been defined by the operator, these should be located at the end of the last TRX (Figure 3). Additionally, the PDCHs that have been assigned to a terminal for transmission or reception must belong to the same TRX Upgrading/downgrading the GPRS domain A GPRS domain upgrade is issued when a certain percentage (e.g. 8%) of the USFs of the existing PDCHs have been allocated to data sessions. In the case that available slots are found in another TRX, an intracell handover shall be performed first (Figure 3). If the GPRS domain upgrade fails, the request is reactivated as soon as a slot is released. A GPRS domain downgrade occurs whenever a PDCH preemption occurs PDCH/USF assignment mechanism The proposed PDCH/USF Assignment Mechanism is responsible for allocating the proper number of USFs to packet calls, determines which packet call(s) shall transmit a radioblock and when. It is obvious that such a mechanism shall guarantee the QoS negotiated of the active packet calls, ensuring at the same time that high priority services have a special treatment. Services prioritization is implemented by a (fixed length) queue per PDCH, containing all the packet calls to which a USF has been assigned (see Figure 7). It is evident that a packet call may have many instances (entries in queues) depending on the number of USFs currently allocated to it. Packet calls that do not have any assigned USF(s) (e.g. during congestion periods) are stored in a separate queue, the so-called pending queue. The handling of a new uplink packet call is depicted in Figure 8(a). As shown, as soon as the contention phase is completed successfully, the system checks whether there is an available USF jjjj in one of the existing PDCHs. ž If available, a PAM is sent to the MS, which starts monitoring the assigned PDCH in order to listen its USF. As soon as it detects its USF, it transmits a radioblock starting from the next frame. It shall be stressed that a radioblock transmission implies that the packet call has been placed first in the queue of the corresponding PDCH, as indicated by the applied RRM scheme. Additional USF/PDCH (if available) may be assigned to the corresponding packet call, if the QoS-related system measurements (e.g. mean throughput) dictate so. Release of a USF may occur when (i) the corresponding USF/PDCH is preemptied by a circuitswitched call (Note 1), (ii) a higher priority packet call, for which the QoS-negotiated profile is not respected, is assigned the USF, or (iii) the (so-far) jjjj In the PDCH currently offering the maximum throughput, if possible.