WiMAX-A Study of Mobility

1. Introduction

The wireless market is growing rapidly; being pushed through wireless connectivity demand various wireless connectivity means are emerging (WLAN/802.11, WMAN/802.16a, WWAN/802.16d) [IEEE, 2005]. Wireless networking has enabled Wireless Internet Service Providers (WISPs) to reach to such remote areas, which were expensive or impossible to reach. These wireless standards suppose to provide higher bandwidth for wireless voice and data rate.

WIMAX or 802.16 is working group number 16 of IEEE 802, specializing in point to point (P2P) or point-to-multipoint (P2MP) broadband wireless access [Smith & Meyer, 2004]. WiMAX has better range-radius and power of the signal, and it also does not conflict with Wi-Fi and other legacy Network technologies.

Due to the broadcasting nature of the 802.16 PHY/MAC protocols, WiMAX/802.16 maintains a network hierarchy to maximize efficiency; minimize co-channel interference [Lin et al, 2006]. WiMAX promises to improve the network security, QoS* and to fill the gap between 3G and wireless LAN. The core components of a WiMAX system are the subscriber station (SS) and the base station (BS). On air, the BS controls activity within the cell, including access to the medium by SS, allocations to achieve quality of service (QoS) and admission to the network based on network security mechanisms.

As WiMAX is still under development stages with parts of 802.16 being completed, so it will prove an interesting case to study and analyze. WiMAX technology and architecture will be the main focus of this thesis and not least but the most important aspect will be analysis of design and structure of WiMAX/802.16 protocols regarding mobility.

2. Problem Description

The wireless Technology is a rapidly growing segment of the telecommunication sector and the number of users is increasing day by day both in public and private sectors with the main aims to provide secure, robust, higher data rate and mobile wireless connection to portable devices located anywhere around the world.

Networking has been through massive evolution stages, as networking access has gone from indoor wired technology to wireless out door mobility; even wireless technology has gone from fixed access point (AP) to mobility. Wireless standards have gone from fixed point 802.11 to 802.16e or 802.20 standards to provide mobile user better and more secure access to Base Station (BS) [Rubin, 2003; Balachandran et al, 2005]. All these drastic changes are occurring to strive for customer expectation and market demands.

As a load of wireless technologies are trying to strive for market demand e.g. Blue tooth, HIPERLAN* XMAx etc. WIMAX protocol were not designed for mobility from the start; now keeping an eye on user mobility issues, IEEE task group have developed IEEE802.16e to support mobility and seamless handover [Sunderam, 2005]; however IEEE802.16e protocols are still under testing at the moment and IEEE 802.20 protocols are under designing phase.

This paper will provide in depth analysis of mobility cases in wireless technology; those already have been deployed in market and future challenges for wireless mobility deployment.

2.1  Problem Statement

Wireless means transmitting invisible radio signals ranging from in door to open area wirelessly instead of wire through open air (medium). Wireless technology can be ranged as simple as to make a phone call to sales force information of Enterprise Resource Planning (ERP) systems.

In wireless world, end users are looking for persistent and seamless handover from one wireless network to another network with blink of eye, even if they are flying across the border or in remote area of the same country.

The future of wireless lies in meeting end users demands and expectations in terms of seamless handover, faster browsing, security, encryption of wireless data, and interoperable with existing and emerging wireless technologies.

• What are the mobility capabilities of WiMAX?
• Is WiMAX/802.16 standards are interoperable with its existing standards and other wireless networking standards?
• How many users can one WiMAX tower support simultaneously?
• Does the throughput lower significantly as more users go online on the same tower?
• What is the mobility cost in WiMAX?
• What are security enhancements in WiMAX upcoming standards?

WiMAX technology and architecture will be the main focus and not least but the most important aspect will be analysis of design and structure of WIMAX/802.16 protocols regarding mobility.

2.2 Ethics Statement

The following ethical behaviour must be observed during the course of this research.

• Abide by BCS code of conduct
• Remain impartial in data analysis and professional consulting
• Preserve academic honesty
• Avoid software policy
• Avoid plagiarism

3.  Communication Mediums:

In the “information age” there is great need of data transmission within and outside of the premises, portable devices and wireless communication has become an integral part of our daily life and business routines; moreover these potable devices with wireless base unit provides so much control that a person can access, store and synchronize the information abroad and remote areas using various means i.e. Wi-Fi, hotspots, Fixed wireless Access Points (AP) etc.

Accelerating technology developments are driving changes throughout the communication industry (Fixed or Wireless) at an unprecedented rate, ranging from cellular to satellite communication.

3.1 Wired Access/Medium

Fixed wireless Access refers to an infrastructure, where both sender and receiver remain fixed in wired communication and transfer data through the guided medium e.g. twisted pair cable, co-axial cable etc. In wired communication, data is transmitted through Public Switched Telephone Network (PSTN); which also called world Telephone System. Public Switched Telephone Network (PSTN) infrastructure runs a copper wire until your home and business premises and than home and business are connected to Subscriber Local Carriers (SLCc) known as “Local Loop” [Prentice-Hall, 2006]. This backhaul connect your home, offices and other Equipments through physical cables.

3.2 Physical Medium

For wired communication various medium has been used over the time, various physical mediums can be used depending in terms of bandwidth, delay, cost, and ease of installation and maintenance.

-Twisted Pair cable*
-Coaxial Cable+
-Fiber Optic

Figure 3.1: Wired Medium (L to R) Coaxial cable, Twisted Pair and Fiber Optics [Prentice-Hall, 2006]

3.3  Wired Systems:

Broadband access provides higher data transfer rate and always on for uplink and downlink for internet communication. Wired broadband (DSL, ADSL and PON[2]) are competing for consumer’s markets with wireless and other new emerging wired/wireless technologies. There are various wired technologies operating at the moment i.e.

-DSL

-ADSL

-PON

-Cable Modem

-BRI*

The wired topology, physical medium and selection of wired system depends upon the size of the network, cost and scale of the network. Although, wired medium still occupies great share in telecommunication, but higher data rate, mobility, cost effectiveness, easy installation, flexibility are driving forces for wireless medium.

The wireless networking technology is making great strides in respect to cost, WLAN throughput, interoperability and security concerns have been addressed by new and stronger features that effectively guard sensitive data and network access. New wireless 802.16e standards; are under testing which will provide mobility, extra security features and stronger data encryption and seamless handover from one network to other. Fixed Wireless Access (FWA) networks are alternative of wired networks i.e. DSL/ ADSL/ Fibre Optics etc. Main features of wired network are shown in the below table 3.1.

Table 3‑1: Comparison of Wired Medium [Smura, 2004]

4.  Fixed Wireless Access

Fixed Wireless Access (FWA) can be classified as licensed or non-licensed system(s). There are two principal subcategories of licensed system called Local Multipoint Distribution System (LMDS) and Multi-channel Multipoint Distribution System (MMDS) [4^th wave Inc, 2007; University of Engineering Hong Kong, 2004]. Local Multipoint Distribution System (LMDS) are considered suitable for dense urban area for business and offices; Local Multipoint Distribution System (LMDS) operates at higher frequencies though they have limited range.

4.1           Types of Fixed Wireless Access

Fixed Wireless Access types can be grouped into three main types: Point to Point, Point to Multipoint or mesh depending upon number of users, radius, local backhaul etc.

v  Point to Point

v  Point to Multi Point

v  Mesh Networking

4.2  FWA Architecture

FWA logical architecture comprises of several basic components like Station (STA), Access Point (AP), Fixed Wireless Access Point (FWAP), which act as bridge between STA and existing network bone.

4.2.1  Fixed Wireless Access/802.11 Protocol

Fixed Wireless Access/802.11 Protocol are defined by IEEE 802 committee, components of wireless protocol architecture either fall into Data Link Layer or Physical layer of OSI reference Model. Wireless architecture is defined into two separate layers, Logical Link Control (LLC) and Medium Access Control (MAC), which is similar to Data Link layer and PHY is similar to Physical layer of OSI reference model as shown in the figure 4.1

Figure 4.1: OSI reference Model and 802.11 Architecture [Ennis, 1996]

4.3           FWA Handoff/Roaming

Roaming in IEEE 802.11/802.16 series can be called as Handover/Handoff [Smith et al, 2003], process of moving from one Base Station (BS) or Mobile Station (MS) to other BS/MS between air interfaces. There are two basic reason of handoff/handover in wireless communication, if device has move out of the range of the base station and can get a better radio link from other Base Station or Base Station is full with number of Subscriber Station (SS) and it will handover to different Base Station (BS).

In seamless Handoff (HO) mechanism of 802.16, Base Station (BS) continuously keeps on broadcasting advertisement management messages to identify the network and neighbour BS to Subscriber Station (SS) [Kim et al, 2004]. Subscriber Station may acquire further information from targeted Base Station before performing Handover mechanism. Hand Over process comprises of two phases

• Pre-registration
• Releasing connection with current BS and re-associates with target BS.

In Broadband Wireless, handoff mechanism is handled by network itself. However, in contrast, 802.11 dictate handoff mechanism should be controlled by client/SS autonomously and independently itself with no prior knowledge of targeted Access Point [Misra et al, n.d.; Ramani et al, 2005]. During the process of mobility, Subscriber Station (SS) keep on listen the beacon signals (passive signals from AP). Subscriber Station sends additional probe broadcast packets to Access Point (AP) to get response. There are two main type of handoff/handover mechanism.

4.3.1      Horizontal Handoff,

Horizontal Handoff (HO) is also known as Intra-System Handover [Zahran, et al, 2006; Tisgaonkar et al, 2005], in which mobile device is handed over between similar or homogenous Access Point/Base Station.

Figure 4.2 Horizontal Handover

4.3.2      Vertical Handoff

Vertical Handoff (VH) is also known as Inter-System [Zahran, et al, 2006; Tisgaonkar et al, 2005], in which mobile device is handed over between different Access Point and Base Station (Heterogeneous network). See figure below for more illustration

 

Figure 4.3: Vertical Handoff in Wireless Communication

Homogenous network are such netwroks in which different componenets use the same technologies e.g. Mobile phones ( system based on Wideband Code Multiple Access) homogeanous networks are easire to mainain as the mobile phone users can easily switch from one netowrk to other. Drawback of this simple technology is that it force to adapt to this technology.

Heterogenous Network is a network which consist components that connects the different technologies. Heterogenous technologies has more complex architecture and difficult to maintain as different technologies are incorporated in this sort if network. Although these network are difficult to maintain but they provide more flexibilty to the end-users. Different network types have different QoS and this can lead to incorporate different technologies.

5. WiMAX/802.16

The technology known as WIMAX is a Broadband Fixed Wireless Access (BFWA) and with main aim to deliver “last mile” fixed, nomadic and mobile users at metropolitan level. WiMAX initially came out as fixed Broadband and since than it is focusing on two areas, basic and full mobility.

WiMAX basic mobility is natural evolution of IEEE 802.16-2004 with addition of sub-channelization to improve indoor CPE performance, coverage and signal throughput verses distance.

By comparison, WiMAX full mobility will have totally different and complex architecture for MAC and PHY. Based on SOFDMA, IEEE 802.16e will provide seamless hand off with in homogenous and heterogonous network.

Figure 5.1: Evolving Wireless Technologies [Giles, 2006]

5.1  Medium Access Control (MAC)

IEEE 802.16/WiMAX operates on same Logical Link Controller as other Local Area Networks (LANs) and Wide Area Networks (WANs) [Bahai et al, 2004]. IEEE 802.16 is developed in the form of a protocol stack having well defined interfaces, so that each task can be well differentiated from each other.

Physical layer (PHY) offers significant flexibility e.g. initial ranging, registration, bandwidth requests, and connection-oriented channels for management and user’s data.

Figure 5.2: 802.16 MAC Stack Protocol [Tanenbaum, 2002]

WiMAX MAC is different from Wi-Fi and other legacy network technologies, as Subscriber Station (SS) keeps on fighting for Access Point (AP) on random basis [Johnston et al, 2004]. Whereas, WiMAX operates on the basis of MAC scheduling, so Subscriber Stations (SSs) have to fight once for network entry than they will not struggle to access the Access Point (AP) or Base Station (BS).

MAC supports Point-to-Multipoint topology [Boom, 2004]; with main aims to provide efficient sharing of the physical medium. IEEE 802.16 uses scheduled transmissions to ensure collision free access of open air medium.

5.2  WiMAX/802.16 Network Architecture

IEEE defined only PHY and MAC layer for 802.16, which worked fine for technologies such as Ethernet and Wi-Fi, which rely on other bodies such as the IETF (Internet Engineering Task Force) [Wimax-Forum, 2006]. WiMAX provides certifications and profiles for 802.16e and WiMAX NWG provides Access Network architecture.

In wireless world, higher wireless bodies like 3GPP and 3GPP2 are trying to develop wide range of interfaces and standards not only for air link interoperability but also for roaming and seamless Handover (HO).

WiMAX forum Network Working Group (NWG) is focusing on creating higher-level networking specifications for fixed, nomadic, portable and mobile WiMAX systems. Network Reference Models (NRM) contains entities such as Mobile Subscriber Stations (MSS), Access Service Network (ASN), Connectivity Service Network (CSN), Network Service Provider (NSP), and Application Service Provider (ASP)

Figure 5.3: WiMAX Reference Model [Wimax-Forum, 2006]

5.3  WiMAX Handover

WiMAX handover is the process where a Subscriber Station (SS) migrates from one air-interface Base Station to other air-interface Base Station. WiMAX architecture extends the 802.16 architecture and it support the mechanism of HandOver(HO). The WiMAX architecture shall support mechanisms such as intra/inter ASN handover, roaming between NSPs, seamless handover at vehicular speed and micro/macro mobility.

In order to provide session continuity of IP layer during handover, the IP handover between different subnet is available [Jang et al, 2005]. After consideration, WiMAX recently released set of specification for new mobility node; called Access Service Network (ASN) typically resides in operator’s equipment and enables to connect multiple WiMAX Base Stations (BSs).

5.3.1 Access Service Network (ASN)

ASN comprises of one or more Access Service Network Gateways, SS and at least one BS, ASN associates with CSN and ASN-GW associates with one or more BS. This segregation of ASN enables multi vendors to operate while still producing their own functional parts.

5.3.2 Anchoring

Anchoring is the process to attach the MSS to ASN and all incoming data will be sent to the anchoring ASN-GW and the CSN does not need to know at which ASN GW the MS’s current BS is located, which makes mobility transparent and changes to IP- address become less frequent.

A number of references are used to identify the different interfaces used to communicate with in ASN as shown in figure 5.4. These reference points indicate the different protocols and functions required to communicate. Intra/inter-ASN Mobility is to ensure minimal delay and data loss during the transition/handover from serving ASN to target ASN.

Figure 5.4: WiMAX handover and anchoring procedure [Rigel, 2007]

 

5.3.3 Intra ASN Handover

The handover performed between BSs belonging to the same ASN though BSs can connect to the same ASN-GW or different ASN-GW. In case of only one BS within an ASN an intra-ASN handover can not be performed unless the BS has several antenna sectors. The intra ASN handover is to minimize the delay and data loss during mobility and if MSS has services like IP or MIP there will be no need for a change of IP address.

5.3.4  Inter ASN Handover

A handover between BSs belongs to separate ASNs and ASN GWs also need to coordinate to make the smooth handover through anchoring and re-anchoring. The purpose of anchoring is to avoid a path update and hence a redirection of the data path, where in the re-anchoring case an update will be performed.

6. Handoff

WiMAX/802.16-2004 is not implemented to support the handoff between Base Stations; it allows only fixed (indoor or outdoor) or nomadic access. Fixed access to Mobility introduces network infrastructure changes i.e. break-before-make, micro- and macro-mobility handovers across BSs, cross-operator roaming.

WiMAX early version support fixed or nomadic support. Fixed access allows no mobility to the end users and remains fixed to one geographical location. Nomadic access provides mobility with in the cell but no handoff support, so user has to establish a new connection with the AP/BS once out from the footprint of Home Base Station.

According to IEEE 802.16e handover operation, MSS in handover process has to complete the network re-entry process [Malki, 2004]. There are different types of handover.

6.1 Hard Handoff

This sort of handoff is also called break-before-make, where user disconnect from the Host Base station before make new connection to the target Base Station. In hard handoff base station communicate only one Base station at a time. This sort of handoff occurs once the signal strength of the target BS exceeds than serving BS as shown in figure below. Hard handoff cause high latency, data loss and every time new IP address is assign to the user.

Figure 6.1 Handovers in the Mobile WiMAX [Becvar et al, n.d.]

Hard handoff technique is suitable for non-real time applications like email or file transfer.

6.2  Soft handoff

Soft handoff also called connects-before break. In SHO, MSS can communicate more than one Base Stations simultaneously. Though soft handoff is resource intensive but it decreases the data loss and latency factor and is suitable for delay sensitive applications e.g. video streaming or Voice over IP (VOIP).

Figure 6.2 Soft Handoff [Becvar et al, n.d.]

6.3  Horizontal Handoff

Horizontal handoff is also known as homogenous handoff. Horizontal handoff is consider less complex as MSS roams among similar architecture and lead to less complicated solutions and will discuss with greater details in chapter 7.

6.4  Vertical Handoff

Vertical handoff is also known as heterogeneous handoff, which allows the MSS to roam seamlessly among different network and access technologies, maintaining same connection. MIP was used to overcome the problem of loosing stateful connection problem. It will be discussed with greater details in chapter 8.

6.5  Macro Diversity Handoff (MDHO)

In MDHO, both MSS and BSs maintain diversity list of each other. MSS keeps on communicating with all the BSs in diversity list. In case of downlink two or more BSs transmit data to MSS in such way that diversity combining can be performed at MSS. Case of Uplink MSS sends the data to two or more BSs where selection diversity of the information received is performed.

Figure 6.3: Macro Diversity Handoff [Becvar et al, n.d.]

6.6           WiMAX Mobility

Mobile WiMAX extends the 802.16 architecture and includes the mechanism of handoff as well. IEEE 802.16e will support the mechanism of inter/intra seamless handover at vehicular speed. Although 802.16e architecture will based on same 802.16 and other legacy architecture but it promises to provide secure seamless Handover to next Base Station (BS) to receive higher QoS.

6.6.1              Network Acquisition:

To perform successful and secure handover, MSS needs to perform network acquisition through scanning neighbouring BSs (Base Stations) with or without optional association [IEEE, 2005]. There are three levels of association, scan/association without coordination, association with coordination and network assisted association reporting.

Base Stations keeps on sending Beacon signals or Broadcast messages, which contain information about neighbouring BS radius, channels and BSID to make Subscriber Station’s (SSs) synchronization simple with targeted BS. The host BS receives the information of targeted BS through network backbone.

MSS keeps on scanning for targeted BS on the basis of channel quality after getting permission from host BS. During the scanning interval the host BS keeps on buffering incoming data (UCD/DLD) until the MSS finishes the targeted BS scanning and transfers the buffered data back to the MSS[3].

 

 

Figure 6.4: Hand off messages/ initialization

The process of handover, transferring from host BS to target BS can be triggered either from MSS or BS. To commence actual handover process, BS and MSS sends and receives messages to establish synchronization among MSS, target BS, and host BS.

6.6.2  Network Topology Acquisition

BS or MSS tries to acquire network topology before initiating HO mechanism. Network topology acquisition is based on number of messages exchanged between serving BS, SS and targeted BS through network bone. BS keeps on transmitting beacon signals/MOB_NBR-ADV, which include network information, signal strength, Signal to Noise Ratio (SNR). In next step, MSS will scan and select the neighbour BSs using MOB_SCN_REQ message without synchronization to DCD/UCD. Serving BS allocates time slots for MSS to scan target BS and keep on buffering all incoming data to MSS. Once scanning has done than either MSS or BSS can perform actual HO process.

6.6.3  Handover Process

Handover process consist of following six stages,

• Cell reselection “The Process of acquiring BS information to evaluate the chances of possible handover to acquired BS. Cell reselection acquires intervals from host BS to scan targeted BS but this phase doesn’t need to occur in relation to handover decision.”
• Handover decision and initiation “The process of initiating handover process to migrate the MSS to target BS. To commence handover the requesting party (MSS or BS) sends a handover request which will trigger a sequence of handover messages to be sent between MSS, target BS and host BS [IEEE, 2006]
• Synchronization “This phase synchronizes the MSS to target BS downlink channel. During this phase MSS receives downlink and uplink transmission parameters. If MSS already has information about the target BS than the interval for scanning can be shortened.
• Handover ranging “This phase receives the correct transmission information of BS like power level, signal strength, BS radius. Target BS can get information of MSS either through network bone or Synchronization process.”
• Termination of service “This phase marks as the end of services with current BS. Target BS will remove the MSS services with current BS like queues, counters etc.
• Handover cancellation “MS also has the right to cancel the handover process with target BS and can continue the normal processes with current BS.

7. Homogenous Mobility

Performance of handoff is most important issue in mobile environment Homogenous mobility is mobility of MSS (Mobile Subscriber Station) roams among same technologies. Homogenous mobility is simple as compared to Heterogeneous mobility as MSS roams among the networks, those has similar technologies or architecture.

This sort of mobility is simple as the user or MSS roams between the similar technologies, which leads to less complicated system since less complicated measurement like Signal to Noise Ratio (SNR). Example of homogenous network is speech oriented system like Global System for Mobile Communication (GSM) or UMTS, which is widely used in world today. Every person who owns a mobile phone uses these homogenous networks and when user comes out from the footprint of one Base Station (BS) than system performs Horizontal Handover (HHO). This handover takes places in such smooth and seamless manner that users never comes to know that he has been switches to different Base Station (BS) footprint and users continues his conversation without any interruption.

WiMAX operates on packets based network mobility, there is great need of work to done as the this area is new. IEEE and WiMAX are trying to develop such high level layers and protocol which only not provide security but also seamless, low or no latency with lesser packets of data lost e.g. IEEE 802.16, IEEE 802.20.

Global System for Mobile Communication (GSM) and UMTS are two more popular standards used in the world for speech communication in cellular communication. When cellular phone users moves from the range of one Base Station to other Base Station than intra system/ homogenous mobility will be performed.

Homogenous Handoff can be divided into two broad categories:

• Cross Layer handoff “Handoff between two Base Stations (BSs) under the same Foreign Agent (FA), see figure 7.1 Mobile Terminal movement from BS10 to BS11.
• Intra System handoff “Roaming of MSS between two or more BS which belong to different Foreign Agents (FAs) but belong to same system and Foreign Gateway (FGW)

Figure 7.1: Handoff scenario among Base Stations of same FA or different FA [Motanty,S. et al, n.d.]

7.1  Soft Handover and Fast BS switching

During soft handover two or more BSs transmit the similar MAC/PHY PDUs to MSS like mobile phones can get connected to different Subscriber stations. This situation happens if more than one BS falls in the same footprints and they transfer the beacon signals to the MSS and performs the diversity combining and vice versa from Base Stations (BSs) to enables better coverage, link quality and Qos [IEEE, 2004].

In FBSS/SHO, MSS will interact with only one BS from the active BSs list but they will maintain a list of each other. The BS which interacts with MSS from the active list of BSs is called Anchor Base Station (ABS), although MSS can change the BS on per frame basis like CDMA mobile phone. All Base stations in active set and MSS need to synchronize with each other because if BSs send different PDUs or send the same PDU but with a difference in time which MSS can not interpret the received data

Figure 7.2 Soft Hand off and Fast Base Switching[Becvar et al, n.d.]

7.2    Hard Handoff

In this type of hand off as explained in chapter 6 as well, MSS break before connect and MS communicate with only one BS at a time and this type of handoff is simple. The example of hard handoff in homogenous network is cellular network/ mobile phones, where small area is covered by large number of Base Stations (BSs) and MS can select the BS depending on Base Station strength. One advantage of hard handoff is that it is allocated with only one channel. Mobile WiMAX is designed for triple play services (voice, video, and data) but they are designed to tolerant for traffic delay. The reason of HHO selection for WiMAX is that it is more bandwidth efficient than SHO, although it causes more delays.

7.3    802.16-2004 mobility

Although 802.16-2004 just covers only fixed network, which lead to 802.16e for mobility support for MSS. An approach has been defined in 802.16-2004 with main goal to provide mobility without modifying the standards through hierarchal MIP, and pre-defined messages.

Existing messages are used for handoff mechanism, MSS sends a request to perform handoff and BS uses the same existing messages for request acknowledgment. These messages will not affect the state of MSS or BS even if MSS doesn’t perform handoff.

7.4    802.16e Mobility

IEEE introduced 802.16e standard to enable seamless handoff, described in [Li, P. et al, 2006]. IEEE introduced seamless HO mechanism as Last Packet Marking (LPM) for intra-domain. LPM also merges MAC layer HO to minimize delay and safe HO because of limited bandwidth availability in wireless communication [Kim et al, 2004].

IEEE 802.16e uses hard handoff as default mode and to reduce handoff delay, MOB- NBR-ADV, MOB-SCN-REQ, and MOB-SCN-RSP messages were defined [Kim et al, 2006]. MDHO and FBSS are optional handoff procedure in IEEE 802.16e. In these procedures MSS maintains diverse number of BSs. 802.16e mobility can be divide in to following parts.

• Cell selection
• HO decision initiation
• Synchronization
• Handoff ranging
• Termination of MSs context

8. Heterogeneous Mobility

Heterogeneous mobility also referred as vertical handoff, “in which mobile devices roam among different technologies or they handover between different technologies” see figure 8.1. One of the main features of next generation wireless technology will be seamless inter-system handover. Therefore, in future wireless technologies need to be deployed in way so that they can keep an interaction through network backbone to maintain connectivity all the time.

There are different technologies like WCDMA, UMTS, GPS, and GPRS and they differ to each other in terms of QoS, bandwidth, security, power consumption, radius etc.

In this chapter special focus will be given to resources management in heterogeneous network, in particular to mobility management among heterogeneous networks.

Figure 8.1: Heterogeneous network mobility [Misra A. 2002]

8.1           Background

With the emergence of next generation wireless technologies e.g. Mobile Broadband Wireless Access (MBWA), offering better data rate, security, QoS, mobility, inter-operability as compared to Fixed Wireless Access (FWA).

BFWA inter-operability with UMTS, HYPERMAN, WiFi, superior range and high scale performance is making it popular among end-users[4]. The proliferation of wireless technology is driving force for wireless connectivity, which WiMAX can fulfil by its long range, seamless mobility and QoS.

This chapter will provide in depth literature review of seamless heterogeneous mobility provided by WiMAX

8.2           UMTS and WiMAX

The Universal Mobile Telecommunication System (UMTS) is the IETF standard for the cellular networks, with more than 30 UMTS network in 25 countries [UMTS. 2007]. UMTS utilizes WCDMA radio access with basic IETF features and security features are based on GSM.

On general, UMTS and WiMAX network reference model look quite similar to each other as show in figure 8.1, with well defines network interfaces, core architecture and user equipment.

Figure 8.2: Similarities in UMTS and WiMAX Network Reference Model

CN in UMTS and CSN in WiMAX play a vital role to roam between to and fro the either network. WiMAX and UMTS also support almost the same class of QoS, so lead to map the data from one network to other. HSDPA and HSPU both enhance the uplink and downlink speed of UMTS, so it will not degrade the video or voice quality while roaming from WiMAX to UMTS [SPG Media PLC, 2007].

WiMAX network architecture with additional inter Access network make it inter-operable with UMTS and other wireless technologies [WiMAX Form, 2005]. Similarly, UMTS continuous under going standard changes has made it inter-operable with Wi-Fi and WiMAX. There are two types of coupling between networks, loose and tight, depending on network integration.

8.2.1  Loose Coupling

In case of loose coupling integration, WiMAX and 3GPP network providers enter roaming agreement and keep individual traffic separate with own mechanism of mobility, authentication and billing see figure 8.2.

Loose coupling allows to not to adopt any of the other wireless coupling technology standard in its Base Station. This approach enables minimum changes in existing infrastructure, which lead to easy integration among wireless technologies.

Figure 8.3: Loose coupling between UMTS and WiMAX [carlberg demander]

WiMAX always refer to go for loose couple [WiMAX Forum, 2006] which enable to maintain there own mechanism but extend covering radius. In case of loosely coupled network, wireless technologies to share the network features like radius, security, latency, architecture to make the right decision and time for quality handoff.

8.2.2   Tight Coupling

In tightly coupled networks, integrated wireless networks share the core components i.e. AAA, gateways and infrastructure. In UMTS and WIMAX tight coupling scenario, WiMAX connects to the GPRS Support Node (GGSN) and the packet switched domain of the UMTS core network. WiMAX or other Wi-Fi appears as a RNC to the UMTS network. This requires the user equipment to run UMTS protocol stacks [Sang et al, 2006] as shown in figure 8.3

Figure 8.4 Tight Coupling between UMTS and WiMAX/ WLAN [Sang et al, 2006]

Tight coupling has less flexibility as compare to loose coupling but allow WiMAX to tailor the wireless need like UMTS. WiMAX traffic is introduced to the UMTS network, through IP and cellular network traffic is different and network core may have to adapt to changes to avoid capacity conflicts.

8.3  WiMAX and Wi-Fi

Much like WiMAX is a MAN radio access technology, WiFi comes as WLAN radio access based on IEEE 802.11 standards. WiFI access has serious security vulnerabilities, even tough its simple network architecture makes easy to integrate with other legacy or competent wireless technologies.

In WiMAX- WiFi integration, WiMAX works as backhaul for Wi-Fi WLAN access points. WiFi/802.11f standard provides inter AP roaming but due to limited wireless radius with higher latency makes handoff impractical. WiMAX and WiFi handoff procedures resembles, need of handoff happens when MS moves from one IP-subnet to other subnet.

WiMAX layer manager handoff at higher level in layer 3, while mobile device roams between different ASNs. In 802.11, handoff is controlled by layer 2, when mobile devices moves from one access point to different IP access point. WiMAX and WiFi architecture both woks on IP based infrastructure, integration doesn’t require any translation. Like UMTS and WIMAX integration, there are two type of coupling between WiMAX and WiFi called loose and tight coupling.

Figure 8.5: WiMAX and Wi-Fi inter-operability [Premec , 2006]

8.4 IEEE 802.21/ MIH

IEEE is currently working on 802.21 stands, also referred as Media Independent Handover (MIH), a cross layer standard will work like middleware to make integration simple with 802.xx or non 802 standard/ heterogeneous network.

 

Figure 8.6: IEEE 802.21 Layering Architecture [Duttam et al, 2006]

9. Conclusion

This paper has presented literature review of handoff procedures and seamless mobility and WiMAX deployment effects on homogenous and heterogeneous mobility. Due to technical differences and QoS importance while roaming between different networks. WiMAX architecture is rich, but features rich environment provides seamless mobility to end users. It has many characteristics similar to cellular network like billing, seamless mobility and QoS and inter-operability with 802.xx and non 802 standards.

There are many proposals for network acquisition, network scanning and hand off mechanism depending on adaptive algorithm(s) for seamless mobility between homogeneous and heterogeneous mobility.

WiMAX has potential of proliferate the broadband into residential and SOHO through flexible solutions like mobility and FWA. Future direction for WiMAX and other wireless technologies will base on low latency, high QoS, inter-operability and adaptive algorithm(s)[5].

From IP point of view, the low level handover in heterogeneous network has no difference to homogeneous network. MSS need to has a binding update, when it calculates it’s new CoA, it need to done after physical handoff has done and no need to done before handoff. We will recommend following points to consider important while designing mobility management.

• Lower Signals overhead
• Low latency
• Low handoff failure
• Minimum packets loss during handoff
• Efficient network resources usage.
• Adaptive network scanning algorithm
• Heterogeneous network integration

10.      References

4^th wave Inc, 2007, ‘Fixed Wireless’, Webpage Visited 1 January 2007,

<http://www.wave-report.com/Tutorials/FixedwirelessTutorial.htm>

Bahai, A., R., Saltzberg, B., Ergen, M., 2004,’Multi-Carrier Digital Communications: Theory and Applications of Ofdm’, Springer Publisher, Pages 357-362

Becvar, Z. Zelenka, J. , Bestak, R. COMPARISON OF HANDOVERS IN UMTS AND WIMAX , http://fireworks.intranet.gr/fireworks_docspublic/Fireworks_6CTUPB005a.pdf

Boom, D., 2004, ‘Denial of Service Vulnerabilities in IEEE 802.16 Wireless Networks’,

<http://www.ieee802.org/16/tge/contrib/C80216e-04_406.pdf>

Cho,S., Kwun,J., Park, C., Cheon, J., Lee,C. and Kim, K., 2006, Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems, http://ezproxy.staffs.ac.uk:2195/iel5/11096/35443/01683032.pdf?tp=&arnumber=1683032&isnumber=35443

Duttam A., Das, S., Famolari, D., Obha, Y., Kodama, Y., Schulzrine, H., 2006, Seamless Handover across Heterogeneous network-Am IEEE 802.21 centric approach,

http://www1.cs.columbia.edu/~dutta/research/wpmc-final.pdf

Ennis, G., 1996, ‘802.11 Architecture by Greg Ennis, Symbol Technologies’, IEEE,

<http://grouper.ieee.org/groups/802/11/Tutorial/archit.pdf>

Giles, M., 2006, ‘Intel Wireless Broadband EDUCAUSE 2006’, Visited 25^th Feb, 2007,

<http://www.educause.edu/upload/presentations/E06/MTG13/Intel.%20Giles.%2010.9.06.pdf>

IEEE, 2005 IEEE Spectrum Magazine, July 2005, page – 10

IEEE, 2004, Soft Handover and Fast BS Switching Procedure, http://wirelessman.org/tge/contrib/C80216e-04_171r1.pdf

IEEE, 2006, IEEE Std 802.16e/D9. Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems – Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands. Technical report, June 2005, http://ezproxy.staffs.ac.uk:2195/iel5/10676/33683/01603394.pdf?tp=&arnumber=1603394&isnumber=33683

Johnston, d. & Walker, J., 2004, ‘Overview of IEEE 802.16 security’, <http://ezproxy.staffs.ac.uk:2195/iel5/8013/29015/01306971.pdf?tp=&arnumber=1306971&isnumber=29015

1. El Malki, ”Low Latency Handoffs in Mobile IPv4,”, Internet Draft,” Network Working Group, 2004

Kim, K., Kim, C., and Kim, T., 2004, A Seamless Handover Mechanism for IEEE 802.16e Broadband Wireless Access, http://popeye.snu.ac.kr/publication/file/05_ij_01.pdf

Li, P., Yi, X., 2006, A Seamless Handover Mechanism for IEEE 802.16e Systems, http://ezproxy.staffs.ac.uk:2195/iel5/4146258/4117911/04146293.pdf?tp=&arnumber=4146293&isnumber=4117911

Lin, P., Ngo, H. Qiao, C. Xin, W., Wang, T. & Qian, D., 2006, ‘Minimum Cost Wireless Broadband Overlay Network Planning’, IEEE Computer Society,

<http://ezproxy.staffs.ac.uk:2070/10.1145/1140000/1139410/25930228.pdf?key1=1139410&key2=8740913611&coll=portal&dl=ACM&CFID=4262948&CFTOKEN=39094370>

Misra A. 2002, “IDMP – Based Fast Handoffs and Paging in IP-Based 4G Mobile Networks”, IEEE Communications Magazine

Misra, A., Shin, M., Arbaugh, W., n.d., ‘An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process, Webpage Accessed 25^th Feb 2007 <http://www.cs.umd.edu/~waa/pubs/handoff-lat-acm.pdf>

Mohanty, S., Akyildiz, F., Fellow, Intersystem Handoff Types [A Cross-Layer (Layer 2 + 3) Handoff Management Protocol for Next-Generation Wireless Systems]

Rigel, M., 2007, WiMAX Networking Implications for IETF 16ng, http://www3.ietf.org/proceedings/07mar/slides/16ng-1.pdf

Premec, D., 2006, Siemens WiMAX CSN MM, Page 33, http://www.ikr.uni-stuttgart.de/Content/itg/fg524/Meetings/2006-09-29-Ulm/02-060925-mobile-wimax-network-arch.pdf

Prentice-Hall, 2006, Computers are your future, Viewed 23-Feb 2007, <http://www.cameron.edu/~moinian/cis1013/chap3.ppt>

Rubin, D., A., 2003, Wireless networking security, Volume 46, Issue 5, visited 24^th Feb 2007,

<http://ezproxy.staffs.ac.uk:2070/10.1145/770000/769821/p28-rubin.pdf?key1=769821&key2=9749384611&coll=portal&dl=ACM&CFID=5741514&CFTOKEN=97194095

Smith, C., Gervelis, C., 2003, Wireless Network Performance Handbook’, McGraw-Hill Professional, Pg 157-158

Smith, C. & Meyer, J., 2004, 3G Wireless with WiMAX and Wi-Fi, McGraw-Hill Professional, 177-192

Smura, T., 2004, ‘TECHNO-ECONOMIC ANALYSIS OF IEEE 802.16a-BASED FIXED WIRELESS ACCESS NETWORKS’, Department of Electrical and Communication Department

<http://www.netlab.tkk.fi/u/tsmura/publications/smura_thesis.pdf>

SPG Media PLC, 2007, HSUPA (High Speed Uplink Packet Access), http://www.mobilecomms-technology.com/projects/hsupa/

So, J., Wang, J., 2006 HIP Based Mobility Management for UMTS/WLAN Integrated Networks, http://www.ee.unimelb.edu.au/atnac2006/papers/58.pdf

Sunderam, V., S., 2005, Computational Science — ICCS 2005: ICCS 2005 : 5th International Conference, Atlanta, GA, USA

Tanenbaum, A., S., 2002, ’Computer Network’, Pg 305

UMTS, 2007, Universal Mobile Telecommunications System. UMTS Forum, http://www.umts-forum.org

WiMAX Forum, 2005, WiMAX End-to-End Network Systems Architecture. Technical report. Draft. Stage 2: Architecture Tenets, Reference Model and Reference Points. Pg 39

Wimax-Forum, 2006, ‘Mobile WiMAX – Part I: A Technical Overview and Performance Evaluation’,

<http://www.wimaxforum.org/news/downloads/Mobile_WiMAX_Part1_Overview_and_Performance.pdf>

Zahran, A., H., Liang, B., Saleh, A., 2006, ‘Signal threshold adaptation for vertical handoff in heterogeneous wireless networks’,

<http://ezproxy.staffs.ac.uk:2070/10.1145/1150000/1147588/p625-zahran.pdf?key1=1147588&key2=6416165611&coll=portal&dl=ACM&CFID=6416591&CFTOKEN=74066081>

* WiMAX maintains QoS through dynamic resource allocation protocol (DRAP) that provides admission & congestion control within the wireless domain of the network

* HIPERLAN is a European alternative for the IEEE 802.11 standards (the IEEE is an American organization) defined by the European Telecommunications Standards Institute (ETSI). In ETSI the standards are defined by the BRAN project

* two insulated wires twisted around each other – used for telephone wires

+ centre copper wire surrounded by insulation, surrounding a layer of braded wire

[2] Passive Optic Network

* Basic Rate Interface “provide a means for a residential or small business customer to access WAN”

[3] PDUs are used as indication for accessing or terminating interval for BS scanning.

[4] ZDNET research predict WLAN market to reach 5bn by 2006 and 70 min users by end of 2005

[5] The algorithms must consider user preferences, network load conditions (current and predicted), QoS and the priority of users within the heterogeneous/homogeneous network(s).