In this chapter, we’ve seen that wireless networks differ significantly from their wired counterparts at both the link layer (as a result of wireless channel charac- teristics such as fading, multipath, and hidden terminals) and at the network layer (as a result of mobile users who change their points of attachment to the network). But are there important differences at the transport and application layers? It’s tempt- ing to think that these differences will be minor, since the network layer provides the same best-effort delivery service model to upper layers in both wired and wireless networks. Similarly, if protocols such as TCP or UDP are used to provide transport- layer services to applications in both wired and wireless networks, then the applica- tion layer should remain unchanged as well. In one sense, our intuition is right—TCP and UDP can (and do) operate in networks with wireless links. On the other hand, transport protocols in general, and TCP in particular, can sometimes have very dif- ferent performance in wired and wireless networks, and it is here, in terms of perfor- mance, that differences are manifested. Let’s see why.

Recall that TCP retransmits a segment that is either lost or corrupted on the path between sender and receiver. In the case of mobile users, loss can result from either network congestion (router buffer overflow) or from handover (e.g., from delays in rerouting segments to a mobile’s new point of attachment to the network). In allcases, TCP’s receiver-to-sender ACK indicates only that a segment was not received intact; the sender is unaware of whether the segment was lost due to congestion, during handover, or due to detected bit errors. In all cases, the sender’s response is the same—to retransmit the segment. TCP’s congestion-control response is also the same in all cases—TCP decreases its congestion window, as discussed in Section 3.7. By unconditionally decreasing its congestion window, TCP implicitly assumes that segment loss results from congestion rather than corruption or handover. We saw in Section 7.2 that bit errors are much more common in wireless networks than in wired networks. When such bit errors occur or when handover loss occurs, there’s really no reason for the TCP sender to decrease its congestion window (and thus decrease its sending rate). Indeed, it may well be the case that router buffers are empty and packets are flowing along the end-to-end path unimpeded by congestion.

Researchers realized in the early to mid 1990s that given high bit error rates on wireless links and the possibility of handover loss, TCP’s congestion-control response could be problematic in a wireless setting. Three broad classes of approaches are possible for dealing with this problem:

• Local recovery. Local recovery protocols recover from bit errors when and where (e.g., at the wireless link) they occur, for example, the 802.11 ARQ protocol we studied in Section 7.3, or more sophisticated approaches that use both ARQ and FEC [Ayanoglu 1995] that we saw in use in 4G/5G networks in Section 7.4.2.

• TCP sender awareness of wireless links. In the local recovery approaches, the TCP sender is blissfully unaware that its segments are traversing a wireless link. An alternative approach is for the TCP sender and receiver to be aware of the existence of a wireless link, to distinguish between congestive losses occurring in the wired network and corruption/loss occurring at the wireless link, and to invoke congestion control only in response to congestive wired-network losses. [Liu 2003] investigates techniques for distinguishing between losses on the wired and wireless segments of an end-to-end path. [Huang 2013] provides insights on developing transport protocol mechanisms and applications that are more LTE- friendly.

• Split-connection approaches. In a split-connection approach [Bakre 1995], the end-to-end connection between the mobile user and the other end point is broken into two transport-layer connections: one from the mobile host to the wireless access point, and one from the wireless access point to the other communication end point (which we’ll assume here is a wired host). The end-to-end connection is thus formed by the concatenation of a wireless part and a wired part. The trans- port layer over the wireless segment can be a standard TCP connection [Bakre 1995], or a specially tailored error recovery protocol on top of UDP. [Yavatkar 1994] investigates the use of a transport-layer selective repeat protocol over the wireless connection. Measurements reported in [Wei 2006] indicate that split TCP connections have been widely used in cellular data networks, and that significant improvements can indeed be made through the use of split TCP connections.Our treatment of TCP over wireless links has been necessarily brief here. In-depth surveys of TCP challenges and solutions in wireless networks can be found in [Hanabali 2005; Leung 2006]. We encourage you to consult the references for details of this ongoing area of research.

Having considered transport-layer protocols, let us next consider the effect of wireless and mobility on application-layer protocols. Because of the shared nature of the wireless spectrum, applications that operate over wireless links, particularly over cellular wireless links, must treat bandwidth as a scarce commodity. For example, a Web server serving content to a Web browser executing on a 4G smartphone will likely not be able to provide the same image-rich content that it gives to a browser operating over a wired connection. Although wireless links do provide challenges at the application layer, the mobility they enable also makes possible a rich set of loca- tion-aware and context-aware applications [Baldauf 2007]. More generally, wireless and mobile networks will continue to play a key role in realizing the ubiquitous com- puting environments of the future[Weiser 1991]. It’s fair to say that we’ve only seen the tip of the iceberg when it comes to the impact of wireless and mobile networks on networked applications and their protocols!

Wireless and mobile networks first revolutionized telephony and are now having an increasingly profound impact in the world of computer networks as well. With their anytime, anywhere, untethered access into the global network infrastructure, they are not only making network access more ubiquitous, they are also enabling an exciting new set of location-dependent services. Given the growing importance of wireless and mobile networks, this chapter has focused on the principles, common link technologies, and network architectures for supporting wireless and mobile communication.

We began this chapter with an introduction to wireless and mobile networks, drawing an important distinction between the challenges posed by the wireless nature of the communication links in such networks, and by the mobility that these wireless links enable. This allowed us to better isolate, identify, and master the key concepts in each area. We focused first on wireless communication, considering the characteristics of a wireless link in Section 7.2. In Sections 7.3 and 7.4, we examined the link-level aspects of the IEEE 802.11 (WiFi) wireless LAN standard, Bluetooth, and 4G/5G cellular neworks. We then turned our attention to the issue of mobility. In Section 7.5, we identified several forms of mobility, with points along this spectrum posing dif- ferent challenges and admitting different solutions. We considered the problems of locating and routing to a mobile user, as well as approaches for handing over the mobile user who dynamically moves from one point of attachment to the network to another. We examined how these issues were addressed in 4G/5G networks and in the

Mobile IP standard. Finally, we considered the impact of wireless links and mobility on transport-layer protocols and networked applications in Section 7.7.

Although we have devoted an entire chapter to the study of wireless and mobile networks, an entire book (or more) would be required to fully explore this exciting and rapidly expanding field. We encourage you to delve more deeply into this field by consulting the many references provided in this chapter.