1. Handoff: Overview
The recent years have been marked by a growing need for advanced applications and Internet-related services at high throughput and low costs while guaranteeing continuous and open access to such services. Besides, mobile access to Internet services requires access to packet-switched wireless networks, which are generally divided into an access part and a core part. The access part contains the link-layer devices while the core part contains network-layer devices mainly acting at the network layer. A mobile node (MN) wishing to access Internet services needs to attach to the first link-layer device in the access network, also called, link-layer point of attachment (PoA). Nevertheless, the communicating MN may change the link-layer PoA during an active session due to particular reasons. This process is called handover or handoff. Handoff may be mobility driven when link conditions change due to mobility. In such case, the handoff occurs when the MN leaves the radio coverage of the actual PoA and becomes threatened by losing the network connectivity. On the other hand, handoff may be policy driven when the change in the PoA is argued by higher data rates, better services, lower costs, etc. After selecting the new PoA, a particular mobility mechanism implementing the handover process is executed. Such a mobility mechanism updates the mobile device and the network states to reflect the new PoA and redirects the ongoing data packets that were addressed to the old PoA during the handoff. If the MN has switched to a new link-layer PoA belonging to the same access network, a link-layer mobility mechanism will be executed. However, if the mobile device changes between link-layer PoAs belonging to different access networks, a network-layer mobility mechanism will be executed. When both PoAs (the old and the new) implement the same physical layer technology, the handoff is referred to as intratechnology handoff or horizontal handoff. Besides, when the new PoA implements a different physical layer technology compared to the old PoA, an intertechnology handoff, vertical handoff, or roaming takes place. In the latter case, the MN should be equipped with more than one network interface card (NIC). On the other hand, the handoff, which involves connecting to a new link and disconnecting from an old one, may be hard, soft, smooth, or seamless depending on the implemented mobility solution. Hard handoff takes place when the MN receives data packets from only one PoA during the handoff as the current link is disconnected first and then the connection to a new link is established. Controversially, soft handoff enables the MN to receive data packets by more than a PoA simultaneously as first a connection is established to a new link and then the old link is disconnected. Finally, a smooth handoff is a handoff where packet loss is minimized while a seamless handoff is a handoff transparent to the application. Handoff is always triggered by the occurrence of a certain event at the mobile device or in the network. If the handoff trigger occurs in the mobile device, the handoff is mobile initiated; otherwise, it is network initiated. As there is more than one candidate PoA to switch to, the decision to select the new PoA may be taken by the MN or the network; thus the handoff may be mobile controlled or network controlled, respectively. The handoff is mobile controlled and network assisted when the handoff decision is taken by the mobile based on information received from the network. Controversially, if the handoff decision is taken by the network upon the reception of information from the mobile such as the signal quality of neighboring PoAs, the handoff is network controlled and mobile assisted.
2. Roaming: Overview
Roaming appeared with the Global System for Mobile (GSM) technology. It may be defined as the set of mechanisms that allows extending the connectivity service to a location that is not covered by the home network (HN) where the service is registered but is covered by a visited network (VN). The VN always asks the HN for the authentication and authorization data associated with the visiting MN to allow or deny service access. Roaming also refers to changing the access network while on move. For instance, an MN which is GSM enabled and equipped with a wireless local area network (WLAN) NIC may roam between the two networks depending on its location without interrupting an active session. Roaming support is provided through the implementation of mobility management, authentication, authorization, and billing procedures. Different service providers managing different networks need to negotiate a roaming agreement to define the legal aspects related to service availability and billing. Often, the roaming process consists of three steps. First, the HN detects the MN as an unknown device which is not registered; therefore, the VN tries to identify the MN’s HN. If there is no roaming agreement between the HN and the VN, the MN will be not able to access VN services. However, if a roaming agreement exists, the VN requests MN’s service information to deduce whether the MN is allowed to roam. If it is the case, the VN maintains a subscriber record for the device while the HN updates the MN-related information so that any information destined to the MN will be correctly routed. The subscribers activity is registered in a file maintained by the VN. This file includes the details of the initiated calls, the subscribers visited locations, the calling parties, the volume of the exchanged data in case of data calls, the time of the calls and their duration, etc. Based on such information, the HN will be able to perform billing. Interstandards roaming allows MNs to seamlessly move between mobile networks with different access technologies. Nevertheless, this roaming type is particularly challenging as communication technologies have evolved independently across continents and were implemented by different industry bodies.
