Chapter9.Networking

Chapter 9. Networking

Michelle X. Gong and Scott F. Midkiff

The previous chapters explored issues related to the creation of a functioning communications link. However, a link is but a small part of a communications network. The transition between establishing a functional communications link and an operational network is nontrivial and remains a subject of research even when not faced by the unique challenges offered by a UWB communications link. This chapter discusses the steps and issues involved in the creation of a network of UWB devices. As we shall see, there are aspects of UWB networking that are similar to traditional communications networks, while other aspects are quite different.

To begin our discussion of UWB networking, we review some fundamental networking concepts. A network is a collection of interconnected devices; a UWB network is collection of devices interconnected by UWB links. These links operate in the same wireless medium, so even the most rudimentary networks must define a way of sharing the wireless medium whether by coordination or by contention. More complex networks that provide indirect connections typically supplement these medium access schemes with methods for coordinating the end-to-end flow of information through the networks. Networks employ protocols to formalize the processes for medium access, end-to-end information flow, and other functions in the network.

For example, when communication occurs only between directly connected nodes, that is, over a single hop, a data link layer protocol (or a set of data link layer protocols) is needed to coordinate the transfer of information in any of several ways, such as one-to-one (unicast), one-to-many (multicast), one-to-all (broadcast), or many-to-one. Minimally, this requires the introduction of an addressing scheme to ensure that data gets to the proper recipient(s). Additionally, some scheme is needed to coordinate the use of the shared wireless channel by multiple nodes; this is typically performed by medium access control (MAC) protocols, which are discussed in more detail later in this chapter. This view of a network (direct connections between wireless devices) is analogous to the network provided by the IEEE 802.11 wireless local area network standards [1] and the WPAN specifications for a piconet [2].

However, when nodes are also connected indirectly, that is, over multiple hops, additional protocols need to be introduced. Specifically, network devices need to be able to determine how to route information between indirectly connected devices, a process that is complicated by the movement of devices, as in a mobile ad hoc network (MANET), and the lack of any network infrastructure for routing, as in a Bluetooth scatternet [3] or a MANET. This task typically falls to network layer protocols. Further, an indirect link, such as an intermediate hop, may fail, thus necessitating the use of additional protocols to ensure reliable communications between the indirectly connected nodes. Reliable end-to-end communications is typically provided by a transport layer protocol.

Ultimately, the performance of a UWB network depends on the performance of its links, its point-to-point protocols, and its end-to-end protocols and on the interaction between these protocols. Predicting the performance of a network can be quite complicated due to the mutual dependencies and interdependencies of the network protocols.

To simplify the construction of network protocols, the International Organization for Standardization (ISO) promulgated the Open Systems Interconnection (OSI) model [4]. The OSI model separates the functionality of the network protocols into seven layersapplication, presentation, session, transport, network, data link, and physical. By hiding the operations of each layer behind well-defined interfaces, a process known as encapsulation, this layering approach facilitates the independent development of the protocols for each layer. Through the use of well-defined interfaces, the protocols in each layer of this model interact with the protocols only in layers immediately above and below its layer. In other words, a data link layer protocol may interact (perhaps by passing packets) with protocols in the network or physical layers, but not with protocols in the transport or application layers. Indeed, if a network is designed properly, protocols remain blissfully ignorant of the protocols that are not immediately adjacent in the stack. However, in practice, the seven-layer model is commonly simplified to a five-layer TCP/IP reference model, as shown in Figure 9.1, by aggregating the presentation and session layers into the application layer. This five-layer model is similar to the four-layer U.S. Department of Defense (DoD) network model that was used in the development of the Internet. In that model, the application layer is referred to as the process layer, the transport layer is called the host-to-host layer since it deals with end-to-end connectivity, the network layer is the internet layer, and the data link and physical layers are combined into the network access layer.

Figure 9.1. A Five-Layer Protocol Reference Model.

In the TCP/IP reference model of Figure 9.1 that is considered in the remainder of this chapter, the physical layer handles modulation, error coding, transmission, and reception. The purpose of the physical layer is to efficiently transmit and receive data bits with as few errors as possible. In effect, the physical layer comprises all aspects of the UWB communications covered to this point. The data link layer groups data bits into data link layer protocol data units (PDUs), called frames, that handle frame errors and control the flow of frames. The basic service provided by the data link layer is moving a frame from one node to a neighboring node via a single communication link. As previously discussed, this requires the use of a form of addressing and, when multiple nodes must share the physical medium, a mechanism for controlling access to the medium; this is typically performed by a MAC protocol. The primary duty of the network layer is to determine appropriate paths between indirectly connected devices. The algorithm that determines these paths is appropriately called a routing algorithm, and it is realized by a routing protocol. For Internet Protocol (IP) networks, the network layer is realized by IP and associated routing protocols, such as the Border Gateway Protocol (BGP) or Open Shortest Path First (OSPF) protocol. To facilitate the network protocol's routing algorithm, another level of addressing is typically provided by the network layer. Residing between the application and the network layers, the transport layer provides an end-to-end logical connection between the source and destination nodes. As illustrated in Figure 9.1, transport layer protocols function in the end systems, the source and destination nodes, but not in the intermediate nodes that perform routing at the network layer. Transport layer protocols can also be used to ensure reliable end-to-end communications, but this is not always the case. For instance, the User Datagram Protocol (UDP) provides unreliable best-effort connectionless service to the application layer, while the Transmission Control Protocol (TCP) provides a reliable connection-oriented service. Sitting atop these layers is the application layer. The application layer runs application processes like electronic mail and web services and provides the interface presented to the user.

Broadly, the remainder of this chapter focuses on the data link layer, including medium access control, and the network layer, especially as it relates to ad hoc network formation and routing. This is not to downplay the importance of the application and transport layers, but rather is due to the relative lack of maturity of UWB networks and the fact that UWB networks are developing in a bottom-up manner. Indeed, the transport layer is important in wireless networks as the error characteristics of the wireless link can significantly influence the performance of TCP [5]. Also, novel applications may present new requirements that affect the design of the network and data link layers.

Specifically, the remainder of this chapter is organized as follows. Section 9.1 addresses the issues that make UWB networks different from more traditional networks. Section 9.2 briefly summarizes physical layer issues that affect UWB networking. Section 9.3 discusses data link layer design and several medium access schemes that are suitable for UWB systems. Section 9.4 presents two possible architectures for UWB multiple hop ad hoc networks and corresponding routing schemes. Section 9.5 provides an overview of other networking-related issues.