Performance Evaluation of Transport Layer Protocols for Video Traffic over WiMax Hafiz Muhammad Omer Chughtai 1, Shahzad A. Malik 2, Muhammad Yousaf 3 1,2
COMSATS Institute of Information Technology, Islamabad, Pakistan Center for Advanced Studies in Engineering (CASE), Islamabad, Pakistan 1 2 3
[email protected],
[email protected],
[email protected] 3
Abstract-With a variety of access technologies available, the demand for mobile multimedia applications has increased enormously. Supporting these multimedia applications with varied quality of service (QoS) requirements while maximizing the resource utilization is a challenging task. One important factor is the transport protocol that significantly affects the offered QoS and efficient utilization of network resources in the inherently varying transmission conditions due to the wireless medium. Currently most of the multimedia applications use UDP as the main transport layer protocol. However, UDP performance has not been satisfactory in meeting the varied QoS of diverse multimedia applications. A number of new protocols are being developed to meet the diverse needs of emerging multimedia applications. SCTP and DCCP are two important developments that are being considered in this regard. In this paper, through simulations, performance of UDP, SCTP and DCCP protocols has been analyzed for the transport of MPEG-4 video traffic over WiMAX as underlying access technology. Considering single cell WiMAX network, performance metrics such as throughput, delay and jitter have been determined for each of the three protocols in varying WiMAX network topologies/scenarios. On the basis of this study, it has been found that both SCTP and DCCP outperform UDP by large extent. Further, DCCP performance is better than SCTP in terms of delay and jitter.
I.
INTRODUCTION
With the rapid proliferation of wireless access technologies like WiMAX, WiFi, GPRS, UMTS HSPA and LTE, the demand for mobile multimedia applications has gained significant importance. A challenging and demanding requirement of these multimedia applications is to fulfill quality requirements as well as optimal utilization of network resources. One important factor in meeting QoS requirements and efficient resource utilization is the transport layer protocol. UDP is being used for transport of the multimedia traffic in IP based networks like Internet. However, due to the lack of quality of service (QoS) features in UDP, several other transport protocols have been developed, to overcome the deficiencies like reliability, in-sequence delivery, congestion control, and flow control etc. Two such protocols, SCTP and DCCP provide features that may be used for multimedia applications. Various wireless access technologies (WiMAX, WiFi, GPRS, UMTS HSPA and LTE) differ from each other in terms of their range, offered data rates, and QoS features. Recently WiMAX has attracted a great deal of attention and a large number of WiMax networks are being deployed all around the world. WiMAX has gained considerable importance due its
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elegant features such as last mile, long range, accessibility, integrity, flexible TDD/TDMA frame structure and QoS. WiMAX uses TDMA as an access mechanism so there are fixed time based frames. WiMax has been designed from the outset to operate in packet mode and includes mechanisms for QoS provisioning. One of the most demanding multimedia applications is the video as it imposes stringent bandwidth and delay requirements on the network. Video data is encoded using the MPEG (Moving Picture Experts Group) standard. Different versions of MPEG have been introduced until now. Currently MPEG-7 is under development phase whereas MPEG-4 is considered to be more suitable for Internet as compared to the MPEG-1 and MPEG-2 [8]. MPEG-4 produces better video quality as compared to MPEG-1 and MPEG-2 while operating at low bit rate in the same network conditions. The range 4.8kbps to 64kbps is the bit rate of MPEG-4, 1.5 Mbps for MPEG-1 and 15 Mbps for MPEG-2. MPEG-4 stream is composed of three frames e.g. 'I', 'P' and 'B'. Sequence of each frame is dependent on the encoding procedure [1]. Performance study of transport layer protocols for multimedia traffic over Ethernet and WiFi has been carried in [7]. In [1] performance analysis has been conducted by comparing Transport Layer Protocols like PRSCTP (Partial Reliable Stream Control Transmission Protocol), TCP and UDP for MPEG4 traffic in mobile network. Other performance analysis has also been done to transport MPEG4 for SCTP over wired technology [8]. As WiMax deployment is taking place at an extremely rapid pace, extensive analysis is needed to determine and characterize the performance of various protocols for high end applications like video in WiMAX networks. In this work, we have focused on performance analysis of transport layer protocols (SCTP, DCCP, UDP) for video traffic over WiMax. The paper is organized as follows: Section II provides a brief introduction of SCTP and DCCP while in section III, WiMax has been discussed. Section IV includes the simulation setup, scenarios and the simulation results. Section V concludes the work and identifies some future work. II.
TRANSPORT LAYER PROTOCOLS FOR MULTIMEDIA APPLICATIONS
Transport protocols are characterized by various features such as reliability, in-sequence delivery, congestion control,
and flow control that essentially contribute to performance and quality offered by the underlying network. For multimedia applications such as video, the quality requirements include low delay, jitter, loss rate and a minimum throughput. The choice of a transport protocol affects these performance measures. As mentioned earlier, UDP is currently being used. However, UDP performance is not satisfactory. SCTP provides a reliable transport service, ensuring that the data is transported across the network without error and insequence. A unique association has been established between the two end points before sending the data. Datagram orientation of UDP, reliability and the sequencing of TCP combine to form SCTP. Due to its features like multistreaming and multi-homing, it utilizes bandwidth in an efficient way [3]. In multi-streaming each stream has a unique stream ID, any errors in a particular stream will not affect the others which can then reduce the head of line blocking (HOL) during the whole association [4]. Datagram Congestion Control Protocol (DCCP) is another transport layer protocol, which provides congestion-controlled mechanism with Explicit Congestion Notification [5]. DCCP offers two different congestion control mechanisms including TCP-Like and TCP friendly rate control [6]. Response of received packets in DCCP is similar to selective acknowledgment (SACK) of TCP. Streaming applications over DCCP provides less delay, reliable and in-order delivery. III. IEEE 802.16 - WIMAX IEEE 802.16 defines a broadband wireless access technology, built to replace or complement its counterpart wired technologies such as DSL. IEEE 802.16, well known as WiMAX (Worldwide Interoperability for Microwave Access). WiMAX offers amazing data rates up to several ten of Mbits/s and radius in Kms. In WiMAX, Base Station (BS) has the responsibility to centrally control the Subscriber Stations (SSs), means that the SSs have to communicate with each other through BS. WiMAX is capable of providing a minimum bandwidth of 1.5 Mbps and can theoretically go beyond 75 Mbps. Furthermore the QoS and resource access grant mechanism of WiMAX makes it a better choice in delaysensitive and high bandwidth-based video transmission. In WiMAX radio signals have been utilized efficiently. Some of the Features that WiMAX provides include wide coverage, mobility, flexible architecture, low cost, high capacity, high security, quick deployment and quality of service (QoS) [12]. WiMAX Implementation for ns-2: NIST WiMAX implementation used in this study is based on the IEEE 802.16-2004 standard [11]. Standard features of NIST WiMAX implementation are as follows [10]: • Duplexing technology is Time Division Duplexing (TDD) • Management messages for the node to enter in to the network • Different schedulers have been provided in this implementation, By Default, round robin scheduler is used.
Fragmentation and reassembly of the uplink and downlink frames. • Orthogonal Frequency Division Multiplexing (OFDM) feature at physical layer with changeable modulation techniques. •
No authentication mechanism has been defined in this implementation when the node enters into the coverage area of a base station [10]. Figure 1, represents the flow of traffic from upper layers towards the WiMAX layers. VIDEO TRAFFIC (MPEG-4) UDP/SCTP/DCCP IP WiMAX Figure 1. Traffic Flow from upper layers to IEEE802.16 Layers.
WiMAX implementation offers the flexibility to control user data rate by configuring parameters like downlink to uplink ratio (dlratio), modulation type, cyclic prefix etc. By changing the values of these parameters, results have been analyzed. Here 0, 0.25 (1/4), 0.125 (1/8), 0.0625 (1/16), 0.03125 (1/32) are the valid values of cyclic prefix. Increase in cyclic prefix, reduce the maximum throughput and vice versa [10]. Several study scenarios have been simulated through combination of the above mentioned parameters. IV. SIMULATION SETUP AND RESULTS For this work, a generic simulation setup has been designed using ns-2 simulator [2], as shown in Figure 2; setup includes single cell WiMAX network connectivity between base station (BS) and subscriber station (SS). BS is directly connected with server using a wired link, whereas SS are connected with the BS through a wireless link.
Figure 2. Simulation Setup.
Different scenarios have been configured to upload and download transport of MPEG-4 Video traffic between BS and SS. Performance analysis is conducted for each of the three transport protocols: UDP, SCTP and DCCP.
1.
Performance Metrics Four performance metrics have been evaluated and analyzed in this study for various Transport Layer Protocols like UDP, SCTP and DCCP. These performance metrics directly affect the video traffic [9] and are described as below; a.
Throughput Throughput is the received rate at which a computer receives data in a given amount of time or simply a rate at which a file is transferred from one system to another per unit time. Equation (1) shows the received rate (throughput). Throughput=
i.
ii.
Numberof ReceivedPackets Last packetsentTime− First packetsentTime
−−−−−−1
Instantaneous Throughput Instantaneous throughput is the received rate at a particular moment or at a specific instance of time. Average Throughput Average throughput is computed as the total received rate divided by total time. Total time may be the Simulation run time with reference to the simulation start and stop time or the total time may be the time between last packet received and first packet received.
2.
Simulation Scenarios Four simulation scenarios have been considered for the evaluation of Transport Layer Protocols for video traffic (MPEG4) over WiMAX. Scenarios are listed below; i. ii. iii. iv.
Maximum achievable rate of the WiMAX link Impact of variable packet size on performance of Transport Layer Protocols (UDP, SCTP and DCCP). Impact of number of subscriber stations on performance of UDP, SCTP and DCCP Impact of variable video rate on performance of UDP, SCTP and DCCP.
1) Maximum achievable rate of the WiMAX link Figure 3 shows the setup that is used to determine the maximum achievable data rate or total capacity of the WiMAX download link. Total capacity of a particular WiMAX link is the maximum data rate that can be reliably achieved over a communication link. In this scenario, a CBR/UDP traffic flow is established between the SS and the Server. The send rate of flow is varied until the WiMax link saturates. This is indicated by a constant throughput in spite of increasing send rate.
b.
Delay Delay is the time experienced by a packet to travel or move across the network from one device to another. Expression for delay is shown in equation (2). Delay= Tr− Ts
−−−−−− 2
'Ts' is the sending time of a particular packet and ‘Tr’ is receiving time of that packet. Mean delay is the average delay computed using the relation shown in equation (3). MeanDelay=
TotalDelay N
−−−−−− 3
Where 'N' is the total number of packets received during simulation time. c.
Jitter Jitter can be calculated using equation (4). Jitter= Delayj − Delayi
−−−−−− 4
Where 'Delay (j)' is a current packet delay and 'Delay (i)' is a previous packet delay. Similarly mean jitter can be computed as in equation (5). MeanJitter=
TotalJitter N
−−−−−−5
'N' is the total number of packets received. d.
Packet Loss Packet Loss= Ps j− Pri
− − −− − − 6
Where Ps (j)' is the total numbers of packets sent and 'Pr (i)' is the total number of packets received.
Figure 3. Capacity of WiMAX download Link.
For downlink and uplink sub-frames, dlratio, that is downlink to uplink ratio has been configured, with value equal to 0.5, which indicates that 50% of the frames are for downlink and 50% are for uplink. Parameters tabulated in Table I, are used to find maximum achievable rate of WiMAX link in downloading scenario. TABLE I - PARAMETERS FOR CAPACITY OF THE LINK Downlink to Uplink ratio (dlratio) 0.5 Frame duration Frequency Cyclic prefix Packet size Modulation Protocol Traffic Wired Link BW Wired Link Propagation Delay Queuing Mechanism
0.004 sec 5 MHz 0 1000 Bytes 64 QAM UDP CBR 100 Mb 1 msec Drop Tail
(FIFO) scheduling algorithm has been used in this scenario. In this scheduling algorithm the packet dropping probability increases when the buffer is full. Scheduling algorithm plays an important role in avoiding the congestion. UDP comprises no congestion control mechanism so a packet in the congested link reaches at the destination by acquiring extra time so experiences greater delay.
Figure 4. Total Capacity of WiMAX download Link.
Figure 4 represents that, with the increase in send rate, received rate or throughput also increases. Received rate increases with a ratio of 0.98, up to 7Mbps and beyond this remains constant if dlratio is equal to 0.5. Similarly we can depict that by varying the dlratio the capacity of the WiMax link also varies. This is because of the unreliable behavior of the UDP. 2) Impact of variable packet size on performance of Transport Layer Protocols (UDP, SCTP and DCCP) Figure 5 reflects an uploading scenario. In this scenario, Subscriber Station (SS) sends MPEG-4 traffic towards a server through WiMAX link. Transport Layer Protocols like UDP, SCTP and DCCP for MPEG-4 traffic are evaluated by varying the packet sizes from 300 Bytes to 4000 Bytes with a constant send rate equal to 2Mbps. Other parameters used in this scenario are tabulated in Table II.
Figure 5. Uploading MPEG-4 Traffic over WiMAX.
Both SCTP and DCCP have a congestion control mechanism hence experiences less delay. From the table; DCCP is better than SCTP up to 2000 Bytes as it includes Explicit Congestion Notification (ECN) mechanism along with congestion control mechanism. In Figure 6, Jitter has been calculated using variable values of packet sizes. Analysis reveals that, in case of UDP, Jitter increases with the increase in packet size. Initially up to 1500 Bytes DCCP shows less values of Jitter, reflecting better results to some extent compared with other protocols. Jitter is directly proportional to delay. 16 14
Packet size (Bytes) 300 500 1000 1500 2000 2500 3000 3500 4000
TABLE III - PACKET SIZE VS DELAYS UDP Delay SCTP Delay DCCP Delay (msec) (msec) (msec) 54.5989 15.9972 14.7959 56.3003 62.8326 65.5782 66.4418 70.6263 72.8518 79.4664 85.4193
54.5904 64.5906 63.6859 65.3676 62.8738 63.4738 62.8738 63.2278
47.1528 55.7291 58.822 65.9017 71.1802 68.6437 70.5793 71.8793
Values shown in Table III depict that, in case of UDP an increase in packet size increases the packet delay. SCTP and DCCP reflect lower delay as compared to UDP. Drop-Tail queuing mechanism, which implements First-In-First-Out
12
Jitter (m sec)
TABLE II - PARAMETERS USED IN 2ND SCENARIO Packet size Variable Modulation 64 QAM Protocol UDP/SCTP/DCCP Traffic MPEG4
10 8 6 4 2 0 300
500
1000
1500
2000
2500
3000
3500
4000
Packet Size (Bytes)
UDP Jitter (msec)
SCTP Jitter (msec)
DCCP Jitter (msec)
Figure 6. Jitter Comparison of UDP, SCTP and DCCP.
3) Impact of number of subscriber stations on performance of UDP, SCTP and DCCP Downloading scenario in Figure 7 is composed of multiple Subscriber Stations (SSs). These SSs are connected with a server through base station using WiMAX links. Video traffic (MPEG4) with a constant rate equal to 2Mbps has been
transported over each Transport Layer Protocols UDP, SCTP and DCCP by varying number of SSs. Details of parameters used in this scenario is given in Table IV. TABLE IV - PARAMETERS USED IN 3RD SCENARIO 0.6 Downlink to Uplink ratio (dlratio) Frame duration 0.005 sec
12
1000 Bytes 64 QAM UDP/SCTP/DCCP MPEG4
10
Throughput (M bps)
Packet size Modulation Protocol Traffic
Similarly the other performance measures like Delay and Jitter are also being considered in this scenario. Received rate and sent rate depends upon the congestion over the network; apparently delay and jitter also changes accordingly. From Table VI, it is observed that DCCP has lower values of delay and jitter compared with UDP and SCTP.
8 6 4 2 0 1
2
4
6
8
10
No. of SSs UDP Received Rate
SCTP Received Rate
DCCP Received Rate
Figure 8. Number of Subscriber Stations Vs Received Rate. TABLE VI - DELAY AND JITTER WITH RESPECT TO THE SUBSCRIBER STATIONS No UDP SCTP DCCP Figure 7. Downloading Scenario for variable Subscriber stations.
No. of SS 1 2 4 6 8 10
TABLE V - PARAMETERS USED IN 2ND SCENARIO UDP SCTP DCCP Received Received Received Packet Packet Packet Rate Rate Rate Loss Loss Loss (Mbps) (Mbps) (Mbps) 0.9 1.8 3.4 5.1 5.5 6.2
10 89 151 623 1270 1944
1 2 4 5 7 8
0 0 0 30 44 45
1 2 4 6 8 10
0 0 0 0 0 3
Table V, shows the received rate (Throughput) and packet loss with reference to the number of subscriber stations for UDP, SCTP and DCCP protocols. Configuring the above scenario shown in Figure 7; we have analyzed that as we go on increasing the number of SSs, received rate also increases accordingly however it is realized that the rate achieved by UDP is less than SCTP and DCCP. SCTP and DCCP behave alike to number of subscriber stations equals to 4. Beyond that DCCP behaves better for throughput and as well as for packet loss. This is because of flow control and congestion control mechanisms employed in DCCP. Congestion is a stage where packets sent to the network are greater than available capacity of the network. Flow control is the size of traffic being sent by a sender up to the limit it receives response from the receiver. Figure 8 shows the graphical view Throughput as in Table 7.
of
Delay
Jitter
Delay
Jitter
Delay
Jitter
SS
(msec)
(msec)
(msec)
(msec)
(msec)
(msec)
1
0.0063
0.0011
0.0067
0.0014
0.0060
0.0039
2
0.0078
0.0012
0.0076
0.0017
0.0095
0.0053
4
0.0126
0.0020
0.0142
0.0029
0.0146
0.0077
6 8 10
0.0173 0.0192 0.0281
0.0023 0.0025 0.0063
0.0295 0.0445 0.1618
0.0073 0.0123 0.0292
0.0236 0.0368 0.0391
0.0120 0.0227 0.0266
4) Impact of variable video rate on performance of Transport Layer Protocols (UDP, SCTP and DCCP) TABLE VII - PARAMETERS USED IN 3RD SCENARIO 0.7 Downlink to Uplink ratio (dlratio) Frame duration 0.005 sec Packet size 1000 Bytes Modulation 64 QAM Protocol UDP/SCTP/DCCP Traffic MPEG4
Topology in this scenario is same as shown in figure 3, however parameters used are tabulated in Table VII. Throughput and packet loss for transporting video traffic (MPEG4) for each Transport Layer Protocols i.e. UDP, SCTP and DCCP over WiMAX has been scrutinized in this scenario. Subscriber Station (SS) receives MPEG-4 traffic from server through WiMAX link as shown in Figure 3. Results composed for each protocol i.e. UDP, SCTP and DCCP are evaluated using variable video send rate as depicted in Table VIII.
5
simultaneous flows as compared to UDP. It is noticed that in case of UDP and SCTP the number of packets lost increases and hence badly affects the video quality beyond a send rate of 4 Mbps. Further, we find that up to 4Mbps of video send rate, both SCTP and DCCP give same received rate however beyond that DCCP maintains its received rate and its packet loss is minimum as compared to SCTP. However, DCCP gives higher throughput and smaller packet loss ratio. DCCP is not a reliable transport layer protocol but it performs much better than UDP for traffic like MPEG-4. SCTP is a reliable transport layer protocol and offers excellent throughp8t but after a certain rate (4Mbps) its performance decreases. Based on the analysis in this study, DCCP seems better suited to video streaming application and a better choice for fulfillment of QoS requirements for the transport of streaming video traffic over WiMAX. Because received rate and the delay in case of DCCP are much better than SCTP and UDP and DCCP experiences minimum packet loss as compare to both SCTP and UDP for video traffic. If reliability is not a major concern then DCCP is the best alternative of UDP which has no congestion control mechanism. The work can be extended by including mobility aspect, additional combination of various WiMAX physical layer parameters and considering other access technologies as well.
4
REFERENCES
From Table VIII, it is clear that the received rate increases with the increase in video send rate. In case of UDP the maximum received rate is 5.43 Mbps for 7 Mbps send rate, whereas DCCP has the 100% received rate for send rate up to 7Mbps as shown in above Table. Below is the graphical view of received rate shown in Figure 9 which has been examined in this scenario.
Send Rate (Mbps) 1 2 3 4 5 6 7
TABLE VIII - PARAMETERS USED IN 2ND SCENARIO UDP SCTP DCCP Received Received Received Packet Packet Rate Rate Rate Loss Loss (Mbps) (Mbps) (Mbps) 0.97 1.94 2.87 3.80 4.55 5.17 5.43
10 26 94 164 450 763 1128
1.00 1.98 2.96 3.99 4.39 4.45 4.45
0 0 0 0 0 0 12
Packet Loss
1.0 2.0 3.0 4.0 5.0 6.0 7.0
0 0 0 0 0 0 0
8
Throughput (M bps)
7 6
3
[1]
2 1 0 1
2
3
4
5
6
7
Video Send rate (M bps)
UDP Received Rate
SCTP Received Rate
DCCP Received Rate
Figure 9. Video Send Rate Vs Received Rate.
Same is the case with packet loss, as revealed from Table 10. In case of UDP, increase in send rate consequently increases the packet loss. This is due to the unreliable mechanism of UDP. But in case of SCTP and DCCP, the ratio of packet loss is too much low. However behavior of DCCP in case of packet loss is very almost negligible as shown in Table VIII. V. CONCLUSIONS AND FUTURE WORK In this work, we have analyzed the performance of three transport layer protocols i.e. UDP, SCTP and DCCP over WiMAX for supporting MPEG-4 video traffic. The performance measures like delay, jitter, packet loss and throughput have been evaluated using four different simulation scenarios. Results indicate that both DCCP and SCTP exhibit better performance than UDP. Both SCTP and DCCP are reasonably fair in distributing bandwidth among the multiple
Hongtao Wang, Yuehui Jin, Wendong Wang, Jian Ma, Dongmei Zhang, "The performance comparison of PRSCTP, TCP and UDP for MPEG-4 multimedia traffic in mobile network", Broadband Network Res. Center, Beijing Univ. of Posts & Telecommun., China, Volume: 1, 2003, Pages:403-406 [2] The Network Simulator – ns-2, available online www.isi.edu/nsnam/ns/ [3] L. Ong, J. Yoakum, "RFC 3286: An Introduction to the Stream Control Transmission Protocol (SCTP)", IETF, May 2002 [4] Rumana Alamgir, Mohammed Atiquzzaman, William Ivancic, "Effect of Congestion Control on the Performance of SCTP and TCP in a Wireless Environment", School of Computer Science University of Oklahoma, Norman,Volume 4: June 2005 [5] E Kohler, M Handley, S Floyd, "RFC 4340: Datagram congestion control protocol", IETF, May 2006 [6] S Floyd, E Kohler, "RFC 4341: Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control", IETF, March 2006 [7] Nosheen, S. Malik, S.A. Bin Zikria, Y. Afzal, M.K, "Performance Evaluation of DCCP and SCTP for MPEG4 Video over Wireless Networks", Multitopic Conference, 2007. INMIC 2007. IEEE International, Pages: 1-6, Dec. 2007 [8] Ashraf Matrawy, Ioannis Lambadaris, Changcheng Huang, "MPEG4 Traffic Modeling Using The Transform Expand Sample Methodology", In Proc. of 4th IEEE IWNA4, 2002 [9] Zhao Lifen, Shang Yanlei, Liu Ju, " The performance study of transmitting MPEG4 over SCTP", Sch. of Inf. Sci. & Eng., Shandong Univ., Jinan, China, Volume: 2, 2003 [10] National Institute of Standards and Technology – The Network Simulator ns-2 NIST, Draft 1.2 add-on IEEE 802.16 model (MAC+PHY), June 2007 [11] IEEE std 802.16-2004 (Revision of IEEE Std 802.16-2001) (2004), pp. 0_1-857.ISBN: 0-7381-4070-8, 2004 [12] WiMax Explained: System Fundamental by Dr. Kalai Kalaiachelvan Lawrence Harte, 2007, ISBN-13:978-1932813548
