HANDOFF WITH UMTS AND WIFI

Mobile WiMAX intends to combine the advantages of both WiFi networks and third generation networks such as UMTS while intercommunicating with both.

1 Handoff with UMTS

If we carefully examine the reference models of both WiMAX and UMTS networks, we will notice that they share many similarities. For instance, the WiMAX ASN may be directly mapped to the UMTS terrestrial radio access network (UTRAN) while the CSN may be mapped to the UMTS core network as their functionalities are distributed in the same fashion. 

Moreover, the QoS classes of service provided by both technologies enable a direct transfer of traffic flows from one network to another. In fact, the UMTS conversational class may be mapped to the UGSs class, the UMTS streaming class to the nrtPS, the UMTS interactive service class to the (extended) rtPS, and the UMTS background class to the BE class. Regarding security, both UMTS and WiMAX networks support mutual authentication and provide integrity, replay protection, confidentiality, and nonrepudiation guarantees. Nevertheless, moving from WiMAX to UMTS will induce degradation in the provided data rates. Network planning authorities may choose between deploying loose or tight coupling between WiMAX and UMTS networks depending on the degree of integration of both networks. 

When loose coupling is implemented, access to the 3G AAA services is guaranteed by the packet data gateway (PDG) edge routers connected to the WiMAX network or routed through the Internet. PDG routers provide a tunnel termination gateway (TTG) between the WiMAX network and the mobile core network while providing charging gateway interfaces, IP address allocation, authentication in external networks, and single access to mobile core network packet data domain services. 

Meanwhile, WiMAX and UMTS traffics are separated so that WiMAX providers can implement their own mobility, authentication, and billing mechanisms while signing roaming agreements with 3G providers. 3G providers will benefit from extending the coverage of their networks while reducing the deployment costs and serving rural or inaccessible areas. However, loose coupling requires the implementation of higher level mobility management protocols that need to be media independent; thus adding complexity and latency when roaming between different access networks. To address this issue, each network should share its admission control and resource management information with the neighboring networks.

When tight coupling is implemented, WiMAX and UMTS networks will use the same core network components including gateways, AAA entities, and infrastructure. More specifically, the WiMAX network will be connected to the UMTS gateway GPRS support node (GGSN) and the packet-switching domain of the UMTS core network. The UMTS considers the WiMAX network as a radio network controller (RNC); therefore, mobile terminals need to implement the UMTS protocol stacks. Compared to loose coupling, tight coupling facilitates the management procedures as the same billing and authentication, and mobility protocols can be used for both networks. Besides, vertical handoff may be easily initialized and optimized as the core network possesses a clear view of resource status in both networks. However, introducing WiMAX traffic into the UMTS network should be preceded by adapting some core network entities to handle new types of load and traffic patterns to avoid capacity conflicts. Last but not least, the tight-coupling approach is less flexible in coverage extensions than loose coupling; consequently, it is generally practical to adopt it when WiAMX and UMTS networks are owned by the same operator and the integration is implemented by means of adding patches to the existing components.

2 Handoff with WiFi

IEEE 802.11e standard aims at providing QoS guarantees at the LAN scale by specifying differentiations mechanisms at the WiFi MAC layer. However, WiFi networks-limited coverage prevents them from providing continuous Internet-related services and real-time applications support anywhere and at any time. On the other hand, IEEE 802.16e, from which inherits mobile WiMAX, faces some problems related with energy saving and quality providing in some indoor areas. Consequently, integrating WiMAX and WiFi is a hot topic that is currently attracting an increasing number of researchers. A multilayer network protocol architecture that aims at addressing QoS and mobility issues for integrating WiFi and WiMAX. In fact, they designed a two-tier network that considers a 802.16e cell as overlay cell that overlays a few WiFi cells and 802.11e cells as underlay cell cluster. When the MS stills under the coverage of WiFi cells, handoff will be performed horizontally as WiFi can provide high bandwidth and good performance. The AP, which offers the highest bandwidth value and reduces the unnecessary handover probability due to signal strength dropping down, will be selected as target AP. Analyze multiple parameters in a cross-layer fashion to optimize the handoff decision. Examples of such parameters, which are adopted during the simulation, include residential time, WiMAX-cell capacity, and blocking and dropping probability.

The proposed network stack supports three IEEE 802.11 physical layers which are 802.11b, 802.11g, and 802.11n, providing data rates of 11, 54, and 250 Mbps. The network stack also includes IEEE 802.16 and IEEE 802.16e physical and MAC layers. A handover layer based on the IEEE 802.21 standard lies between the lower layers and the network layer implementing the Fast Mobile IPv6 protocol. Management entities are present at each layer and implement all the required provisioning, maintenance, operation, and administration functions. They are also in charge of communicating with servers processing the QoS policies, the mobility decisions, and the profiles storage. To define unified layer 2 abstractions in the IEEE 802.21 to support layer 3 fast handoff. Moreover, WiFi/WiMAX and Fast Mobile IP interaction is achieved by the IEEE 802.21 primitives. They also base the handoff decision on the analysis of different link parameters such as signal strength, velocity of MNs, delay, service level prediction, etc. For instance, the target cell selection is speed sensitive, which means that the MNs are directed to the appropriate cell layer according to their velocity to decrease the blocking and dropping probabilities. A handoff protection mechanism consisting in reserving two free guard channels for handoff usage in advance to minimize the dropping of handoff calls. The proposed mobility management scheme depends on three types of arrival hosts in the WiFi cell.

 If a filtered MN arrives in the WiFi cell from neighboring WiMAX cells and overlaid WiMAX cell, the scheme does not change its state. However, if a nonfiltered MN arrives in the WiFi cell from overlaid WiMAX cell and initial session itself, and if the residential time is longer than the residential time threshold and the WiMAX cell has enough capacity, then it will be overflowed to the WiMAX cell to reduce the handoff probability. Otherwise, it will be assigned to the WiFi cell. Overflow thresholds to reduce the dropping of a handoff call and the blocking of a new call. When a nonfiltered MN arrives in the WiFi cell and the blocking and dropping probabilities of the WiMAX cell are less than the overflow threshold, it will be overflowed into the WiMAX cell. Otherwise, it will be assigned to the WiFi cell.

IEEE 802.21 HANDOFF STANDARD | HETEROGENEOUS HANDOFF

IEEE 802.21 specifications aim at defining a framework that supports information exchange between entities belonging to different access networks and helps in taking handoff decisions; they also define a set of functional components that have to execute those decisions. The proposed framework is built around the media independent handover function (MIHF), which lies between layer 2 and layer 3 and provides higher layers with abstracted services through a unified interface. Three services can be distinguished: the event service, the command service, and the information service.

Media independent event services (MIES): In general, handover is triggered by multiple events that may occur at the terminal or the network level. The MIES provides event classification, event filtering, and event reporting corresponding to dynamic changes in link characteristics, link status, and link quality. Figure 1 shows that events can be classified into link events and MIH events. Besides, the event model follows the subscription/notification procedure. More specifically, the MIHF registers link event notifications with the interface while any upper-layers entity may register for an MIHF event notification. Link events are generated by both the physical and the MAC layers and then sent to the MIHF which has to report them to any entity that has registered an MIH event or a remote MIH event. MIH events are generated by the MIHF and may be local or remote. Local events occur at the client level while remote events take place in the network entities. Both link and MIH events are classified into six categories, which are administrative, state change, linkparameter, predictive, link synchronous, and link transmission. The most common events include link up, link down, link parameters change, link going down, link handover imminent, etc. Upon being notified about certain events, the upper layer uses the command service to control the links to switch to a new PoA when needed.

Figure 1: Media-independent event services.

Media independent command service: The media independent command service (MICS) enables MIH users to manage and control link behavior relevant to handovers and mobility. More specifically, MICS commands aim at gathering information about the status of the connected links along with transferring higher-layer mobility and connectivity decisions to lower layers. As shown in Figure 1, upper layers generate MIH commands which may be local or become remote depending on the destination entity. These MIH commands are translated by the MIHF to link commands which differ depending on the access network being used. Examples of MIH commands include MIH poll, MIH scan, MIH configure, and MIH switch. These intend to poll connected links to get informed about their most recent status, to scan for newly discovered links, to configure the new links and then to switch between the available links.

Media independent information service: Mobile terminals need to discover the heterogeneous neighboring networks and communicate with their entities to optimize the handoff process and facilitate seamless handoff when roaming across these networks. To address this issue, media independent information service (MIIS) provides the capability for obtaining the necessary information for handovers. Such information includes neighbor maps, link-layer information, and availability of services related to a particular geographic area. MIIS enables all layers to share IEs required for making handoff decisions; it also specifies the information structure and its representation while implementing a query/response type mechanism. Information may be either static such as the names and providers of the mobile terminals neighboring networks or dynamic such as channel information, MAC addresses, security information, etc. MIIS fulfils quick data transfer understood by different access technologies with very little decoding complexity. In fact, reports can be transferred either by means of type-length value (TLV) or using a schema referred to as resource description framework (RDF) which is represented in eXtensible Markup Language (XML). As layer 2 information may not be available or may not be rich enough to make intelligent handoff decisions, higher-layer services may be consulted to assist in the mobility decision making process.

Service access points: service access points (SAPs) represent the API through which the MIHF can communicate with the upper-layer and the lower-layer entities using IEEE 802.1 primitives. More specifically, we distinguish three types of SAPs: lower-layer SAPs, upper-layer SAPs, and management plane SAPs. Lower-layer SAPs vary according to the network access type as defined by each MAC and PHY relative to the corresponding access technology. On the other hand, upper-layer SAPs define the interface between the MIHF and upper-layer mobility management protocols or policy engines implemented within a client or a network entity. Finally, management SAPs describe the interfaces between the MIHF and the management plane of various networks.

MEASUREMENTS FOR HANDOFF OPTIMIZATION

Handoff optimization is a key challenge for the network management as it results in the enhancement of the network performances by optimizing throughput, routing, delay profiles, delivered QoS, and communication costs. Therefore, mobile WiMAX should take into consideration numerous parameters that implement a particular handoff policy to optimize the handoff performance, especially in case of interaction with heterogeneous networking technologies. It is valuable to note that IEEE 802.16e specifications consider the handover decision algorithms beyond the scope of the standard.

1 Monitoring Parameters

Radio resource management (RRM) specifications should guarantee efficient resource utilization in WiMAX networks by assisting functions such as QoS provision, service flow admission control, and mobility management. RRM mechanisms shall be implemented in ASNs either with BSs that directly communicate between them or with BSs having no direct communication between them or at a centralized RRM entity that does not reside in a BS but that collects and monitors radio resource indicators from several BSs. Each BS should collect data about the neighboring BSs either through static configuration data or through a different RRM entity aware of the dynamic load of neighboring BSs. RRM mechanisms should support network functions in taking the required decisions. For instance, RRM may assist handoff preparation and control to improve the overall performances. 

For example, RRM mechanisms may optimize the system load control by selecting the most suitable target BS during handoff or updating the list of recommended BSs to handoff. The radio resources available at a BS where a BS-ID defines a sector with a single frequency assignment may be a good hint for BS selection during network entry or handoff. More specifically, the available radio resource indicator, which gives the percentage of reported average available subchannels and symbols resources perframe, may be used to select recommended target BSs that achieve approximately equal load. Averaging is calculated over a configurable time interval with a default value of 200 frames [8]. Choosing the more suitable BS as a target BS is done through scanning and association activities. The criteria that enlighten the choice may include the link quality in the UL direction. Such handoff monitoring parameters are measured by the MSs and then sent in a form of report to the managed BS, which may deliver them to the radio resource controller (RRC). The RRC, which is generally implemented at the ASN GW or at a BS, controls multiple radio resource agents (RRAs) located at the BS level. RRAs maintain a database of collected radio resource indicators received by the managed MSs or by the respective BSs. In fact, IEEE 802.16d amendments define the receive signal strength indicator (RSSI) and the CINR as two main signal quality indicators that help in assigning BSs and selecting adaptive burst profiles. The specifications also define the mean and the standard deviation because the channel behavior varies in time. These indicators enable the channel quality monitoring and may be augmented to include measurements related to QoS parameters such as the burst error rate. 

It is valuable to note that the RSSI measurements do not require receiver demodulation lock; therefore, they provide reliable channel-strength estimation even at low signal levels. Each BS has to collect the RSSI measurements while each MS shall obtain an RSSI measurement in an implementation-specific fashion. After performing successive RSSI measurements, the MS should derive and update estimates of the mean and the standard deviation of the RSSI and then report them in units of dBm via a REP-RSP message. To obtain such a report, statistics are quantized in 1 dB increments ranging from−40 to −123 dBm while the values outside that range are assigned the closest extreme value within the scale. IEEE 802.16d specifications do not impose how to estimate the RSSI of a single message, but they claim the relative accuracy of a single-signal-strength measurement taken from a single message to be 2 dB with an absolute accuracy of 4 dB. 

Each BS has to collect the CINR measurements while each MS shall obtain a CINR measurement in an implementation-specific fashion. After performing successive CINR measurements, the MS should derive and update estimates of the mean and the standard deviation of the CINR and then report them in units of dB REP-RSP messages. IEEE 802.16d specifications do not impose how to estimate the CINR of a single message, but they claim the relative accuracy of a single signal strength measurement taken from a single message to be 1 dB with an absolute accuracy of 2 dB.

2 Optimization Functions

The handoff process should not decrease the overall performance; therefore, it should implement mechanisms that optimize the delay to support real-time applications such as VoIP and video streaming. Connection dropping should also be reduced. IEEE 802.16e and mobile WiMAX specifications define prescan mechanism to measure the radio connection and select the target BS before handoff execution. Nevertheless, the handoff procedure should include not only layer 2 handoff but also the IP layer handoff for IP-based services. 

Figure 1 below shows that layer 3 handoff, which includes movement detection, IP configuration, and location registration subphases, highly affects the overall handoff delay and requires much more time to be executed than layer 2 handoff. Optimizing such delay is a hot research issue. For instance, the fast handoff procedure proposed proposes to process the configuration of CoA, duplicated address detection (DAD), etc. in advance to reduce the handoff latancy. However, the mobile IP procedure always begins after ending layer two handoff so that the handoff delay is the summation of the layer 2 handoff delay and layer 3 handoff delay. Consequently, it is imperative to consider the correlation between layer 2 and layer 3 within a cross-layer scheme. Analyzed the signaling message flow sequence and the format of both IEEE 802.16e and fast MIPv6 to correlate both handoff procedures and minimize the required signaling overhead. For instance, propose to integrate some layer 3 handoff information with the MOB_HO_IND message and the RNG_REQ as they share semantic characteristics while performing the handoff.

 

Figure 1: The handoff process for IP based services.

Regarding QoS, mobile WiMAX specifications define different classes of services having different requirements in the quality of the traffic delivered to MSs. This quality is generally measured in terms of data integrity, latency, and jitter. The handoff process should be optimized to not highly affect these parameters. For instance, maintaining data integrity during handoff means that the packet loss, duplication, or reordering rates will not be considerably increased while the impact on the DP setup latency/jitter should be minimized. From the QoS point of view, there are controlled handoffs and uncontrolled handoffs. A controlled handoff should respect the following conditions:

• If the handoff is initiated by the MS, the latter should send to its serving BS a list of potential targets.
• Network should base its target selection on the list of potential targets provided by the MS.
• Network should inform the MS with the list of available targets for handoff. If that list is empty, the network will refuse to accept MS handoff. The available targets list should be a subset of the one requested by the MS or reported by it.
• MS should process handoff by moving to one of the provided targets or it should cancel the handoff. The decision is sent via the MOB_HO-IND message.

When any of these conditions is not respected, the handoff is considered uncontrolled and does not provide any QoS guarantees. In the worst case, the MS can connect to the target BS without any indications given to that target BS inducing an unpredictive handoff. To provide data integrity and optimize the handoff process, several mechanisms are provided and classified into two main groups: DP setup mechanisms and DP synchronization mechanisms. DP setup mechanisms refer mainly to buffering and bi-multicasting. Buffering consists in saving the traffic of the services for which data integrity is required at the DP originator or the DP terminator level. 

Nevertheless, the buffering point may change during handoff based on the data integrity mechanism selection. Moreover, buffering may be done only during handoff or for simplicity within the lifetime of a session. On the other hand, multicasting refers to multicasting downstream traffic at the originator endpoint of the DP while bicasting consists in bicasting traffic to the serving element and to only one target. It is worth noticing that bicasting achieves better performances when it is combined with buffering. DP synchronization mechanisms aim at guaranteeing data delivery in different data functions, which buffered the different DPs (serving and target) used to deliver the data during handoff. It is achieved by using either sequence numbers, data retrieving, or Ack window with sequence number disablement. 

A sequence number is attached to each SDU in the ASN DP and then incremented by one every time it is forwarded in the DP. Data retrieving does not require the definition of sequence numbers; the anchor DP function buffers or copies the data during handoff preparation, and when a final target BS is identified, the serving BS will push back all the nonsent packets to anchor/target DF. When Ack window with sequence number disablement is implemented, data storage buffers in anchor DP are released by full or partial ACKs from the serving BS without requiring sequence numbers. Vertical handoff between mobile WiMAX and other wireless networks represents a hot research topic as it requires the implementation of optimization algorithms that are able to decide when to initiate handoff and which access network to choose while minimizing the handoff latency and preserving the security context and the required QoS.