A Low Latency Handoff Algorithm for Voice over IP Traffics in the Next Generation Packet-Based...

Wireless Personal Communications 23: 353–378, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

A Low Latency Handoff Algorithm for Voice over IP Trafficsin the Next Generation Packet-Based Cellular Networks:Cellular Mobile IPv6

HAN-CHIEH CHAO 1, WEI-MING CHEN 2 and YEN-MING CHU 1

1Department of Electrical Engineering, National Dong Hwa University, Hualien, Taiwan, Republic of ChinaE-mails: [email protected] and [email protected] of Computer Science and Information Engineering, National Chung Cheng University, Chiayi,Taiwan, Republic of China

Abstract. In the mobile communication environments, Mobile IP is defined to provide users roaming everywhereand transmit information freely. It integrates communication and network systems into Internet. The Mobile IPv6concepts are similar to Mobile IP, and some new functions of IPv6 bring new features and schemes for mobilitysupport. Two major problems in mobile environments are packet loss and handoff. To solve those problems,a mobile management scheme - the cellular mobile IPv6 (CMIv6) is proposed. Our approach is based on theInternet Protocol version 6 and is compatible with the Mobile IPv6 standard. Besides, it also combines with thecellular technologies which is an inevitable architecture for the future Personal Communication Service system(PCS). In this paper, Cellular Mobile IPv6 (CMIv6), a new solution migrated from Mobile IPv6, is proposed formobile nodes moving among small wireless cells at high speed. This is important for future mobile communicationtrends. CMIv6 can solve the problems of communication break off within smaller cellular coverage during high-speed movement when packet-switched data or the real-time voice messages are transmitted. Voice over IP (VoIP)packets were chosen to verify this system. The G.723.1 Codec scheme was selected because it has better jitterresistance than GSM and G729 in a packet-based cellular network. Simulation results using OPNET show smoothand non-breaking handoffs during high-speed movement.

Keywords: IPv6, Cellular, Mobile, VoIP.

1. Introduction

The explosive growth of the wireless devices market is stimulating the research community toclone almost any technology developed for desktop computers to wireless handheld devicesconnected to a wireless network.

In the mobile communication environment, Mobile IP is defined to provide users roamingeverywhere and integrate communication and network systems into Internet. The IETF MobileIP defines three functional entities, and its mobility protocols must be implemented. There aremobile node (MN), home agent (HA) and foreign agent (FA). At the beginning, Mobile IP isbased on and is compatible with IPv4. The Mobile IPv6 concepts are similar to Mobile IP, andsome new functions of IPv6 bring new features and schemes for mobility support.

Numerous breakthroughs have occurred in telecommunications technology. From the earlyAdvanced Mobile Phone system (AMP) to the present Global System for Mobile Communi-cation (GSM) and the third generation mobile communication system, people have graduallymoved away from hard wire based communications. There are still many problems that need to

354 Han-Chieh Chao et al.

be overcome, for example, resource management, larger bandwidth, lower power consumptionand security issues [1].

The next generation mobile communication will be the so-called Mobile Internet. All com-munications and network systems will be integrated into the Internet [2]. Enough bandwidthand a good protocol that can be used in various systems are in demand.

One of technical challenges for a cellular system is the handoff procedure that occurswhen a mobile node traverses across the boundaries of neighboring cells. Using a complicatedhandoff process the handoff can be made without degrading the on going communicationquality [3].

A number of micro- or pico-mobility solutions have been discussed in the literatureincluding:

– “Optimized Smooth Handoff in Mobility IP” [4], the authors proposed further strategiesthat are compatible with route optimization and it’s a security model for Mobile IPv4.

– In “Low-Latency Handoff for Cellular Data Networks” [5], the authors examined variousmethods to allow mobile users to roam without interruption to their network communi-cations. The handoff protocol that they presented achieves latencies between 8 and 15ms with no data loss in the common case when handoffs are made between base stationsthat are topologically close to one another.

– “Fast and Scalable Wireless Handoffs in support of Mobile Internet Audio” [6], thispaper makes two main points. First, it argues that a hierarchical mobility managementscheme is necessary for latency and scalability in a world of ubiquitous portable devicesthat communicate over a large wireless network. Second, it shows that a simple handoffmechanism at the lowest level of the hierarchy can be made fast and reliable enough tosupport the stringent demands of interactive audio applications.

– In “Handoff Enhancement in Mobile IP” [7], Woo and Leung considered buffering andalso proposed a special extension to Mobile IP and to the Internet Mobile Host Protocol(IMHP). They proposed a handoff enhanced extension to the route optimized schemethat delivers optimal performance at all handoff rates.

– In “Distributing Mobility Agent Hierarchically” [8], the authors presented a distributionfor the mobility agent functionalities of foreign agents. The mobile bindings are cachedin the access network and the system protects their use with a session key protocol. Thisenables secure localized location updates with efficient signaling. When signal-basedhandoffs are used in wireless environments, the system presented can provide the handoffspeeds needed for glitch less multimedia streaming.

– “An IPv6-based Location Management Scheme” [9]. This paper introduced a scheme toaddress that issue by manipulating the inherent client server interaction, which exists inmost applications to provide the correspondent node with the current binding node.

– “IPv6 Flow Handoff In Ad Hoc Wireless Networks Using Mobility Prediction” [10].The special distinction between this research project and others is that it applies an AdHoc wireless network. They proposed a “Flow Oriented Routing Protocol” (FORP) forrouting real-time IPv6 flows in a highly mobile ad hoc flow. A new concept, “Multi-hopHandoff”, was introduced to anticipate topological changes and perform rerouting, thuslimiting the disruption of the flow due to the changing topology.

– “Mobility and QoS support for IPv6-based Real-time Wireless Internet Traffic” [11]. Themain issues in this work involved providing smooth real-time RSVP-base services andexploiting the features of IPv6 for mobile nodes.

A Low Latency Handoff Algorithm for Voice over IP Traffics 355

– “IPv6 Mobility Support for Micro-Cell networks” [12]. The preliminary goal in thispaper was to investigate the signaling loads and packet delays under different networktopologies and mobility characteristics. The authors attempted to design and implementa micro-cell mobile IP scheme without modifying or extending the existing supportprovided by IPv6.

– “A Hierarchical Mobility Management Scheme for IPv6” [13]. In this research, ahierarchical mobility management architecture for IPv6 was presented. The authors be-lieved that 69% of a user’s mobility is local and a hierarchical scheme that separatesmicro-mobility from macro-mobility is preferable.

– “Hierarchical Mobile IPv6 Mobility Management” [14]. Hierarchical mobility manage-ment reduces the performance impact of mobility by handling local migrations locallyand hiding them from home agents. In this case, a home agent knows gateway addressrather than a mobile host’s exact point of attachment. The difference between this re-search and the proposed one (CMIv6) is in the handling of MN’s identity. In CMIv6 aMN is allocated an IP address in the cell it first connects to the network and a new IPaddress in each new cell. Packets addressed to the MN use one of these IP addresses asdestination address. The FHA intercepts these packets and checks whether the MN isstill in the same cell (F flag is 0). If not, it finds out the MN’s current location based onits MAC address and forwards the packet accordingly. (This differs from HierarchicalMobile IP which allocates to MN a care-of address from FHA’s domain as well as a newlocal care-of address in each cell.) One concerns about CMIv6 is that as the MN moveson to new cells, its entries using the old IP addresses must remain in the registry. Thisresults in a continuously expanding list of registry entries in the FHA. When packetsarrive, the FHA must search this list first based on IP address or flow label, then on MACaddress. In addition, the network must make sure that different mobile nodes, visiting acell at different times, do not use the same IP address. Otherwise there could be confusionin the FHA in forwarding packets. However, CMIv6 is able to solve these problems andthe detailed explanation is presented in Section 4.

– “Cellular IP” [15]. Cellular IP, a new lightweight and robust protocol that is optimizedto support local mobility works efficiently with Mobile IP to provide wide area mobilitysupport.

The comparison between the work discussed above and the CMIv6 is shown in Table 1. Thecontributions of CMIv6 are threefold:

1. A hierarchical architecture is adopted. The top of this hierarchy is rooted at the edge ofthe access network and defined by the care-of address registered with the home agent.Upon receiving a packet, the foreign agent at the top of the hierarchy interacts with a localdatabase to determine which lower level foreign agent is located in the access network inorder to forward the packet.

2. We eliminated the usage of buffer in CMIv6 so that the delay time for re-forwardingpackets to the current location is small. The base station in our design is merely a bridgeor switch as defined in IEEE802.11. It is not a proprietary base station.

3. An IP multicasting technique is used to support fast handoff. CMIv6 does not relyentirely on multicasting. Even if this function is removed, FHA can still work well.


Table 1. Summary of the Mobile IP Proposals. (RF: Re-Forwarding, F: Forwarding, B: Buffering, M: Multicast,P: Plane network, HR: Hierarchical routing, H: Hierarchical network, O: Overlap network.)

Proposal For Special Machine Network Topology Reduce Tips BetterMobile BU HandoffIPv4/v6 Scheme

Cellular Mobile MIPv6 FHA/Cellular P(H/O) (Yes for M/F YesIPv6 Multicasting/ Overlap

Flow Label Networks)Handoff

Optimized Smooth MIPv4 FA Buffering/ Hierarchical ? B LocalHandoff [4] Smooth Handoff (not tightly handoff/

coupled) bufferhandoff

Low-Latency MIPv4 Hierarchical B/M/RFHandoff for (Overlay Network)Cellular DataNetwork [5]

Fast and Scalable IPv4 ARP proxy/ Hierarchical B YesWireless Handoffs gratuitous ARP[6] message

Handoff Enhance- MIPv4/IM B/RFment in Mobile-IP HP[7]

Distributing MIPv4 HFA/LFA/SFA Hierarchical YesMobility Agent[8]

IPv6-base Location MIPv4 EMIPv6-RF/ B ?Management [9] Redirection Agent

Border Router/Local Agent

IPv6 Flow Handoff MIPv6 Flow Oriented Ad Hoc wireless NA Flow Yes (Flow[10] Routing Protocol network SETUP Handoff,

(FORP) Multi-hophandoff)

Mobility and Qos MIPv6 Care-Taker (CT) Plane Yes RF ?Support [11]

MIPv6 for MIPv6 Hierarchical H Yes HR YesMicro-cell [12]

Hierarchical Mobility MIPv6 H Yes M YesManagement [13]

Hierarchical Mobile MIPv6 Gateway/ H Yes Gateway YesIPv6 Mobility location dataManagement [14] base with timer

Cellular IP [15] MIPv4 Paging/ Hierarchical Yes F YesPaging Cache (Semi-Routing Cache soft

Handoff)


This paper is structured as follows. From Section 2 to Section 5, the Cellular Mobile IPv6is studied. In Section 6, network modeling and simulation issues are studied. In Section 7,simulation results are analyzed.

2. Cellular Mobile IPv6

CMIv6 is proposed first time in June 1999 [16]. It is considered as a better solution thanHierarchical Mobile IPv6 protocol [14] or the flow based routing (draft-soliman-mobileip-flow-move-00.txt proposed in July 2001). Figure 1 clearly illustrated that the IETF MobileIPv6 cannot solve the breaking problem during handoff in cellular networks. For all cellularnetworks, this is a location determination and management problem. The handoff latency timeis the basic issue in this problem and it is solved using minimum delay and the least amountof packet loss. Details of the analysis can be referred from [17].

In Figure 1, the “BS-A” means the base station connecting to the interface A of the foreignrouter and “BS-B” is to interface B. MN@A means the mobile node is located at “Cell-A”.So-called “Cell-A” is the area covered by BS-A and “Cell-B” is the area covered by “BS-B”.MN@B is the mobile node at “Cell-B”.We define the environment parameters as follows:

– CoA-A: the care-of address of the mobile node in the foreign link A. Its prefix is3FFE:2F0A::/96.

– CoA-B: the care-of address of the mobile node in the foreign link B. Its prefix is3FFE:2F0B::/96.

– CN’s Address is 3FFE:2F11::0080:C883:7861.– HA’s Address is 3FFE:2F05::00A0:C978:AAE1.– MN’s MAC address is 0040:0568:F240.

By using those functions of the IETF Mobile IPv6 protocol, the correspondent node contactswith the mobile node successfully. And by routing optimization, the connecting packets aredelivered from the correspondent node to mobile node directly. This process is illustrated inFigure 1, the correspondent node sends packets to mobile node continuously. The mobile nodewants to roam from Foreign Link-A into Foreign Link-B at Time-α. The mobile node entersthe overlap region between Cell-A and Cell-B, then keeps on moving to Cell-B. At Time-β,the mobile node are out of Cell-A completely. In the other words, the mobile node can notreceive any packet or beacon signal form BS-A since Time-β. From Time-β to Time-γ , thisis the phase one of handoff breaking, “Breaking I”. This handoff latency time is not too long.The factors that effect the latency time are the speed of mobile node, the size of the overlapregion, the interval time between beacon signals and the time for the mobile node to obtaina new care-of address. The mobile node obtains a new care-of address “CoA-B”, its prefix is“3ffe:2f0b::/96”. Then it sends the binding update packets with CoA-B to its home agent andcorrespondent nodes as well.

In Figure 1, that is, the MN completes the processes of stateless autoconfiguration andsends binding update packets at Time-γ . But due to the delay and packet loss in the Internet,the CN does not receive the binding update packet until Time-δ. After receiving the bindingupdate, the correspondent node updates its Binding Cache and sends the Binding Acknowl-edgement packets, Packet-φ, with the current destination address, CoA-B, the actual locationof the mobile node. Packet-φ is the first packet arrives at BS-B and received by the MN againsent from correspondent node after receiving the Packet-ν. But the difference between Packet-


Figure 1. Timing diagram during handoff in Mobile IPv6 scheme.

φ and the previous other receiving packets is the destination address field of the Packet-φwhich is CoA-B (3FFE:2F0B::0040:0568:F240). So we can observe the phase two of handoffbreaking, “Breaking II”, since Time-γ to Time-δ. This handoff latency time is longer than thelatency time of “Breaking I”. The major effective factors for the latency time are the delay andpacket loss in the Internet. After receiving binding update. The correspondent node sends thepackets with current destination address, CoA-B, to the mobile node.

Form the above explanation; we can see that the IETF Mobile IPv6 can’t solve the breakingproblem during handoff in the cellular networks. For all cellular networks, this is the problemof location determination and management. The handoff latency time is the basic issue of theproblem.

For solving those problems, the CMIv6 algorithm is proposed in this paper. There aretwo main machines in CMIv6, the first is the Foreign Home Agent (FHA) and the second is


Figure 2. Cellular Network with FHA.

Cellular Multicasting (CM). The Foreign Home Agent, as implied by its name, is the agent inthe foreign network. The reason that we do not use the word, “Foreign Agent”, is that FHAis different from the FA in Mobile IPv4 even if their missions are forwarding packets. In ourdesign, the FHA is not a host or server. This is quite different from FA. FHA is an improvedIPv6 router. Inherently, the FHA can be an IPv6 Router or Layer 3 Switch supporting the IPv6protocol. Figure 2 shows the wireless cellular networks with FHAs. Because the FHA is aswitching router, it can be located at any the place at which a former router resides.

Figure 3 is used to explain the FHA operation in the Cellular Mobile IPv6.At Time-γ , the mobile node sends the binding update messages and the mobile node is

detected in the handoff-state by the FHA through the various ways that will be introduced inthe Section 4. After detecting the state, the FHA records it on the Location Table or RoutingTable within the FHA and acts as the packet forwarding agent. So as Packet—o sent fromCN arrives at the FHA, the FHA will forward the packet to the mobile node according to therecord of the mobile node’s current location in the tables, rather than using the destinationaddress of the packet. The details of the FHA processes for forwarding a packet are given inthe next section.

At Time-δ, the CN receives the packet, Packet-φ, then sends Packet-φ with the BindingAck. message. After the packet passes through the FHA (or the mobile node notifies the FHA),the FHA discharges the forwarding mission for the mobile node.

Judging from the above, the handoff latency time is substantially reduced by CMIv6.CMIv6 uses FHA to eliminate the Breaking II state. In other words, the key factor in thehandoff performance is within Breaking I only.


Figure 3. Timing diagram of Cellular Mobile IPv6.

To reduce “Breaking II”, we recommend that the BU packet traffic class field be set withas high priority as possible.

LOCATION-TABLE AND ROUTING TABLE

There are two key tables, the Location Table and Routing Table, within the FHA.The Location Table (LT), by its name, is a table for recording the mobile node’s locations.

It does not record the locations of all of the mobile nodes within its domain, only those in thehandoff state. This is to keep the number of entries as small as possible so that the look-up timecan be kept to a minimum. The structure of the Location Table is illustrated in Figure 4(a).


Figure 4. (a) The structure of the Location Table; (b) The structure of the Routing Table.

After the FHA receives a packet, the FHA looks for the destination address field of thepacket in the Location Table. If it matches any of the entries in Location Table, the FHAforwards the packet to the interface according to the record in the LT. If there is no entry thatmatches it, the FHA then looks it up in the Routing Table. The “Routing Table” is a generalrouting table in an IPv6 router. The operation for routing and forwarding packets in FHA issimilar to a general IPv6 router.

3. Flow Assistance Handoff (FAH) Algorithm

We have presented how the FHA solves a handoff break using Location Table. In this section,we will propose another new mechanism for solving the handoff-breaking problem using thenew field, Flow Label, in IPv6 spec.

To use this proposal, the prerequisite is the flow label in the packets which allows themobile node and the correspondent node to communicate. Packets delivered from the corre-spondent node to the MN should be set as non-zero. In other words, the flow ID should beassigned to the traffic flow from the correspondent node to the MN. The flow label is designed


Figure 5. Register with FHA.

for real time application packets. Real time packets are the most vulnerable to handoff breaks.The flow label field for IPv6 can be found in [18].

As a packet arrives at the FHA, the FHA will look it up in the Routing Table. For a generalpacket without a flow ID, the index is the Destination Address field and the Longest PrefixMatching is necessary. For a packet with a flow ID, the index is the Flow ID plus the SA fieldfor looking up in the Routing Table. FHA forwards the packet to the current interface identityif it matches any of the entries in the Routing Table. The structure of the Routing Table isshown in Figure 4(b).

The advantage of using the flow label is that a packet is looked up in only one RoutingTable, not two. This is especially helpful when there is a great number of mobile nodes in thehandoff-state in the FHA domain.

The Flow ID information is often in the cache memory, which allows the performance tobe substantially improved.

4. The Foreign Home Agent Trilogy

In the next three sections, the FHA process will be explained, including “Register with FHA”,“Work in FHA” and “De-Register with FHA”.

REGISTER WITH FHA

Figure 5 shows the mobile node registration process with a FHA. In Figure 5, three ways fora mobile node to notify the FHA are shown. Parameter “TR_FHA” is the amount of time sincea packet has arrived at the FHA. This is the time for registering the packet into the record.


“BU to FHA”To modify the current IETF Mobile IPv6, it is necessary to include this mechanism. Afterthe mobile node enters a new cell and obtains a new care-of address, it sends the BindingUpdate messages to not only its home agent and the correspondent nodes but also the FHA.The mobile node sends the BU to the FHA using the anycast address or the unicast address ofthe FHA.

“ICMP to FHA”This mechanism is similar to the “BU to FHA” mechanism. The mobile node notifies the FHAto register. The previous mechanism required the BU to send a new message to the FHA. Inthis new mechanism, the IPv6, Router Discovery function is used.

“BU pass FHA”This mechanism differs from the previous mechanisms in notifying the FHA. In this mecha-nism, the FHA will actively check all packets passing through it. A packet passing through theFHA is checked as a binding update message. The FHA checks if the message is recorded inthe table. If it is not in the table, it will be recorded. The advantage of this mechanism is thatit does not modify the current IETF Mobile IPv6 protocol. This is implemented only in theFHA and the protocol in the mobile node can be kept the same as the original IETF MobileIPv6. The main controversy about this mechanism is that it might aggravate the FHA load anddelay time. Because of the high-speed packet processing capability, the performance is notseriously influenced by the delay.

The FHA determines the handoff-state of a mobile node and whether it is recorded in thetable according to the MAC address. If it is, the FHA sets the “F” flag for the entry to “1”meaning that the node is in the handoff-state. If it is not, the FHA adds the entry as a newentry and modifies its “F” flag to “0”. Generally, the entry for the mobile node should berecorded in the Routing Table as the flow is set in the FHA.

A mobile node with the 0040:0568:F240 MAC address has been recorded in the RoutingTable, as shown in Figure 6. As the FHA receives a notice, such as in Figure 7, to register thebinding update message, the FHA records it in the Routing Table. This is entry number 102.The FHA determines the number 101 entry using the MAC address as the index. The flag “F”of the number 101 entry is set to “1”.

WORK IN FHA

After the mobile node registers with the FHA, the FHA performs the role of agent forforwarding packets to the mobile node. The work process in the FHA is indicated in Figure 8.

If the FHA adopts the LT mechanism, the FHA looks the packet up in the Location Tableafter it arrives at the FHA. If there is a matched entry in the LT, the FHA forwards the packetto the interface according to the indication in that entry. If there is not, the FHA looks thepacket up in the Routing Table.

The FAH mechanism, adopted by the FHA, is shown at the bottom of Figure 8:

(1) After a packet arrives at the FHA, the FHA determines the volume of the flow label inthe packet.

(2) If the volume is non-zero, the FHA looks the packet up in the Routing Table. If the FlowID index plus SA matches some entry, check the flag “F” of the entry.


Figure 6. The Routing Table before the register with FHA.

Figure 7. The Binding Update packet from the MN to its CN.

(3) If the flag “F” is “0”, the FHA forwards the packet to the interface that is identified bythe IF (interface port) of the entry.

If the value of the flag is “1”, the FHA finds another entry with the same MAC addressaccording the MAC address. It then goes back to step (2).


Figure 8. Work procedure in FHA.

To continue the example in the previous section, as a packet arrives at the FHA (Fig-ure 9(a)), the FHA looks it up in the Routing Table and detects its Flow ID and Source Addressand tries to match the entry number (101). The FHA then checks the flag “F” of the number101 entry. If the flag “F” is “1”, the FHA finds the entries which the MAC address field isthe same as the number 102 entry is. Then the FHA check the flag “F’ of the number 102entry and the value of he flag is “0”. The FHA forwards the packet to the interface port “B”indicated by the entry.

DE-REGISTER WITH FHA

After the CN receives the binding update message and updates its binding cache, the packetssent by the CN with the correct DA are delivered. When the first correct packet arrives atthe FHA, it will need to help the FHA to complete the de-registration process. The FHAdischarges the forwarding mission for the mobile node.

There are various mechanisms for notifying the FHA to ignore the agent role for the mobilenode.

“Timeout”In this mechanism, the entry will be invalid over a span.


Figure 9. (a) The Packet sent form the CN to MN. (b) The Binding Ack. packet from the CN to the MN.


“BA to FHA”In this mechanism, the correspondent node must send another Binding ACK message to theFHA particularly and notifying the FHA discharges the forwarding mission.

“BA pass FHA”This mechanism is similar to the “BU pass FHA” in the previous section. The BA packet isshown in Figure 9(b). A packet passes through the FHA and is checked as a Binding ACKmessage. The FHA checks if it is recorded in the table. If the Location Table is adopted in theFHA, the FHA looks it up in the LT. As the FHA determines if the entry has a match, it willbe deleted. If the FHA uses the FAH, he FHA resets the flag “F” of the entry in the RoutingTable.

“ICMPv6 to FHA from MN”The difference between this mechanism and the previous mechanisms is that the mobile nodenotifies the FHA to end the agent role. It uses the ICMPv6 message to inform the FHA.

A Binding Update packet, such as the Figure 9(b), arrives at the FHA. The FHA detects itis a BU message according to the “Option Type” field. The FHA determines the 101 and 102entry number with the same MAC address as the BU packet. The FHA delete the number 101entry due to its prefix (PF) field isn’t the same as the BU message and its flag “F” is “1”.

PARAMETERS AND ANALYSIS

In this paragraph, we will discuss some parameters and analyze the FHA role in CMIv6.– Toverlap: The time that the MN cross the overlap between two cells.

We define it as follows:

Toverlap = Size_of_the_overlap

Speed_of_the_MN

From above equation, we can determine that the factors that affect the Toverlap are theoverlap size between two cells and the speed of the MN.

– Tmove: The time that is starting from MN first receives the beacon signal from the newBS (BS-B) the first time and end with the MN last receiving the beacon signal from oldBS (BS-A) last time.

Tmove ≤ Toverlap

The interval between two beacons from the same base station is the major cause for theabove inequality. In later discussions in this paper, we will assume that Tmove is equal toToverlap.

– Tstateless: The time that the mobile node costs to complete the stateless auto-configuration.About Tstateless, we will discuss it further in the next paragraph.

– TD_Internet: The delay time in the Internet.– TD_Cell: The propagation delay between the FHA and the BS in its domain.– TD_RADIO: The propagation delay time in the radio interface between the BS and the

MN.– TDelay_BU_M_F: The delay time that starts at the MN send out the BU message and ends

at the message arrives at the FHA.

TDelay_BU_M_F = TD_Cell + TD_RADIO


Figure 10. Mobile IPv6 Handoff Overhead.

– TR_FHA: The register time in the FHA.The starting with the FHA first receives a BU message and end with finishing theregistration process.

– TB_CMIv6_FHA: The handoff breaking time with the FHA mechanism. It starts with themobile node leaving the original cell (Cell-A). It ends with the mobile node obtaininga new care-of address (CoA-B). Then it registers to the FHA and serves as the mobilenode’s agent.

– TLatency_FHA_disable: The latency time in the FHA does not enable the FHA function. Itmeans that the FHA is an ordinary router and its FHA function is disabled.

– TLatency_FHA: The latency in the FHA that enables the FHA function.– TD_BU_M_C: The delay time in the binding update message from the MN to its CN. From

the previous parameters, we make the following conclusion:

TD_BU_M_C = TDelay_BU_M_F + TR_FHA + TD_Internet

– TUBC: The time the CN costs to update its binding cache, after it receives a new bindingupdate.

– TD_Total_MIPv6: The total delay time in the IETF Mobile IPv6.

TD_Total_MIPv6 = TD_Internet + TLatency_FHA_disable + TD_Cell + TD_RADIO

– TL_TOTAL_CMIv6: The total latency time in CMIv6 mechanism.

TL_TOTAL_CMIv6 = TD_Internet + TLatency_FHA +D_Cell +TD_RADIO

TIME FOR STATELESS AUTO-CONFIGURATION

We will analyze the stateless auto-configuration of the IETF Mobile IPv6 using Figure 10.From the insertion till the card driver is loaded, the value, 310 ms, is not a measured value

for the measurement is within the kernel mode and it is difficult to obtain the results. Weassumed the value to be, 310 msec, referred from [19].


At the 510 msec, the IPv6 stack is notified. The MN will begin proceeding to the statelessauto-configuration for obtaining a new care-of address. We can see the time needed from the“IPv6 Stack Notified” to “Care-of Address Acquired” is 200 msec. At 710 msec, the MobileIPv6 demand is starting and it sends out the Binding Update message at 720 msec.

5. Cellular Multicasting

In this section, we will discuss another mechanism, Cellular Multicasting (CM).From the previous sections, we can see the handoff-break, Breaking I, still occurs even

if the FHA mechanism is enabled. The time during the handoff-break is the TB_CMIv6_FHA

defined in Figure 3. To eliminate this breaking, the FHA can forward the packets to thenew cells before the mobile node completes the stateless auto-configuration process and theregistration with the FHA.

The elimination of multicast breaking or group-based routing has been proposed by manyresearchers [5, 19]. After a mobile node enters a cellular network, it is a member of particulargroup and receives packets through multicasting or group-based routing. Which cells will theFHA forward packets to? This is a very important issue. Moreover, to which group will themobile node be assigned?

As a mobile node enters a new cell, it doe’s not process the stateless auto-configurationbut joins a multicast group using the IGMP Host Membership Report Message or MulticastListener Discovery (MLD). The multicast group ID is the MAC address of the mobile node.

As a packet from the CN arrives at the FHA, the FHA forwards it according to the multicastgroup information using Cellular Multicasting.

If every mobile node joins its cellular multicast group and requests all of their packets tobe multicasted by the FHA, the radio channels will be overloaded. The Cellular Multicastshould be only for those mobile nodes that pass through cells in a high-speed state. For this,we propose two mechanisms.

The first involves setting a counter for counting the handoff rate in the mobile node andusing the IETF Mobile IPv6, MAX_UPDATE_RATE parameter. If the value of the counter isgreater than the MAX_UPDATE_RATE, the mobile node enables the CM function and sendsthe IGMP Host Membership Report message or MLD to the FHA.

The second method involves using a certain mechanism to detect if the speed of a mo-bile node is higher than a certain rate and sustained. If so, the mobile node enables the CMfunction. Figures 11(a) and 11(b) show the operation flow for Cellular Multicasting.

6. Network Simulation

A simulation was implemented using the OPNET tool. G723.1 was adopted because it per-forms well in a wireless environment [20]. This paper only compares CMIv6 to the originalMobile IP protocol because it is very difficult to exactly set up the best simulation environmentfor each method described in section 1. The simulation parameters we were using for wirelessenvironment is listed in Table 2. Figure 12a shows the network scenario for this simulation.In Figure 12b, the topology of “Mobile network in the US” is shown. There are a FHA andfour base stations in this subnet. Every base station connects directly to the FHA. We assumedthere are 10 mobile nodes as the background traffic in a cell. The cell range is 500 meters and


Figure 11. (a) Operation flow of Cellular Multicasting. (b) Operation flow of Cellular Multicasting, contiguous.

Table 2. Simulation attribute.

Attribute Value

Antenna Pattern isotropic

Radio channel modulation bpsk

Radio channel data rate for beacon signal 1200 bps

Radio channel data rate for voice communication 64000 bps

Radio channel bandwidth 200 kHz

Radio channel min frequency 1900 Mhz

the overlap between two cells is 25 meters. The trajectory information for a mobile node is inTable 3. The altitude of the MN is set at 1.35 meters.

Table 3 shows the Mobile Node moving from position (500,450) to (898,450) at 45 km/hr.It then waits at the position (898,450) for 1 sec. The Mobile Node then moves from position(898,450) to (898,945) at 90 km/hr and waits at that position (898,945) for 1 sec. Route 3shows the Mobile Node moving from position (898,450) to (504,945) at 180 km/hr. It thenwaits at the position (504,945) for 1 sec.

The simulation parameters of the FHA are listed in Table 4.


Figure 12a. The Simulation top subnet.

Table 3. The trajectory information of the mobile node.

Route Point of Destination Altitude Ground speed

departure (meter) (km/hr)

1 (500,450) (898,450) 1.35 45

2 (898,450) (898,945) 1.35 90

3 (898,945) (504,945) 1.35 180

Table 4. Simulation parameters of FHA inCMIv6.

Processor Speed Processing Scheme

(packets per sec)

50,000 Central Processing


Fig

ure

12b.

The

mob

ile

node

inU

.S.A

.


Figure 13. Number of Packet Loss.

7. Results and Analysis

Figure 13 shows the number of lost packets. We can observe the results from various handoffscenarios.

As the MN’s speed rate is 45 km/hr (since 0th to 31.84th sec) in Mobile IPv6, the handoffdoes not affect the number of packets lost. As the MN’s speed rate is 90 Km/hr (since 32.84thto 52.64th sec) in Mobile IPv6, the handoff affects the packet loss. The handoff packet loss (atabout 43rd sec) is about 20 packets. As the MN’s speed rate is 180 km/hr in Mobile IPv6, thepacket loss rate increases to 33 packets (at about 59th sec). This means that the handoff-breakinterval is about one second. For Cellular Mobile IPv6, regardless of the MN’s speed, the FHAcan forward packets to the MN seamlessly.

Figures 14 and 15 show the End-to-End delay. Figure 15 is displayed in the sample holdformat. From Figure 15, we can find discontinuities in the Mobile IPv6. The discontinuity isnear the number 1430th and 1900th packet. According to the analysis in [9], [20], we can seethat the G.723.1 Codec sends out one packet every 30 msec. It is said that the discontinuitiesare at about 43rd and 58th sec. Referring to Figure 13, serious packet loss occurs at these twopoints.

Figure 16 shows the CPU utilization in FHA. The blue line indicates that only the MobileIPv6 protocol is enabled in FHA. The red line indicates that the Mobile IPv6 and CMIv6protocols are enabled in the FHA and there are handoff-states in its domain. The green lineindicates that the Mobile IPv6 and CMIv6 protocols are enabled but there are no handoff-states in the domain. We can see the CPU utilization as FHA takes on the responsibility forforwarding packets to the mobile nodes in the handoff-state is about the same as an IPv6router.


Figure 14. End-to-End Delay.

Figure 15. End-to-End Delay during handoff.


Fig

ure

16.

FH

AC

PU

util

izat

ion.


8. Conclusion

In this paper some of the challenges that existing and future cellular mobile systems willmeet in an environment of ubiquitous wireless Internet availability were addressed. Cellularnetworks in the new environment will face new challenges in terms of network throughputrequirements and service quality constraints. This paper described the status of the currentInternet regarding mobility and cellular networks. How the future wireless environment willlook like was forecast. The Pico-cellular network will be dominant in the future so that the IP-layer handoff will be the dominant source for handoff latency. The security issues regardingCMIv6 are introduced somewhere else [21]. More detailed explanation regarding this wholepaper can be reached at [22].

The drawback in the IETF Mobile IPv6 in global cellular networks was discussed. Thehandoff-breaks caused by the latency time for messages is the problem solved in this paper.A new protocol, Cellular Mobile IPv6, was proposed to cope with this problem. The ForeignHome Agent, a new agent-based architecture proposed in CMIv6, is careful with the bindingmessages form the mobile node in its domain and forwards the packets to the mobile nodehome agents. Cellular Multicasting, a multicast-based mechanism in CMIv6, forwards thepackets to the multi-cell before the mobile node completes the stateless auto-configurationprocess.

Acknowledgement

This paper is a partial result of project no NSC 89-2119-E-259-004 conducted by the NationalDong Hwa University under the sponsorship of National Science Council, ROC.

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18. J. Finney and A. Scott, “Implementing Mobile IPv6 for Multimedia”, in Proceedings of the GEMI-SIS/IEE/BCS Symposium on Multimedia Network Technology, Digest Number G/MNT/1/1998(e), Salford,U.K., May 1998.

19. R. Ghai and S. Singh, “Maintaining Seamless Communication between Mobile User – An Architecture andCommunication Protocol for Picocellular Networks”, IEEE Personal Communication, 3rd Quarter, 1994.

20. H.C. Chao, Y.M. Chu and T.G. Tsuei, “Codec Schemes Selection for Wireless Voice over IP (VoIP)”, LectureNotes in Computer Science, Vol. 2195, pp. 622–629, Springer, 2001.

21. K.M. Liu and H.-Ch. Chao, “Virtual Private Networks in Cellular Mobile IPv6 Architectures”, TANET 2001,Chia-Yi, Taiwan, ROC, 2001.

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Han-Chieh Chao is a full professor of the Department of Electrical Engineering and the di-rector of the University Computer Center at National Dong Hwa University, Hualien, Taiwan,R.O.C. His research interests include High Speed Networks, Wireless Networks and Ipv6based Networks. Dr. Chao received his MS and Ph.D. degrees in electrical engineering fromPurdue University in 1989 and 1993 respectively. Dr. Chao has published about 70 journal andconference papers and participated in many international academic activities. Dr. Chao hasreceived many research awards, including Purdue University SRC awards, and NSC researchawards (National Science Council of Taiwan). He also received many funded research grantsfrom NSC, Ministry of Education (MOE), Industrial Technology of Research Institute, andFarEasTone Telecommunications Lab. Dr. Chao has been invited frequently to give talks atnational and international conferences and research organizations. Dr. Chao is also serving asthe executive editor of the Journal of Internet Technology.


Wei-Ming Chen serves as a technician and system division chief in University ComputerCenter of National Dong Hwa University. He is responsible for all the programming jobs oncampus. He holds a M.S.E.E. degree from National Chung Cheng University, and is currentlypursuing his Ph.D. degree at the same institute.

Yen-Ming Chu currently serves as a associate researcher in Telecommunication Labs ofChanghwa Telecom Co. Ltd., Taiwan, where he is responsible for research and developmentOSS of wireless communication. His research interests include mobile networks, IP routing,network information processing, and optical networks. He holds a B.S.E.E. degree from Fun-Jen University, Taiwan, and an M.S.E.E. degree from National Dong-Hwa University, Taiwan,in June 2000.

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