EVALUATION AND ANALYSIS OF SIP AND VOIP PERFORMANCE WITH PRESENCE TRAFFIC OVER HSPA

Transcription

1 EVALUATION AND ANALYSIS OF SIP AND VOIP PERFORMANCE WITH PRESENCE TRAFFIC OVER HSPA Fan Rui¹, Wang Min¹, Mårten Ericson², Stefan Wänstedt² Ericsson Research ¹Beijing, China; ²Luleå, Sweden Abstract This paper investigates the SIP based Presence traffic impact on the VoIP capacity, assuming an HSPA system. The results show that 2000 subscribers per cell with VoIP and presence enabled can be accommodated with a minor capacity loss of 5%. The uplink is the bottleneck for the system. The major reason for the capacity decrease is not the presence packet itself, rather the control channel overhead from DPCCH in UL and A-DPCH in DL due to the channel release delay. The paper investigates different priority based scheduling schemes. The simulations show that the best performance is achieved when VOIP and SIP (including presence and call setup) have equal priority. Prioritizing SIP over VoIP decreases SIP call control setup delay at the expense of lower VoIP capacity. Prioritizing VoIP over SIP or granting Presence the lowest priority results in very large overhead from the control channels, especially the uplink DPCCH. This leads to both decreased VoIP capacity and increased SIP setup delay. However, when UL DPCCH gating is applied, the performance of the scheme to prioritize VoIP over SIP call control and Presence is improved quite a lot and achieves the best trade-off between VoIP capacity and SIP setup delay. Keywords-VoIP; Presence;Capacity; HSPA I. INTRODUCTION IP Multimedia Subsystem (IMS) defined by 3GPP [1] is believed to be the architecture for future communication networks. In an IMS based network, the packet based VoIP service replaces the traditional circuit switched voice service. SIP signaling is used to initiate and release VoIP communication sessions. Presence is a kind of service that enriches the end user perception. Since the network resources are limited, the emergence of Presence traffic is sure to impact SIP call control and VoIP performance, i.e. the SIP call control setup delay and the voice capacity. The objectives of this paper are to investigate the impact of Presence traffic on SIP call control and VoIP performance over HSPA both in the uplink (UL) and in the downlink (DL), and to investigate whether there are some methods to improve the overall SIP call control and VoIP performance in the mixed traffic scenarios. The rest of the paper is organized as follows. Section II shows an introduction of the traffic model and the scheduling schemes considered in this paper. Section III describes the metrics and the simulation assumptions. UL VoIP capacity with theoretical analysis is described in section IV. The simulation results and performance analyses are described in Section V. Finally, discussion and conclusions are presented in Section VI. II. TRAFFIC MODELS AND SCHEDULING SCHEMES A. Traffic Models Three different types of traffic are investigated in this paper: SIP call control traffic, VoIP traffic and Presence traffic. SIP call control traffic is responsible for the setup and release of the VoIP sessions. The SIP message sequence for call setup includes INVITE, 100 Trying, 183 Session in Progress, PRACK, 200 OK, 180 Ringing, 200 OK, and ACK. The SIP sequence for call release includes BYE and 200 OK. The VoIP traffic model is similar to the one used in [2], i.e. AMR 12.2kbps codec rate with 50% voice activity. The session duration follows an exponential distribution with mean value of 30s. Four types of Presence transactions are modeled in this paper: presence update, watcher request, registration and reregistration/de-registration, see Figure 1. Each Presence transaction includes 2-4 Presence messages. The four transactions have intensities of 0.93, 1.34, 0.34 and 0.5/0.18 per user and hour, respectively. The exact intensities of the Presence messages is of course difficult to know, and may vary depending on e.g. user behavior and application. Figure 1. The four types of Presence transactions The Presence traffic, but not SIP call control and VoIP traffic, needs to consider the HS-DSCH channel downswitch timer. This is because generally the HS-DSCH downswitch timer, which is used to control how long a user can occupy resources even when it does not need them, is about 10 times as large as the duration of Presence transactions but less than 1/20 of SIP/VoIP sessions. Until the HS-DSCH down-switch timer expires, DPCCH/F-DPCH channels still exist and consume power even when no Presence messages need to be transmitted /07/$ IEEE

2 The activity of a Presence user is rather low. A Presence user is active only from the time when the first Presence message is transmitted between the user and the Presence server to the time when its channel down-switch timer expires. Assuming the mean duration of one Presence transaction is about 100 ms and the down-switch timer is set to 2 s, the activity factor of one Presence user is approximately ((0.1s + 2s) /3600s)*3.3 = 1.9 merlang where 3.3 is the considered mean number of transactions per user and hour. Assume that all Presence users are VoIP subscribers, the number of Presence users per cell can then be roughly derived from the voice activity per user and the maximum number of active voice users a cell can accommodate. Assuming that a cell typically accommodates up to 50 active voice users and the voice activity per subscriber is 20 merlang, the corresponding number of voice or presence subscribers is then 2500 per cell. In this study the number of Presence subscribers (or users) per cell is varied from 2000 to 8000 as we believe presence subscribers are not necessarily VoIP users. This is done to investigate the effect on SIP call control and VoIP performance. B. Scheduling schemes Scheduling schemes play an important role in HSPA systems. In this paper, EUL employs the so-called nonscheduled mode while a delay sensitive scheduler is deployed in downlink [3]. Non-scheduled mode in uplink means each UE itself decides when and how much data to transmit to the network without any limitation from NodeB. Since different kinds of traffic are mixed in the system, priority based scheduling is natural to be considered here. The prioritization is done between users. Traffic of lower priority is scheduled only when there is no higher priority traffic in the scheduler queues. Depending on whether SIP call control signaling could be distinguished from Presence traffic at the scheduler or not, three priority schemes are considered. SIP call control together with Presence is prioritized over VoIP or vice versa when SIP call control and Presence is indistinguishable. SIP call control is prioritized over VoIP which is in turn prioritized over Presence when SIP call control can be distinguished from Presence. For users belonging to the same traffic type, a delay sensitive scheduler determines which user should be scheduled. C. Simulation models and parameters A detailed dynamic multi-cell HSPA system simulator is employed, see [2]. The voice traffic uses RLC UM while the SIP call control and Presence traffic uses RLC AM. The IP headers of voice packets are compressed using ROHC [4]; the resulting header is 3 bytes. SigComp [5] is used for SIP traffic and the mean compression ratio is 4:1. The HS channel down-switch timer is 2 seconds. Detailed simulation parameters and their values are listed in Table I. TABLE I. Variable Number of cells SIMULATION PARAMETERS. Value 21 cells with 2-D antenna ROHC [bytes] 3 Admission Control Code limitation No No VoIP codec rate [kbps] 12.2 Mean VoIP call duration [s] 30 VoIP activity factor [%] 50 Channel donwswitch timer [s] 2 Poweroffset for VoIP E-DPDCH 0 db for 10 ms TTI, 7 db for 2 [db] ms TTI Poweroffset for Presence E- 7 db for 10 ms TTI, 14 db for DPDCH [db] 2 ms TTI Poweroffset for E-DPCCH -10 db for 10 ms TTI, -3 db for (VoIP/Presence) [db] 2 ms TTI UL DPCCH gating [6], which means UL DPCCH only transmits when necessary, e.g. when data or CQI updates are transmitted in uplink, is also considered in this study. UL DPCCH gating can effectively reduce the UL interference and improve UL VoIP performance as indicated in [6]. While in this paper, it is demonstrated that UL DPCCH gating can benefit the system performance even in downlink as downlink and uplink are coupled with each other in this system. III. SIMULATION METRICS AND ASSUMPTIONS The metrics we care for are VoIP capacity and SIP setup delay. SIP setup delay is defined as the duration from the time when UE sends the INVITE message to the other end until the time when it receives 200 OK for the INVITE message. In the simulation, SIP setup delay is just the round trip delay between UE and RNC, not the end-to-end delay. The VoIP capacity is defined as the offered VoIP traffic load at which 95% of the users are satisfied. A user is satisfied if it is accepted by the system and its packet loss ratio is less than 1% per link. Both uplink capacity and downlink capacity are considered. The offered VoIP load and the capacity figures shown in section IV are relative to the downlink VoIP capacity when no Presence load is offered in the system. The number of Presence enabled users per cell increases from 2000 to 8000 to get the corresponding VoIP capacity and SIP setup delay. IV. THEORETICAL ANALYSIS OF UL VOIP CAPACITY We first investigate how Presence traffic affects UL VoIP capacity with a simple theoretical analysis. The CIR requirement for one user in the UL can be written as γ = γ a + a β + a β ) (1) tot c ( c ed ed ec ec where γ c is the DPCCH CIR, β ed and β ec are the linear power offsets for the E-DPDCH channel and E-DPCCH channel, respectively, compared to the DPCCH CIR. α c, α ed and α ec denote the activity factors for the DPCCH channel, E- DPDCH channel and E-DPCCH channel, respectively.

3 For a VoIP user, activity factors of the E-DPDCH channel and E-DPCCH channel can be estimated by α ed = α ec = α voip * (TTI / AMR Frame interval) * R voip (2) where α voip is the activity factor of voice traffic (0.5 is used, same as in the system simulations), and TTI (length) is either 0.002s or 0.01s, AMR frame interval is 0.02s, and R voip denotes the mean transmission number for one MAC PDU of VoIP. The activity factor of the DPCCH channel is 1. Similarly, the activity factors of E-DPDCH channel and E- DPCCH channel for one Presence user can be estimated by a = a = a TTI R 3600 ed, pres ec, pres λ presence presence presence (3) where λ presence is the number of presence messages per hour, TTI is either 0.002s or 0.01s, and α presence is the activity factor of Presence traffic (here 0.5, since only half of the Presence traffic is transmitted in the UL direction), R presence denotes the mean number of transmissions for one MAC PDU of Presence. As mentioned in section 2, the DPCCH channel will exist until the channel down-switch timer (default 2s) expires for the Presence traffic. Because the duration of the Presence transaction is far less than the channel down-switch time, the influence of the duration of the Presence transaction is ignored in equation 4. Thus, the activity factor of the DPCCH channel for one Presence user is estimated by α c = (channel down switch time ) * (average transaction number per hour)/3600 (4) Given (1)-(4), the required CIR for VoIP and Presence, CIR voip and CIR pres can be calculated. So, the CIR loss per presence user is CIR loss = CIR pres / CIR voip (5) Then, assuming the capacity is linear to the CIR [2], the capacity for the VoIP system with Presence traffic is M = M CIR N (6) original where M original denotes the pure VoIP capacity and N pres is the number of presence users in the cell. The comparison between theoretical UL VoIP capacity and the corresponding simulation results is shown in section V. loss pres significant capacity decrease. The DL VoIP capacity is hardly affected at all, 2-3% capacity decrease, while the UL capacity decrease is around 5-6%. Increasing the Presence load further, up to 6000 Presence users/cell, gives a capacity decrease for the UL of around 15%, and around 3-4% in the DL. Figure 2. Theoretical and simulated results (relative VoIP capacity with variation of Presence traffic load). In UL, basically the same capacity is achieved for both 2ms TTI and 10ms TTI when the offered Presence load is less than 6000 users/cell. But a small (5%) difference is found when the offered Presence load is beyond 6000 users/cell. That is because VoIP users with 2ms TTI reach their maximum power limitation earlier than users with 10ms TTI. Thus higher UL noise rise is generated with 2ms TTI and this affects UL VoIP capacity adversely. We can see from Figure 3 that the UE transmission power with 2ms TTI is larger than that with 10 ms TTI when the Presence load is beyond 6000 users/cell while they are quite similar when the Presence load is less than 6000 users/cell. Figure 2 also validates that the theoretical UL capacity matches the simulation results very well. The parameters used in the theoretical analysis are either input to or output from the simulation. About 2-5% difference is found between the simulation results and the theoretical results. 5% capacity difference occurs only when the Presence traffic load is over 6000 users/cell. V. SIMULATION RESULTS AND PERFORMANCE ANALYSIS A. Simulation results without prioritization In this section, the impact of Presence traffic on SIP call control and VoIP performance in an HSPA system with equal priority for SIP call control, VoIP and Presence traffic flows is shown. The delay sensitive scheduler in [3] is used in the DL, and in the UL non-scheduled mode is used. Figure 2 shows the DL and UL VoIP capacities as a function of the Presence load. The first observation of Figure 2 is that the DL capacity is slightly higher than the UL VoIP capacity without Presence load. This difference is enlarged with the increase of Presence load. The second observation is that the system can handle 2000 Presence users without any Figure 3. Uplink power with different traffic scenarios

4 As expected, higher Presence load results in larger SIP setup delay as shown in Figure 4 and although the UL VoIP capacity with 2 ms TTI is lower than that with 10 ms TTI when the offered Presence load is beyond 6000 users/cell, the SIP setup delay with 2 ms TTI is always smaller than that with 10 ms TTI. Figure 5. DL VoIP capacities with different priority schemes Figure 4. Mean SIP setup delay with 10ms TTI and 2ms TTI. B. Priority based scheduling The idea with priority based scheduling is to take advantage of the fact that Presence is not as time sensitive as VoIP. In this section, three different kinds of priority based downlink scheduling schemes are considered, i.e. SIP call control together with Presence prioritized over VoIP or vice versa, and SIP call control prioritized over VoIP, which in turn is prioritized over Presence. As a reference scheduling without prioritization applied for different traffic types is also considered. The offered SIP/VoIP load shown in the figures below is not the absolute one but relative to DL VoIP capacity when no Presence traffic is offered. The same number of Presence enabled users per cell is offered for all the cases, namely 6000 users/cell. We expect to see a smaller SIP setup delay when SIP call control is prioritized and a higher DL VoIP capacity when VoIP is prioritized. As expected, the results in Figure 5 and Figure 6 show that the scheme to prioritize SIP call control and Presence over VoIP decreases SIP setup delay at the expense of lower voice capacity. However, somewhat unexpectedly is that prioritization of VoIP over SIP call control and Presence degrades both VoIP capacity and SIP setup delay. The scheme that distinguishes SIP call control from Presence traffic does not give good overall performance either. The best trade-off between VoIP capacity and SIP setup delay is achieved with the same priority. Figure 6. SIP setup delay with different priority schemes According to Figures 5 and 6, down-prioritization of Presence (Presence traffic is granted with lower or the lowest priority in the mixed traffic scenarios) is not a good idea. The reason is that the actual concurrently active number of Presence users per cell is not only determined by user arrival rate but also by user departure rate. Low priority to Presence traffic prolongs the transaction duration, decreases the user departure rate, and finally increases the concurrently active number of Presence users in the system. With the increase of active number of Presence users in the system, uplink noise rise goes up, which prolongs the transaction duration further as more retransmission are needed. Since both the uplink noise rise and the power overhead is increased as more active users exist in the system, the performance of VoIP and SIP setup is of course degraded. C. Priority combined with UL DPCCH gating DL scheduling methods combined with UL DPCCH gating are investigated in this subsection. It is already known that with UL gating, the UL VoIP capacity increases quite a lot [6]. Here we consider the effect of UL DPCCH gating on DL VoIP performance. Figure 7 demonstrates that when UL DPCCH gating is enabled and VoIP is prioritized over SIP call control as well as Presence DL VoIP capacity is improved a lot and is larger than the scheme with same priority. Furthermore, the

5 corresponding SIP setup delay when VoIP is prioritized over SIP call control and Presence is reduced at the same time and is now comparable to that using the scheme with same priority as shown in Figure 8. The reason is that UL noise rise is reduced considerably from a very high level to a reasonable level with UL DPCCH gating for the scheme when VoIP is prioritized as shown in Figure 9, which gives relatively good radio link quality. The SIP call setup process and Presence transaction can be finished more quickly, which reduces SIP setup delay and the number of active Presence users in the system. Less number of active Presence users in the system means less power overhead due to F- DPCH, which results in increased VoIP capacity as more power can be allocated to the HS-DSCH channel. Actually in the scheme when SIP call control has the highest priority, both VoIP and Presence can benefit from UL DPCCH gating. UL gating does not have apparent effect on DL VoIP performance with same priority in this case. This is because the UL noise rise is reasonable for this scheme without gating. Figure 7. DL VoIP capacity with UL DPCCH gating Figure 9. UL noise rise comparison (No UL VoIP traffic) VI. CONCLUSIONS This paper investigates an HSPA system with VoIP users, where each VoIP user also has a SIP controlled Presence service. The system simulation results in this paper show that an HSPA system without the so called gating concept [6] can accommodate around 2000 Presence subscribers per cell with a minor capacity loss of around 5%. Increasing the number of Presence subscribers to 6000 per cell gives a capacity loss of 15%. Further, the results show that the UL is a bottleneck for a system with presence service. The major reason for the capacity decrease is not the Presence load itself, rather the control channel overhead from DPCCH in UL and A-DPCH in DL due to the channel release delay. Several priority scheduling schemes are also investigated. The simulations show that the best performance is achieved when VoIP and SIP (including presence and call setup) have equal priority. Prioritizing SIP over VoIP decreases SIP call control setup delay at the expense of lower VoIP capacity. Prioritizing VoIP over SIP or granting SIP Presence the lowest priority results in very large overhead from the control channels, especially the uplink DPCCH. This leads to both decreased VoIP capacity and increased SIP setup delay. A shorter channel release time will improve the Presence performance. However, it is important that other services such as chat and web browsing need to have a relatively long channel release time. UL DPCCH gating can improve the Presence performance substantially. With gating applied the channel overhead decreases. Therefore the best performance is achieved when Presence is down-prioritized over VoIP. Figure 8. SIP setup delay with UL DPCCH gating REFERENCES [1] 3GPP TS , IP Multimedia (IM) Subsystem; Stage 2. [2] Stefan Wanstedt, Marten Ericson, Kristofer Sandlund, Mats Nordberg and Tomas Frankkila, Realization and Performance Evaluation of IMS Multimedia Telephony for HSPA, Proc. of PIMRC'06, Helsinki, Finland, September [3] Mårten Ericson, Stefan Wanstedt, Mixed Traffic HSDPA scheduling Impact on VoIP capacity, to be published in VTC 2007 spring. [4] Bormann et al. RFC3095 RObust Header Compression (ROHC), IETF, [5] Price et al. RFC3320 Signaling Compression (SigComp), IETF 2002 [6] 3GPP TR , Continuous Connectivity for Packet Data Users, 3GPP release 7, 2005