Complementing 3G Cellular Networks by Multi Hop Capabilities of an Enhanced MAC Protocol for Wireless LAN

Transcription

1 Complementing 3G Cellular Networks by Multi Hop Capabilities of an Enhanced MAC Protocol for Wireless LAN Frank H.P. Fitzek, Federico Bertocchi, Michele Zorzi Department of Communications Technology, Aalborg University Neils Jernes Vej 2, 92 Aalborg Øst, Denmark, University of Ferrara, Via Saragat, I-44 Ferrara, Italy, [fbertocchi Abstract Interworking between wireless LANs and 3G networks is expected to break new ground for sophisticated business models. The combination of the 3G core network infrastructure (with service centers and accounting capabilities) and the achievable data rate of WLANs give the possibility to offer new services. WLAN hotspots are on the way, but the high installation costs of WLAN, especially for larger hotspots, repress a faster evolution. The installation costs are mainly driven by the cabling infrastructure. In the past we have already introduced an enhanced MAC protocol for multi hop networks without any infrastructure. In this paper we investigate the MAC performance for a multi hop scenario, enabling low costs WLAN networks with infrastructure that are coupled with 3G networks. This approach should allow network providers to have a fast return on their investments. As video services are expected to be one of the future key services, we present a detailed performance evaluation of different network topologies transmitting high quality video traffic. I. INTRODUCTION Both 3G and WLAN networks are installed at the moment all over the world. Because of their disjunctive disadvantages the interworking between wireless LANs and 3G networks is expected to break new ground for sophisticated business models. While 3G systems (in conjunction with existing 2 and 2.5G networks) offer large coverage and a rich network infrastructure (application, billing, mobility), WLAN has its potential in high data rates allowing fast web surfing, high quality video transmission and gaming applications. Even the 3rd Generation Partnership Project (3GPP) has identified this interworking as vital for 3G networks introducing []. The question arises why WLAN without 3GPP interworking has this limited success for public access networks. The vicious circle can be described as follows: i.) WLAN networks have high installation costs ii.) not many customers are willing to pay for WLAN networks without any real services ( and web are the only one at the moment) iii.) network providers are poor-spirited to invest in such a business case. To break this circle up we have to bring services to the WLAN and reduce the installation costs. While the former task is done by the coupling with 3G networks, the latter is realized by multi hop networks. Not yet solved are the high installation costs of WLAN. After spending a lot of money onto the 3G spectrum the potential network operators are not willing to set up a parallel technology such as WLAN at high costs. Even if the services are provided by the 3G core network, the cost of the WLAN infrastructure (cabling, building license, etc) is still high. It is estimated that for very large WLAN hotspots the cabling covers up to 75% of the whole network costs. This percentage becomes even larger in the future if the Access Points (AP) becomes even cheaper. Fig.. Interworking between multi hop WLAN systems and 3G network. Therefore we advocate the usage of multi hop WLAN systems. In Figure (in accordance with []) we present a possible interworking between multi hop WLAN systems and 3G networks. The 3G networks such as Universal Mobile Telecommunications System (UMTS) have their own wireless access such as the UMTS Terrestrial Access Network (UTRAN) and for larger coverage extension the Enhanced Radio Access Network (ERAN). Both networks are connected to the UMTS core network via the Serving GPRS Serving Node (SGSN). The UMTS core supports several services such as billing and charging functionality, mobility, and some sorts of application servers (a prominent representative is the multimedia broadcast multi-cast service (MBMS)). The access performance is rather weak in terms of bandwidth and delay compared to WLAN. But as WLAN does not have any service

2 infrastructure, we advocate the combination of 3G and WLAN networks. Different business models for 3G/WLAN interworking are reasonable. One possibility is that network providers that have already 3G licenses start also to deploy WLAN networks. Some global players have already started it. At the moment they target only the more interesting places such as airports and hotels. Smaller providers have their own WLAN networks. The realization in terms of coverage, data rate, and security differ among the network providers. The WLAN network can be coupled to the 3G network over an entity called Packet Data Gateway (PDG). It is still under discussion if this entity will be part of the Gateway GPRS Serving Node (GGSN) or a stand alone one. This discussion is out of the scope of this paper, but we had to address this functionality as it is a vital one for the interworking. Different WLAN networks can be connected to the PDG gaining from the service infrastructure of the UMTS core network. As stated in the beginning our main focus in this paper is to have low cost WLAN network solutions. An enabler for low cost WLANs is to use multi hop capabilities. As given in Figure we introduce some Virtual Access Points (VAP). These are multi hop enabled terminals that belong to the network providers. These entities have fixed power supply (no energy problem) and no mobility (routing becomes easy). Nevertheless we want to stress that also the multi hop over mobile terminals should be possible. It is out of the scope of this paper to discuss the impacts of such an approach. The network provider can start with only one fixed access points and several VAPs. If the traffic in the WLAN is increasing (and therefore the number of customers), some of the VAPs can be connected in a wired fashion (becoming a normal AP, some investment has to be done but this should be justified by the higher number of users). II. SYSTEM UNDER INVESTIGATION In our former work [2], [3] we have presented a new MAC protocol for multi hop enabled WLAN networks based on IEEE82. technology. Our new scheme is based on the usage of multiple IEEE82. channels using a four way handshake over a common signalling channel, while data transmission occurs on N multiple dedicated channels. In the following we refer to such a channel assignment as dynamic approach. The four way handshake contains two Ready To Send (RTS) packets, two Clear To Send (CTS) packets, and one probing packet. The purpose of the handshake phase is to agree on a certain modulation and coding scheme. By means of the first RTS/CTS exchange the neighboring stations of the sender and the receiver are informed to not use the signalling channel until the whole handshake phase is over. After the second CTS packet is send the neighbors are also informed about the chosen dedicated channel and its reservation time. Two further possibilities were mentioned for channel assignment. The first one is the static assignment, where only one out of N possible channels is used. This approach yields high connectivity due to the related node density. Furthermore we introduced the random approach. Here N multiple channels are used randomly. Therefore higher bandwidth can be achieved, but the node density decreases as one terminal can send or receive only on one channel at the same time. Our performance evaluation done so far focuses on ad hoc networks. Multiple senders convey data to randomly chosen receivers. This may lead to areas in the network topology that are highly loaded. Throughout the paper we refer to this scenario as Scenario A (referring to ad hoc). For the interworking with 3G networks our main focus in this paper relies on multi hop enabled networks with a given infrastructure. Three different topologies are under investigation, namely: i.) multiple senders transmit (with the help of multi hop enabled terminals or VAPs) to one dedicated receiver (access point at location X in Figure ). We refer to this as Scenario R, ii.) one sender (access point at location Y in Figure ) transmits to multiple receivers. We refer to this as Scenario S, and iii.) multiple senders (access points at location Z in Figure ) transmit to multiple receivers. We refer to this as Scenario 5S. We lay more emphasis on the down link case as we believe that this is more business driven. As one prominent candidate for new services in 3G and WLAN, we investigate the MAC performance of different schemes for streaming video. A lot of research has be done in the field of video communication over ad hoc networks or the interworking between ad hoc and cellular systems. In [4], [5] a five phase reservation protocol was introduced using a single channel for ad hoc communication (video services were not addressed). The work of [6], [7], [8] was focusing on video services over ad hoc networks, while [9] and others highlight the interworking of cellular systems and ad hoc. In this paper we try to combine these three research topics and motivate the usage of a new MAC scheme enabling efficient multi hopping. III. METHODOLOGY AND METRIC Our investigation focuses on Orthogonal Frequency Division Multiplex (OFDM). OFDM has been chosen as the access technique for the latest WLAN standard IEEE82.a. Each IEEE82.a channel has 52 sub carriers, where four sub carriers are used for pilot signals while the others convey data. IEEE82.a uses multiple modulation schemes in combination with different coding rates. For modulation, BPSK, QPSK, 6-QAM and 64-QAM are used. Coding rates are /2, 9/6, and 3/4. The combination of coding rates and modulation leads to multiple data rates starting at 6 Mbit/s up to 54 Mbit/s (see also []). We assume that eight parallel channels are available referring to the channel spectrum available in the US for IEEE82.a [] in the lower and medium band. The maximum distance at which two nodes can communicate (at the lowest achievable data rate of 6 Mbit/s) is around 75 m. A wireless Network Interface Card (NIC) is only able to receive or send on one channel simultaneously. However, we assume that packets can be sent and received over different channels. The NIC needs some time to adjust from one channel

3 to another. It has to be done in a timely fashion for higher performance. In case two NICs are available, one NIC is for monitoring the common signaling channel only and the second is transmitting data over a dedicated channel. In this case the switching is inherently no problem. In case a packet has to be transmitted over forwarding nodes (thus multi hop is used) some sort of routing has to be applied. Although other more sophisticated protocols could be used, as a preliminary step we use shortest distance routing for all three approaches. Fig. 2. Frame Trace Friends2x. We assume 6 WTs in a surface area of variable size which varies from 3x3 to 5x5 meters. In the Scenario A out of 6 WTs transmit video content to different receiving WTs. Thus, 4 WTs are only forwarding traffic. In the Scenario R WTs send to one dedicated node (transition to the wired network) using the other terminals to forward their packets. In Scenario S and Scenario 5S each access point transmits to one terminal at a time using the multi hop capabilities over neighboring terminals if necessary. Note that the traffic load differs for the four scenarios. Each source (WT or access point) selects randomly a trace file from the second season of Friends available at [2] and presented in [3]. 24 different trace files are available. All video sequences have a frame rate of 25 fps, a video format of 4x288 pixels, and a mean bit rate around 75 kbit/s. We refer to [2] for further information. For all video sequences variable bit rate encoding was used as can be seen in Figure 2. Using a frame aggregation level of 8 for better illustration, the frame size versus time is depicted. In [3] we have already shown that the peak rate is much higher than the mean bit rate. For the Friends episodes the peak to mean values are between 7 and 22. This kind of traffic imposes a challenging situation for the multi hop network. The sending node chooses the video length (no longer than min as this is the maximum length of an episode) and the starting phase randomly between 6 and s. Random starting phases and lengths are chosen to avoid correlation among the traffic sources. The metric for our performance evaluation are the packet delivery ratio and the transmission delay. Both values are measured at the receiving entity. The delivery ratio is calculated dividing the successfully received packets by the overall amount of packets. The transmission delay is the time needed from the sender to the receiver including the retransmissions. IV. RESULTS In the following we present our performance evaluation for the given scenarios. We compare our new MAC protocol (labeled dynamic in the figures) with an approach which uses the standard MAC protocol (labeled static in all figures) and an approach that selects the channel randomly (labeled random in all figures). A. Scenario A In Figure 3 we give the delivery ratio versus the surface area for the static, random and the dynamic channel assignment. It can be seen that the dynamic approach yields better results than any other approach. The performance of the random approach degrades dramatically as the surface area increases. This is caused by the physical limitation of the network. Only terminals using the same channel can build a multi hop network. In Figure 4 the delay values for the three approaches are given. The best values are achieved by the random approach. But we have to note that the delay values are only calculated for successfully received frames. That means, for the random approach, that if a packet arrives, it will arrive with low delay values. But also the dynamic approach yields good delay values below ms. The static approach has delay values larger than one second if the surface area increases over 7 m. This is due to the fact that for these values the network starts to use multi hop capabilities and the overall network traffic increases. In such a case the limitation to one channel becomes a bottleneck and therefore the delay values increase. The dynamic approach is capable to handle the increase in network traffic using the dynamic assignment of multiple channels. B. Scenario R In Figures 5 and 6 the delivery ratio and the mean transmission delay for Scenario R realizing video services versus surface area for static, random and dynamic channel assignment is given. The delivery ratio for the dynamic approach outperforms the static one for surface area larger than 5x5 m 2. For smaller surface areas the static approach yields slightly better results due to the absence of the hand shake phase. The random approach results in delivery ratios smaller than %. The mean transmission delay given in Figure 4 and Figure 6 are similar. The only change is the delay curve for the dynamic approach increasing for larger surface areas. Still the values are smaller than for the static approach, but for surface areas of 3x3 m 2 the delay values are over one second. This makes it hard to realize video services. The increasing delay is caused by a higher number of hops needed to convey a data packet from the access point to its final destination.

4 Scenario: Ad Hoc Scenario: Ad Hoc Fig. 3. Delivery ratio for video services versus surface area for static, random Fig. 4. Mean transmission delay for video services versus surface area for static, random Scenario R: One Sink Scenario R: One Sink Fig. 5. Delivery ratio for video services versus surface area for static, random Fig. 6. Mean transmission delay for video services versus surface area for static, random C. Scenario S In Figure 7 and 8 the delivery ratio and the mean transmission delay for Scenario S realizing video services versus surface area for static, random and dynamic channel assignment is given. Here as a matter of fact, the produced traffic is only a tenth of the traffic compared to Scenario A and Scenario R. Here the static and the dynamic approach yield nearly the same results for both, the delay and the delivery ratio. Only the random approach is suffering due to its channel assignment. D. Scenario 5S In Scenario 5S we increase the traffic by a factor of five. In Figure 9 we see that the static assignment leads to slightly worse values than the dynamic approach. Still the random approach results in very bad delivery ratios. The mean transmission delay given in Figure for the dynamic approach is always lower than ms, while the static approach comes along with delay values of one second if the surface area is larger than x m 2. V. CONCLUSION AND OUTLOOK In this paper we have once more motivated the interworking between WLAN and 3G networks focusing on low cost WLAN infrastructure. We have introduced a MAC protocol for WLAN multi hop networking, that allows installing WLAN network at low costs. By means of the performance evaluation we have shown that our new proposed MAC scheme can be used for pure ad hoc scenarios as well as scenarios with an infrastructure. Even if the MAC protocol was designed for pure ad hoc communication, it outperforms existing solution even for infrastructure based networks. The performance of such a multi hop enabled network is satisfactory for demanding multimedia services in terms of delivery ratio and transmission delay. We believe that this will open the door for cost effective deployment of WLAN systems complementing 3G networks and their successors.

5 Scenario S: One Source Scenario S: One Source Fig. 7. Delivery ratio for video services versus surface area for static, random Fig. 8. Mean transmission delay for video services versus surface area for static, random Scenario 5S: Five Sources Scenario 5Sg: Five Sources Fig. 9. Delivery ratio for video services versus surface area for static, random Fig.. Mean transmission delay for video services versus surface area for static, random VI. ACKNOWLEDGMENT The work was partially supported by ALCATEL, France. We would like to thank Gerrit Schulte for the fruitful discussions and his help in designing Figure. Furthermore, we would like to thank all anonymous reviewers for their valuable comments. REFERENCES [] Technical Specification Group Services and System Aspects, 3GPP system to Wireless Local Area Network (WLAN) Interworking; System Description (Release 6), Tech. Rep. V2.4., 3rd Generation Partnership Project, Jan 4. [2] F. H. P. Fitzek, D. Angelini, G. Mazzini, and M. Zorzi, Design and Performance of an Enhanced IEEE82. MAC Protocol for Ad Hoc Networks, in VTC3, 3. [3] F.H.P. Fitzek, D. Angelini, G. Mazzini, and M. Zorzi, Design and Performance of an Enhanced IEEE82. MAC Protocol for Ad Hoc Networks and Coverage Extension for Wireless Networks, IEEE Wireless Communications, vol., no. 6, December 3. [4] C. Zhu and M.S. Corson, A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Network, in IEEE INFOCOM 98, 998. [5] C. Zhu and M.S. Corson, A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Network, Wireless Networks, pp ,. [6] S. Mao, S. Lin, S.S. Panwar, and Y. Wang;, Reliable transmission of video over ad-hoc networks using automatic repeat request and multipath transport, in 54th IEEE Vehicular Technology Conference, Oct, vol. 2, pp [7] K. Ban and H. Gharavi, Video transmission for multi-hop networks using IEEE 82. FHSS, in International Conference on Image Processing, Sept 2, vol., pp. 3. [8] S. Lin, Y. Wang, S. Mao, and S. Panwar, Video transport over ad-hoc networks using multiple paths, in IEEE International Symposium on Circuits and Systems, May 2, vol., pp [9] H. Li and D. Yu, Comparison of ad hoc and centralized multihop routing, in The 5th International Symposium on Wireless Personal Multimedia Communications, Oct 2, vol. 2, pp [] Benny Bing, Wireless Local Area Networks, WILEY, 2. [] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications High-speed Physical Layer in the 5 GHz Band, IEEE Standard for Information technology. [2] Reisslein, Fitzek, and Seeling, Video Traces for Network Performance Evaluation, [3] F.H.P. Fitzek, M. Zorzi, P. Seeling, and M. Reisslein, Video and Audio Trace Files of Pre-encoded Video Content for Network Performance Measurements, in IEEE Consumer Communications and Networking Consumer Networking: Closing the Digital Divide. IEEE, January 4, Las Vegas, Nevada, USA.