Persistent Bidirectional Peer Traffic in Fixnetwork augmented Broadband Wireless Access Robert Hsieh, Jari Iinatti University of Oulu, Centre for Wireless Communications FI-90014 University of Oulu, Finland
[email protected],
[email protected] Abstract— Currently, the Internet is experiencing the grid accelerated file transfer phenomenon (swarming). It has gained immense popularity and dominance through BitTorrent and has thus far accounted for a substantial amount of the total Internet traffic. In this paper, we borrow ideas liberally from the literature to argue that the use of swarming protocol for traffic/content delivery within the wireless networking milieu is inefficient. We generalize the problem into the delivery of persistent bidirectional peer traffic in wireless hierarchical topology, and further argue that current approaches for fixnetwork augmented broadband wireless access (e.g. 802.11x) has left the problem ill-addressed. A novel system architecture is sketched to rectify such a shortcoming. Hence, the purpose of this short position paper is to stimulate ideas and proposals that may result into important avenues of future research. Keywords— swarming, cross-layer design, wireless communication architecture
I. INTRODUCTION Grid accelerated file transfer, or known to the technological savvy as swarming, was first introduced by Chapweske [17] in 1999. Traditionally, file transfer involves a client and a server, and that the client establishes a connection to the server to retrieve the file in a single operation. Such an approach works well for small files. However, as the file size or the number of simultaneous clients increases, the file transfer becomes expensive (server being overloaded), unreliable (failed transfer due to overload) and very slow. In contrast, with swarming, rather than sending out the entire file in a single response, the server breaks the file into small pieces and sends different pieces to different (simultaneous) clients. Meanwhile, each client also recursively requests some of the pieces from other clients. Such a ‘network’ of file downloaders (leechers) eventually evolve into a grid of file transfer nodes, all working together to accelerate the file transfer process with increased reliability and efficiency. The idea of swarming caught mainstream attention via the popularity of BitTorrent [6] (the latest Peer-to-Peer (P2P) [11] application), which is a swarm at the protocol level. Recent research suggests that such an application generates approximately 35% of the entire Internet traffic by volume [4], and that that is likely to stay unchanged, if not increased further [21]. The efficiency has been claimed at 900% faster for enterprise file transfer [17] or 0.007% [8] of the cost, when compared to the traditional file transfer. Although popular P2P applications are often hindered by publicized high-profile
action lawsuits, the point is that grid accelerated file transfer techniques will stay and ultimately change the way broadcast (time-unbounded) is carried out [8], with little incentive to return to the traditional client-server paradigm for broadcasting purposes. Already, there is even a proposal [17] for real-time streaming protocol based on grid accelerated file transfer techniques. Therefore, the case for architectural change has never been stronger for the fix-network augmented broadband wireless milieu. It is well known that in such networks, the architecture is of the hierarchical topology where wireless nodes are one hop away from the base station and that they can only communicate with the base station. This separates peer communication between the wireless nodes into two point-topoint links and therefore doubles the bandwidth requirement at the base station for every peer communication. While this is traditionally not an issue for the hierarchical topology as the traffic mostly flows outwards to the fixed network, such a setup evidently hinders the performance of swarming when there are multiple leechers within a single wireless coverage. In addition, given that ubiquitous broadband wireless is becoming a reality in the near future, it is not unreasonable to believe that wireless nodes may run swarm applications in numerous situations, for example, in hotspots, in large multiplayer network game venues or even within wireless equipped aircraft in-flight personal entertainment systems. In fact, it can be generalized that any broadband fix-network augmented wireless environments that are subjected to both a high level of persistent bidirectional peer traffic (swarming) and uplink traffic will result in performance degradation. By persistent, we mean a peer communication that lasts for an extended period of time, and by bidirectional, we mean that peers are both uploading and downloading to/from each other simultaneously. The new architectural model that we refer to aligns at the intersection between the Wireless Local Area Network (WLAN) and the Wireless Personal Area Network (WPAN) [14] architectures. WLAN concerns with the extension of LAN into the wireless domain and is therefore last-hop and non peer-to-peer/ad-hoc in essence. WPAN concerns with the temporary formation of peer-to-peer communication in nature while its connection to the ‘outside’ world is not a mandatory consideration. The swarming phenomenon has therefore created an architectural void to be filled, of which the model must cater for. The purpose and contribution of this paper is therefore to promote further thoughts into the challenge of
normal adhoc modes of operation for direct communication to avoid unfairly occupying the uplink channel. The idea is to exploit time, frequency and spatial diversities. Adhoc communications between the pairing nodes are on a different (pairing) channel from the uplink (default) channel and possibly at a reduced power requirement due to location proximity. In this paper, we limit our discussion to the single cell environment.
Network/IP Layer
Network-sub Layer
Peer List
Polling
Network-sub Layer
Service trigger
Higher Layers
Network/IP Layer
Link/MAC Layer
Signal Quality
Wireless Node
Higher Layers
Link/MAC Layer
Physical Layer
Physical Layer
Management Entity
Management Entity
Signal Quality
Peer List
Coordination Node
Peer detection
Figure 1 OAPAC Topology
Figure 2 Network-sub Layer persistent bidirectional peer traffic in fix-network augment broadband wireless environments. It should be noted that the 802.11e draft specification includes an optional Direct Link Protocol (DLP) [1] to facilitate basic peer communication. However, no mechanism exists to separate normal traffic from swarm traffic which greatly limits its practicality. Indeed, the impact of performance degradation due to simple peer or persistent peer traffic within current WLAN architecture is significantly less when compared with the persistent bidirectional peer (swarm) traffic. The organization of this paper is as follows. In the next section, the problem definition and design space are outlined. A new system architecture derived from 802.11 WLAN is proposed in section III. We discuss and describe our future plans in Section IV. II. PROBLEM DEFINITION AND DESIGN SPACE The fundamental problem which we are concerned here is the semantic of hierarchical topology. Network information theory introduces that as a ground rule, in such a topology, the wireless nodes must only communicate with the base station. This is to ensure that communication is possible between all stations even if they are out of range of each other. The tradeoff is that the available bandwidth for node-to-node communication is reduced by more than one-half. In this paper, we introduce a new topology termed as Opportunistic Adhoc-Pairing Augmented Cellular (OAPAC) topology as shown in Figure 1. Consider a hierarchical topology as the base model. The fundamental concept is that the coordinator node (base station) intermittently schedules its observed peer communicating nodes into an adhoc pair and these nodes use
There are three basic design spaces to tackle the problem. In the first design space, the coordinator node can be used to buffer all the peer traffic and simply transmits the buffered data to subsequent peer requests. This, however, requires the coordinator node to monitor and buffer all peer traffic which may be impractical for persistent bidirectional peer traffic. With this approach, the changes are completely hidden from the wireless nodes. The second design space is one where multiple network interface cards (NIC) are used per wireless node. This requires, at least, one interface to be used for adhoc communication and the other to be used for uplink (hierarchical) traffic. Intelligent higher layer protocol control is required to direct the appropriate application traffic to the rightful NIC. Although this may be the cheapest in terms of deployment cost, this is the least spectrum efficient approach. The third design space concerns with cross-layer design, i.e. a new physical, medium access and network layer integration that is specifically engineered to address the grid accelerated file transfer problem within fix-network augmented wireless networks. We believe that if high performance (i.e. bit rate) is required, cross-layer design is a necessity and not merely a design feature, particularly given the scarce nature of the available wireless bandwidth/spectrum. In what follows, we outline an architectural composition within this design space. III. ARCHITECTURE This architectural proposal addresses two main criteria. Firstly, it must adhere to the OAPAC topology. Secondly, it ought to exploit the “two weak signals is better than one whole signal” property of the cooperative diversity [5] gain for peer communication. The intention of the first criterion is self-explanatory. The incentive for the second criterion is to exploit spatial diversity gain [19] such that cooperative communication will increase the efficiency of uplink traffic (e.g. bandwidth) between the pairs and therefore free more resources (e.g. time) for node-to-node communication traffic. It has became apparent to us during the design process that the node-to-node relationship at the network layer, constructed from the use of swarming, can be effectively deployed as part of the partner selection algorithm for the physical layer cooperative communication mechanism. We consider a design where all protocols above the network IP layer remain unmodified and hidden from the complexities at the lower layers. Modifications are made to the Link (particularly medium access control) and Physical layers. As depicted in Figure 2, a new Network-sub layer is introduced to abstract the persistent bidirectional peer communication behavior of the higher layer protocols. The architecture is a joint cross-layer design spanning Physical, Link and the Network-sub layers.
Network-sub layer employs ‘agent’ equivalent indirection techniques, used extensively throughout network layer designs such as Mobile IP [7] and i3 [15], to differentiate normal traffic from peer traffic. Its key responsibilities are i) to construct a peer-list identified passively per wireless node, ii) to convey this information to the coordination node through the Link layer, and iii) to prune the various peer-lists into a master pairing-list which is to be used by the medium access scheduler of the coordination node. The initiation of the Network-sub layer functions may be through direct invocation via certain predefined procedures from the higher layers, or it could be turned on continually to monitor the peer activities passively. The Link layer, in particular the Medium Access Control (MAC), is responsible for the fair sharing of the available resources. Two key functionalities of the MAC for OAPAC are i) the scheduling of adhoc pairs and ii) the cooperative communication scheduling of the uplink traffic for these adhoc pairs. It must be noted that cooperative communication can only be achieved for the uplink traffic to the base station while peer communication is simply a direct link between the pair using a different frequency channel but with potentially lower required transmission power. Given these two functional requirements, we propose the outline of a polling based MAC scheme. Both backoff and interframe space time intervals [1] are used for priority access control while polling order is also transformed into the contending order using the idea of multipoll introduced in [20], [3], [16]. Similar to advanced MAC schemes [20], [1], the idea is to split one super frame into two periods: contention (CP) and contention free (CFP), shown in Figure 3. We further split the CFP into three different phases: i) Down phase is for downlink traffic with an optional uplink poll for time bounded traffic, ii) Peer Initiation phase is to set up the pairing nodes based on the master pairing-list and the associated cooperative communication settings, and iii) Up phase is for the transmission of uplink traffic as well as the cooperative communication traffic (i.e. the reception of two copies of a same signal, one copy from the origin node and the other copy from its pairing node). For direct node-to-node communication, each Up phase can be considered as a pairing block where adhoc pairs can be scheduled to a different channel/frequency for direct node-to-node communication. Figure 3 illustrates the design outline of the MAC protocol. Multipoll may be used in the Up phase or during the CP for increased performance while the group acknowledgement [1] technique may also be used in the Down phase to fine-tune the efficiency. The MAC scheduler must juggle between the master pairing-list, transmission power (for adhoc), available channel resources and cooperative communication diversity (for uplink) to maximize the utilization of the scarce wireless bandwidth. There are management entities (see Figure 2) alongside each networking stack layer which monitor and measure wireless conditions (i.e. SNR) and this information is fed to the MAC scheduler for maximum scheduling performance. The Physical layer design aspect which is of imperative relevance here is the cooperative diversity communication. The Physical layer of the base station must notify the failed
Figure 3 Scheduling Model Schematic reception of the ‘second’ signal copy (the relayed signal from the corresponding pairing node) to the MAC scheduler such that the retransmission process can be initiated and vice-versa. In addition, security protection can be provided through cooperative diversity to prevent wireless eavesdroppers from intercepting packets successfully. The original transmitted signal can be coded differently from the relay signal and it can be designed such that they must be correctly combined by the base station in order to be decoded correctly. Another physical layer design requirement is its independence from the link layer and the associated MAC protocol, similar to the decoupling principles of 802.11 between PHY and MAC. In this way, future access technology with a higher bit rate (e.g. 802.16) can be plugged into our architecture with minimal adjustments. Other physical layer attributes of interest are adaptive transmission rate changes, frequency/channel selection and incremental power level adjustments. These are significant physical layer research issues and have gained much attention already within the research literature. A right mix of these techniques is highly advantageous within this proposed architecture, however will not be explored further due to the scope of this paper. IV. DISCUSSION AND FUTURE WORK There are still much broader unaddressed research issues and open questions. For instance, under the fix-network augmented broadband wireless environment, to what extent would the assumption about swarming occupies one-third of Internet traffic hold? Would the traffic volume statistics be much more or much less than expected, and how would such a variation impact on the new architecture? Another open research question relates to spectrum allocation/planning. Would the dynamic spectrum allocation mechanism [10] further impact on the way OAPAC operates with regards to the pairing channel selection for node-to-node communication? Furthermore, OAPAC clearly shifts the bandwidth cost of node-to-node communication from the default channel to the pairing channel. However, the overhead to total throughput tradeoff and their relationship to the overall spectrum efficiency need to be further studied and characterized. In addition, the tradeoffs between physical layer complexities for increased system performance also need to be examined since cooperative communication requires greater redesign of the physical layer.
As briefly mentioned in the introduction, swarming-like time-bounded streaming traffic protocols such as Swarmstreaming [17] and multipoint-to-point streaming [12] have been proposed in the literature. These are all indications which suggest the increasing need for a hierarchical framework that facilitates efficient broadband wireless nodeto-node communication. In fact, these streaming protocols may further serve as the key enabler for the OAPAC topology. We stress again that current architecture designs for WLAN and WPAN are tailored to predominantly hierarchical topology or adhoc topology respectively. It is our intention to offer an architectural design that brings together the gap inbetween these two prominent network architectures. It is also our intention to show that currently, there are substantial traffic patterns which strongly demand a cross hierarchical and adhoc topology. It must also be noted that our work is strictly single hop based akin to the hierarchical topology. It does not address multihop topology nor does it cater for relay based [18] schemes. Our current research status is as follows. We are focused on evaluating our architecture of an in-house cross-layer simulator that encompasses physical, link, network, and higher layer protocols (e.g. swarming protocol). The implementation on the detailed MAC and MAC Scheduler (not presented in this paper) has commenced. The simulator follows the integrated approach where the PHY simulation (Matlab) is separated from the system level simulation (ns-2), but calculated PHY performance traces are subsequently used as the input for system level simulation. We aim to compare the performance between the plain hierarchical topology (using advanced MAC 802.11e) and our proposal at the system level. At the physical layer, our intention is to evaluate our cooperative strategy with current accepted schemes such as PHY 802.11b, 802.11g and 802.11a. REFERENCES [1]
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