TCP has been designed for use in high bit rate, low-error rate communication .... A simplified single-slot EDGE (Enhanced Data for GSM. Evolution) link level ...
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Impact of TCP/IP Header Compression on the Performance of a Cellular System Zoran Kostic, Xiaoxin Qiu and Li Fung Chang Wireless Systems Research, AT&T Labs-Research 100 Schulz Dr., Red Bank, NJ 07701, USA Abstract - This paper reports on studies of the effects of the TCP/IP protocol header on the system performance of a spectrally efficient cellular system. Integrated cellular-system and TCP-protocol simulations are used to examine the degradation in the effective data throughput and packet delay due to the header overhead. Performance improvements achievable by header compression are investigated. Simulations have been designed for an EDGE-like cellular system. Systemwide performance is examined using cumulative distributions for data throughput, delay and packet success. For example, the data throughput of a micro-cellular system with Web traffic is improved about 10% when header compression from 40 to 2 bytes is implemented. More aggressive compression yielded minimal further improvements.
I. INTRODUCTION Cellular systems of the 2nd and 3rd generations are/will be deployed in expensive licensed frequency bands and are therefore being engineered to provide maximal spectral efficiency and revenue/profit to cellular service providers. There has been a realization by equipment vendors and service providers that future-proof designs of cellular systems need to be based on IP-network foundations. This includes a need to support TCP, UDP, RTP and other IP protocols not only over the network infrastructure of cellular systems, but also over the air to the mobile units. Some technical difficulties associated with using IP protocols in radio channels have been studied by the research community. It has been realized and reported in the literature that running TCP (transmission control protocol) over a single Rayleigh faded wireless channel can result in potentially intolerably high data throughput reduction and very high data delivery delay. A part of this degradation is due to the congestion control mechanisms of TCP, whereas the other part is due to the header overhead that TCP protocol creates in the process of data packetization/segmentation. TCP has been designed for use in high bit rate, low-error rate communication media, such as optical fiber, but wireless channels are often subject to high error rates and they provide relatively low bit rates. The TCP congestion control mechanism is particularly sensitive to high error rate channels. The effect of this feature, in error-prone wireless channels, is that TCP responds poorly to rapid radio channel variations. That creates a possibility for a dramatic underutilization of the achievable wireless channel capacity. Specifically, after the congestion control window (which sets the maximum permitted number of packets in transit)
collapses when a (Rayleigh) fading channel gets poor, it can not recover or expand fast enough to benefit from the rapid improvement in signal-to-noise ratio that a Rayleigh channel experiences. TCP/IP header overhead impacts performance in a rather straightforward fashion. Data is segmented in packets, and packets are encapsulated with information needed to support packet-switched transmission - such as destination address and an acknowledgment field. An uncompressed TCP/IP header is 40 bytes long for IPv4 and 60 bytes for IPv6 version of TCP. The smaller the size of the payload data, the more it is impacted by the header size. The impact manifests itself in the inefficient use of radio bandwidth, which numerous users are sharing in cellular systems. Low delay real-time speech (carried with about 10-20 bytes every 10-20ms) is obviously going to be very significantly affected by 40 byte long headers. Namely, only one third of the link-layer bit throughput would be useful. This is considerably worse than today’s circuit-switched voice cellular systems. The focus of the research community and standardization bodies has mostly been on the interactions between a single Rayleigh faded wireless channel and the TCP protocol [1,2,3,4,5]. The work that we present in this paper extends the current knowledge in that it examines the interactions between the TCP protocol and a loaded cellular system. In particular, we focus on the effects of TCP/IP header compression on the performance. Section II describes the study that we have pursued. Section III presents simulation models of the cellular system and the TCP protocol. Section IV shows the performance results and Section V contains the conclusions. II. DESCRIPTION OF THE SYSTEM Cellular systems are limited by interference as well as by the fading and noise which is found on single radio links. Therefore the performance of TCP in cellular systems can not be inferred from its performance over a single faded link. Our focus here is on results presenting performance degradation of data throughput and packet delay in cellular channels from a cellular system perspective. These degradations are determined by the interaction of the cumulative distribution function of Signal to Interference Ratio (SIR) in a cellular system and by the TCP protocol mechanisms. Even with Rayleigh fading on individual links, many users close to base stations will see small degradation
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in the performance with the use of TCP, whereas fringe area users can be very seriously affected. Whereas a single faded radio link has no resemblance to the "congestion channel" that TCP is built for, the "cellular channel" can exhibit "congestion channel" behavior. This behavior is driven by the interference-limited nature of spectrally efficient cellular systems. Recent discussions in two working groups of the Internet Engineering Task Force (IETF), called the performance implications of link characteristics (pilc) group [6] and the robust header compression (rohc) group [7], have been concerned with the issue of IP protocol header compression. A conclusion has been reached that older header compression schemes [8] do not handle wireless channels well enough for UDP-based IP-voice. The work on TCP header compression has recently started in the rohc group.
The system level simulation includes many TCP protocol sessions running on individual radio links. Cellular System Model Frequency reuse of one is simulated with 49 base stations and a wrap-around feature. The layout is shown in Figure 1. A simplified single-slot EDGE (Enhanced Data for GSM Evolution) link level protocol is used. In this protocol adaptive modulation is implemented to support variable data rate transmission, and link level ARQ is used to provide a reliable link to the upper layer. The peak link layer data rate is 59.2 kbps. Signal-based site selection is used in assigning mobiles to base stations. There is no antenna sectorization nor power control in this study. Both reliable and unreliable radio link protocols are simulated.
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With the goal of assisting discussions on the topic of TCP header compression for cellular communications, this paper shows the effects of header compression on data throughput and packet delay. Using simulations we obtain results showing the loss of effective throughput and increase in delay due to the header in TCP packets. The simulations that we created is an amalgam of a comprehensive system level cellular simulation and Reno TCP protocol [9] running on each base to mobile link.
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Whereas IP experts generally are in favor of strictly defined header-compression schemes for IP packets, some spectral-efficiency-focused cellular system vendors are examining header stripping before the wireless link and (partial or complete) header reconstruction. This can be accomplished by sending header information through the radio link protocol or omitting it altogether.
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Figure 2. Link layer data throughput as a function of SIR For each user, independent traffic can be generated with several traffic models: 1) Exponentially distributed interarrival times and fixed-size packets, 2) Web traffic, 3) Exponentially distributed inter-arrival times and packet lengths, 4) Speech traffic. Packets are broken into TCP segments, and TCP segments are broken into link-level blocks defined by the 1999 revision of the EDGE specification. Table 1 : System Simulation Parameters
Figure 1. Cellular System Layout
PARAMETER FREQUENCY REUSE CELLULAR GRID
III. SIMULATION MODEL A comprehensive cellular system level simulation is used in our studies. It models many base and mobile stations, and computes individual user and system level performance in terms of data throughput and delay. It relies on results of link level simulations that represent block error rate as a function of Signal to Interference Ratio (SIR) for a single radio link.
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1/1 49 BASE STATIONS WRAP AROUND HEXAGONAL TIME-EVOLUTIONARY UPLINK/ DOWNLINK SINGLE-SLOT DATA TWO TIERS MICRO-CELL (exp=4.5) SHADOWING σ=8dB TIME-CORRELATED
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RAYLEIGH FADING (0.1Hz) UNIFORM STRONGEST BASE ADAPTIVE MODULATION AND ARQ
The simulation consists of the following steps: a) Positioning base stations and mobiles in particular geographical locations; b) Computing propagation attenuation, shadowing and Rayleigh fading for all basestation to mobile links; c) Once a user becomes active, assigning an appropriate time slot to it, if there is any available; d) Computing the SIR for each active user; e) Determining a suitable transmission mode (modulation and channel coding) from the previous step and from the values of link adaptation thresholds; f) Determining if a particular link-level block is in error according to the block error rate performance of the chosen mode; g) Updating buffer contents, throughput and delay statistics for each user and for the system, at both the link level and TCP level; h) Computing CDF curves for throughputs and delays. B.
TCP Protocol TCP protocol sessions are embedded within the system level simulation. Each link from base to mobile has an independent traffic generator and operates its own TCP protocol session. In the simulation, we implement the full Reno version of the TCP protocol on the downlink, and model the protocol behavior on the uplink. The uplink model captures acknowledgments, TCP segment delays, drops, jitter, arrival times, and provides complete reverse channel behavior. The implemented protocol follows the state machine descriptions from [9]. To reduce the load on the simulation hardware, we do not process information bits, but fully represent TCP segments by headers and tags indicating segment lengths. The default length of segments is 576 bytes. On a per mobile basis, traces showing TCP window size, threshold, timeout occurrences, moments of individual segment transmission and retransmission, growth of packet FIFOs and other data are generated. Statistics are collected as the simulation time evolves for individual users, as well as for the whole system. For a selected user, an enlarged set of parameters/variables can be monitored.
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Although the simulation allows for different scenarios, in this paper we focus on users that are stationary (in a shadowfading sense) for the duration of a session. Once a mobile is geographically located within the system, base station selection is done based on random samples of shadow fading. During the rest of the simulation, the shadowing is considered fixed for SIR computations. The system level simulation contains a model of the time correlated Rayleigh fading for each link, with no correlation between the links. The block error rate curves which we utilize are thereby based on link level simulations for Gaussian channels. Figure 2 shows the link layer data throughput curves used in the simulations. Table 1 summarizes some simulation parameters.
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Figure 3. Data throughput for a system with 10 users per cell, exponential interarrivals and packet sizes IV.
PERFORMANCE RESULTS
To examine the effects of TCP/IP headers and header compression on a cellular system performance, we monitor cumulative distribution functions (cdfs) representing data throughput, packet delay, successful packet transmission and successful TCP segment transmission of individual links in a system. For this paper, the simulation computes and collects link layer data throughput (without TCP induced data rate loss) and post-TCP data throughput (after loss due to header and congestion mechanisms) for each individual mobile. Also, normalized average delay of packets (application layer packets) for each mobile, as well as the probability of successful packet transmission per mobile are recorded. 1
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Figure 4. Normalized delay for a system with 10 users per cell, exponential interarrivals and packet sizes
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We experimented with numerous system parameters to examine how headers and header compression interact and impact the performance. Here we show representative results for three traffic types, several header sizes, two values of system loading (average number of users per cell) and an unreliable radio link protocol (RLP) using no more than 10 retransmissions per RLP block. 1
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Figure 5 shows the cdf of the percentage of application packets successfully transmitted to a mobile station (packets that were successfully segmented into TCP segments and taken inside the TCP window)2, where 100% represents the traffic generated to one user. The qualitative behavior for all sizes of headers is similar. For example, 80% or smaller percentage of application packets are successfully transmitted to about 16 percent of users for zero header size. For about 82 percent of users, all 100% of packets are successfully transmitted. These numbers indicate that under loading conditions of 10 users per cell, the system is incapable of handling the generated traffic for all users. Observe that one could reach this conclusion only after examining the packet success statistics, although both the throughput and delay curves appear to be reasonable.
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In Figure 3, we show the performance for a system with average loading of 10 users per cell, for exponentially distributed packet lengths with average equal to 1.6 kBytes and exponentially distributed interarrival times with average equal to 1 second. Maximum link layer data throughput is around 59 kbps1 and, for the header of the size of 2 bytes, the same throughput is achieved at the TCP layer. Actually, there is negligible difference in throughput at the link layer and TCP layer for all users (throughout is the cdf curve), for header of size 2. When the header is 40 bytes in size (typical length of an uncompressed header), one can observe that the maximum data rate is reduced to around 53 kbps and that there is an almost constant 5 kbps decrease in rate for all users in the system. Figure 4 presents the cumulative distribution function of the normalized average delay experienced by application layer packets, for the system with average loading of 10 users per cell and the rest of the parameters being the same as in Figure 3. For the ninetieth percentile of mobile users, the average delay is less than 200 ms/kbit, both for the case with no header and for the case with a header of 2 bytes. For 40byte headers, the delay at the ninetieth percentile is increased to 300ms/kbit. Observe that the tail is long, which implies that for some users the average delay is very significant.
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Figure 6. TCP data throughput for a system with 5 users per cell, exponential interarrivals and packet sizes 1
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Figure 7. Normalized delay for a system with 5 users per cell, exponential interarrivals and packet sizes 2
Packet success is complementary to the quantity expressed by the term " percentage of self-blocking packets".
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There are also no dramatic differences between various header lengths when packet delays are examined. This is shown in Figure 7. However, when compared to the loading of 10 users per cell, packet delays are reduced from the range of 200 ms for the 90th percentile point to 40 ms for 90 percent of users. Differences between heavier (10 users per cell) and lighter (5 users per cell) loading become pronounced when packet success statistics are compared between Figure 5 and Figure 8. For loading of 5 users per cell, and for RLP with no more than 10 block retransmissions, no application packets get blocked at the entry to the TCP window (There is a floor shown in the figure, which comes from the fact that the simulation is terminated before the traffic sources are stopped.)
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Table 2. Web workload model Figure 8. Packet success for a system with 5 users per cell, exponential interarrivals and packet sizes We therefore reduce the loading of the system to 5 users per cell on average and keep other parameters the same as in the previously described figures. Figure 6 shows two curves with data throughput for headers of size 2 and 40 bytes and for unreliable radio link protocol with no more than 10 retransmissions. Note that for these application packet sizes (average of 1.6 kBytes), 2 byte headers give virtually the same performance as is obtained without headers. Forty byte headers impact the system performance by reducing effective data throughput by about 5 kbps for all users, which is the same as in the case for loading of 10 users per cell. The curves for lighter loading are steeper, indicating that a larger population of users is getting data at more rapid rates. 1
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LINK LAYER THROUGHPUT (kbps) TCP THROUGHPUT (kbps),HEADER=2 TCP THROUGHPUT (kbps),HEADER=40 NORMALIZED DELAY (ms/kb) PACKET SUCCESS(%)
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WEB PAGE SIZE DISTRIBUTION MINIMUM WEB PAGE SIZE MAXIMUM WEB PAGE SIZE MEDIAN WEB PAGE SIZE MEAN WEB PAGE SIZE OFF TIME DISTRIBUTION MINIMUM OFF TIME MAXIMUM OFF TIME MEDIAN OFF TIME MEAN OFF TIME
LOG-NORMAL (u=9.5,σ=1.8) 100 BYTES 100 kB 10 kB 20 kB PARETO (k=2,α=2) 2 seconds 10 min 4 seconds 12 seconds
Figure 9 illustrates data throughput at link and TCP layers, normalized average delay per application packet, and packet success for the Web traffic source. For loading of 5 users per cell, no more than 10 block retransmissions on the RLP layer, and for the header of 40 bytes, TCP imposes a constant 5 kbps data rate loss over the link-layer. Average link layer throughput is 50.9 kbps, average throughput with 2 byte headers is 49.9 kbps, and average throughput with 40 byte headers is 46.4 kbps. Both normalized delay and packet success rate behave well. One can observe that the loss in data throughput due to the header of length 2 is minimal. Parameters of the Web traffic model that we used are summarized in Table 2. This is a simple model closely related to other models that appear in the literature [10].
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Figure 9. Throughput, delay and packet after TCP for a system with 5 users per cell and for the Web traffic
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V. CONCLUSION TCP segment headers and TCP congestion control impose degradations in the performance of cellular systems whose users utilize the TCP protocol. By means of simulations, we have investigated how headers and header compression impact the loss in data throughput, increase in application packet delay, and reduction in packet success rates. The study has been done for a single-slot microcellular version of the EDGE system with peak data throughput of 59.2 kbps. A selection of the results is presented in the paper. For constantstream data traffic sources where the average packet length is 1.6 kBytes, all the performance metrics behave well for a
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system with light loading of 5 users per cell. The full length header of 40 bytes imposes a data throughput reduction of around 5 kbps uniformly for all users. Packet delays are bounded to between 40 and 60ms for the 90th percentile of users, and packet success rate is 100 percent. With loading of 10 users per cell, the packet success metric indicates that even aggressive header compression can not bring the system to an acceptable performance state. Web traffic simulations for loading of 5 users indicate similarly that header compression from 40 to 2 bytes improves the performance in the range of 5 kbps per user. The principal conclusion of this paper is that aggressive TCP/IP header compression is a needed feature for spectrally efficient cellular systems, even when a header is only a fraction of the size of the payload. However, our studies indicate that going to header compression techniques more aggressive than 2 bytes per TCP segment is not needed for TCP applications in cellular systems.
ACKNOWLEDGMENTS The authors would like to thank Shankar Shankaranarayanan, Kapil Chawla and other colleagues from AT&T Labs for fruitful discussions.
REFERENCES
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