THE OSI MODEL AND HANDOFF

A layer 2 handoff consists in a change of the layer 2 information related to the current attachment between the MN and the network. This information may consist in a layer 2 address or a layer 2 PoA. A trivial example of layer 2 handoff is performed when a laptop is associated with a WLAN AP and then switches to a second WLAN AP in the same subnet. Layer 2 handoff induces messages exchange between the affected MN and the previous and target PoAs. The time period of the first and the last layer 2 signaling message is referred to as layer 2 handoff delay, after which higher-layer protocols can proceed with their signaling procedure. This delay is a built-in value for a particular access technology.

Layer 3 handover consists in a change of layer 3 information related to the current attachment between the MN and the network. This information may consist in a layer 3 address such as an IP address, a routing table entry, or a point of service (PoS) such as an access router in WLANs. A trivial example is when a laptop is associated with an AP in one subnet and then switches to a second AP in a different subnet. When an MN changes its PoS during an active data session, all packets will continue to be routed to the old address unless particular mobility solutions are implemented. To address this issue, the IP packets may be reassociated with the same transport layer session; thus inducing important delays. IP tunneling may also be adopted. More specifically, a second address will be used as the tunnel end point while the old IP address will be maintained. IP tunneling, however, induces important overheads along with important delays. Inter-subnet mobility solutions addressing layer 3 handoff may be classified with respect to the protocols they use. One can distinguish IPv4-based, IPv6-based, or MPLS-based mobility solutions. Besides, it is worth noticing that some mobility solutions also called reactive solutions trigger the layer 3 handoff after the layer 2 handoff. Contrarily, predictive mobility solutions trigger the layer 3 handoff before the layer 2 handoff. Last but not least, some hierarchical mobility solutions define one global (interdomain) location update point and one local (intradomain) location update point to keep the intradomain handoff transparent to the global location update point and reduce the signaling overhead.

The Session Initiation Protocol (SIP) was designed to support mobility at the application layer; it is also used as the signaling protocol for real-time multimedia calls including voice over IP ones. As it acts at a higher layer, SIP can be combined with Mobile IP protocols, with the possibility of end-to-end adaptation for vertical handoff. A mobile host (MH) registers with a SIP server in its home domain. When a correspondent host (CH) sends an INVITE message to the MH to initiate a call, the redirect servers (RS), which have the current location of the MH, redirect the INVITE to the new location. When the MH moves during a session, it should send a new INVITE to the CH using the same call identifier in the original call setup, but it indicates the new IP address in the contact field of the SIP signaling message. The MH should also perform a new registration at the SIP server with its unique URI (uniform resource identifier) for all the new incoming calls. When both MH and CH are mobile, the reinvite messages are sent through the SIP server because it keeps track of the current CHs location.

HANDOFF VERSUS ROAMING

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.

DECENTRALIZED SUBCARRIER AND POWER ALLOCATION SCHEMES

The distributed infrastructure of WiMAX mesh and relay networks, and the need for reducing communication overhead between the BS and network nodes are the motivations behind proposing decentralized resource allocation schemes. Potentially, either no central controller exists or it does not influence the allocation decision in a decentralized resource allocation. These facts make the decentralized schemes more scalable.

Resource allocation in decentralized networks is essentially different from centralized one. In centralized schemes, the BS collects CSI from all users, allocates the subcarriers or power to the users, and informs the users of allocated resources. On the other hand, users may not need to know the CSI of the other parties in decentralized schemes. Besides, the parties may not be aware of the decision of each other, so a collision is probable. Accordingly, in each proposed distributed resource allocation scheme for OFDM-based networks, the following questions should be answered:

• How does a user achieve the required CSI? The hidden terminal and exposed node problems are common problems in self-organized and decentralized networks, because it is assumed that no central controller exists to assist in signaling. Besides, the signaling overhead should be reasonable for a practical implementation.
  
  
• How should the node coordinate or compete with the other nodes to attain the resources? A node does not know the requirements of the other nodes, at each instant, so competence or coordination is a “must” for a node which wants to start capturing the resources.

An interference aware subchannel allocation scheme that overcomes the drawbacks of decentralized schemes. 

As the scheme uses updated CSI at the beginning of each MAC frame, the channel does not need to be assumed time-invariant over multiple MAC frames. The scheme is appropriate for OFDM-TDD networks. The MAC frame is divided into mini-slots; at the first mini-slot of each MAC frame the ongoing receiver nodes broadcast a busy signal to inform the respected transmitters of the quality of the allocated subchannel. The inactive nodes does not send any busy signal. The ongoing transmitter nodes listen to the busy signal and adapt their subcarrier allocation to their specific receiver nodes according to the information on the busy signal. Also, a node that wants to start transmission listens to the busy signal and chooses the subcarriers that are not interfered by ongoing transmissions and their interference. In other words, it selects those subcarriers with a received busy signal power less than a threshold. The advantages of the scheme are as follows:

• Signaling overhead is low compared to other radio resource allocation schemes
• The co-channel interference is reduced significantly since the scheme is interference aware
• Full frequency reuse is possible

A decentralized power allocation problem for a cooperative transmission is formulated in Ref. [43]. The objective is to minimize the total transmission power of all users subject to providing minimum rate requirement of each user. The network model uses a time division multiple access with the OFDM multiplexing (TDMA-OFDM), so only one user accesses the total OFDM subcarriers in each time slot. The user may allocate some of the subcarriers to transmit its traffic and the rest of subcarriers to relay the other users’ traffic to minimize total transmission power. In other words, the resource allocation problem determines if a user should cooperate with other users, and in case of cooperation it determines the subcarriers and power allocation. A cooperative user may be allocated more power than a noncooperative user, because its location and channel gain allows it to cooperate with other users. However, the total network power is minimized by cooperation among users. A decoupled subcarrier and power allocation scheme is proposed to solve the problem.

Assuming an access point in the network proposes a distributed decision-making scheme for the resource allocation. Each user measures its CSI upon receiving a beacon signal from the access point. The subcarriers are divided into several groups (equal to the number of users), and the approximate channel gain of each group for each user is estimated. Then, the users contend with each other to achieve the group with the best channel quality of their own. A backoff mechanism is proposed to avoid collision. Each user start contending for the best group of subcarriers after a backoff time which is proportional to the best group gain for that user. After contention, the access point informs the users of the winners which can transmit in the next transmission interval. A frame is divided into three subframes, contention, acknowledge, and transmission subframes. This scheme is actually a distributed resource allocation scheme for a centralized network that aims to reduce the signaling overhead and processing task of the access point.

A collaborative subcarrier allocation using the swarm intelligent. The scheme relies on the central controller to achieve the updated information of available subcarriers and the highest and lowest demands for each subcarrier. In other words, the nodes negotiate with the central controller iteratively in a negotiation phase. In each iteration, the nodes inform the central controller of their demand for a specific subcarrier. Then, the central controller broadcast a message indicating the highest and lowest demands for each subcarrier. Based on the feedback messages that occur several times in each negotiation phase, the nodes intelligently decide upon subcarriers.

A comparison between the capacity performance of OFDM-TDMA and OFDM-FDMA in a two-hop distributed network. The time frame is divided by two in OFDM-TDMA. In each half of the time frame, all subcarriers are devoted for transmission over one hop. A power allocation algorithm is proposed to maximize the end-to-end capacity subject to the overall transmit power constraint of the two hops. For OFDM-FDMA, the subcarriers are assigned to the two hops without overlapping and a joint subcarrier and power allocation algorithm is proposed. The simulation performance results of the algorithms show that OFDM-FDMA achieves a higher end-to-end capacity than OFDM-TDMA.