Mobile Network Routing Demystified: From Nomadic to MANET

1. Routing Delivering frames across wireless link-that is, between adjacent nodes-the course moves on to data communication between non-adjacent nodes. Routing in mobile networks is every interesting problem. The routing solution depends on whether the mobile computing model is nomadic, cellular, or a MANET. Nomadic computing: As we mentioned before, this model assumes no network connectivity during a move. So, each time a node connects to the network, the node could register with appropriate name servers and locate the servers that provide the desired services by probing the known directory services, Subsequently, packets could be routed to the mobile node. If the node's mobility will be hidden, the network could employ Mobile IP. All the communicating nodes could route packets to the mobile node's home address. There, the home agent would intercept the packets and tunnel them to the mobile node. Cellular computing: Cellular networks that provide continuous connectivity and data communication for mobile nodes take a different approach. As a mobile node moves out of one cell into a neighboring cell (or from one wireless LAN to a neighboring wireless LAN), seamless handoff between the cells is needed. A mobile computing course must discuss this problem for three reasons: 1. Unlike with handoff of voice calls in cellular networks, occasional loss of a few data packets can severely degrade the performance of several data communication applications. 2. Applications that rely on data communication in a cellular setting will become prevalent when third-generation packet radio networks become a reality. The cellular networking domain currently doesn't have many such applications, owing to the low available bandwidth. 3. Depending on the nature of the application, data packets might or might not have delivery deadlines and reliability requirements. So, QoS issues are important in routing decisions. The added dimension of mobility makes the problem more challenging. QoS research and solutions related to backbone networks do not necessarily address the issue of source and destination mobility. MANETS: Owing to the absence of any infrastructure and the need for all nodes to collaborate in routing and packet forwarding MANET routing is fertile ground for the application of distributed computing ideas. While no deployed MANETS exist, such networks have interesting potential civilian and military applications. Owing to the dynamic nature of MANETS, any routing protocol for them should undergo these tests: • What volume of control traffic do you need to propagate routing information? • • • • • • • • How much time do the routing tables at all the nodes take to converge after a topology change? Are there situations when the routing tables might not stabilize? Does the protocol determine routes that are optimal with respect to the metric that is being optimized? Are the routes stable? Wireless links might be unidirectional. Can the routing protocol utilize such links? If so, do you need any modifications at the data link layer? What fraction of packets is dropped owing to non-availability of routes even though the source and destination are part of the same connected component? What fraction of packets is misrouted? The answers to these questions will determine if the routing protocol is scalable. Such scalability will be a requirement for the wide deployment of MANETS. 4. The Transport Layer Teaching issues pertaining to the trarnsport layer, especially TCP, can get tricky in the context of mobile networks. At this layer, only end-to-end considerations matter. However, in a mobile network, the data link layer characteristics of wireless links, such as high bit error rate and low bandwidth, can significantly affect the performance of TCP connections. Consider a TCP connection in a cellular system where only one link is wireless. Packets dropped across a wireless link will result in TCP time-outs and re-transmission by the source. Retransmitted packets will travel over the entire path, including the reliable wired links. Need to determine the best possible solutions that do not violate the end-to-end TCP semantics or the layered protocol stack approach. For example, the Snoop protocol relies on an agent at a base station to monitor the reliable delivery of link layer frames carrying TCP traffic. Indirect TCP, another solution, relies on establishing two connections for a TCP connection. One connection spans the relatively reliable and bandwidth-rich wired part; the other spans only the wireless link and is optimized for this link. Intermediate nodes should treat all the packets the same, whether they carry TCP or UDP (user datagram protocol) traffic. For MANETS, the network's dynamism and the resultant changes in path length and propagation time can impact TCP's mechanisms for congestion control in ways that are difficult to predict. Mutually unreachable replicas, the replicas' state might diverge. So, when two previously unreachable nodes with mutually divergent replicas become reachable. It's interesting to nudge the class toward the realization that the insistence on strict replica consistency or cache coherence they learned in database and computer architecture courses, respectively, might be bad for the disconnected mode: data availability will be adversely affected. So, we need a weaker notion of replica consistency. A related issue is network-aware computing. As the mobile device moves between heterogeneous networks (from connection to a wired access point, to a wireless connection through a wireless LAN, to a MANET) and as the speed of motion changes, the quality of the network link and of other available resources might change significantly. So, the application servers and clients should be able to adjust accordingly. For example, a user walks along a street carrying a mobile node with a high-bandwidth wireless link but with a limited energy supply and a very basic I/O interface. Later, the user enters a car, slips the mobile node into its docking port, and starts driving along a highway. Now, the node has a relatively abundant energy supply from the car's battery and a much better I/0 interface, but a much lower wireless bandwidth owing to the cars high speed.