Reduction of Handover Latency Using MIH Services in MIPv6 Yoon Young An1, Byung Ho Yae1, Kang Won Lee2, You Ze Cho2, and Woo Young Jung3 1 ETRI, Daejeon, Korea {yyahn, bhyae}@etri.re.kr 2 Kyungpook National University, Daegu, Korea
[email protected] 3 DGIST, Daegu, Korea
[email protected]
Abstract In this paper, we propose an enhanced handover mechanism with new additional primitives and parameters to the media independent handover (MIH) services defined in the IEEE 802.21. The proposed scheme can reduce handover latency for mobile IPv6 (MIPv6) by removing the router discovery time. Moreover, when the proposed mechanism is applied to the FMIPv6, we can increase the probability that the FMIPv6 can be performed in predictive mode by reducing the handover initiation time, thereby we can reduce the expected handover latency in the FMIPv6. In addition, with the proposed scheme, we can design the network cost-effectively by reducing coverage overlap between adjacent cells because the handover initiation time in the FMIPv6 is decreased.
1. Introduction Mobile communication has become more popular due to the increased availability of portable devices and advanced wireless technology. Moreover, the core network of heterogeneous wireless access networks is evolving into all-IP based network. Accordingly, Mobile IPv6 (MIPv6) has become a global solution to support mobility in the Internet between various access networks [1][2]. But the long handover latency in the MIPv6 degrades the perceived quality of service (QoS) especially in real-time services. To reduce the handover latency in the MIPv6, fast handovers for the MIPv6 (FMIPv6) has been proposed in the IETF. The FMIPv6 reduces packet loss by providing fast IP connectivity as soon as a new link is established. It does so by fixing up the routing during L2 handover and binding update, so that packets delivered to the old care of address (CoA) are
forwarded to the new. In addition, the FMIPv6 performs the care of address configuration by using the link information (such as the subnet prefix) of new access router (nAR) while the mobile node (MN) is still attached to the old access router (oAR). The FMIPv6 can reduce the amount of CoA configuration time in the new subnet. However, the FMIPv6 only concentrates on the protocol operation while it does not consider other issues, such as radio access discovery and candidate access router discovery, which are critical to the handover performance of the FMIPv6. Moreover, during handover initiation, the MN could lose its connectivity to the oAR due to a sudden disruption of the link. In this case, MN performs a normal handover procedure in the MIPv6 or the reactive FMIPv6, resulting in a long handover latency [3]. IEEE 802 is developing standards to enable handover and interoperability between heterogeneous networks. IEEE 802.21 specification, media independent handover (MIH), defines the method to provide the link layer intelligence and other related network information to the upper layer to optimize handovers. But, the existing MIH primitives limit to optimize the handover latency. And there hardly improve the performance of the FMIPv6 because these only suggest the method of link layer trigger. In this paper, to reduce handover latency in the MIPv6 and to increase the probability that the FMIPv6 can be performed in predictive mode, we propose an enhanced handover mechanism with new primitives and parameters to MIH services in the IEEE 802.21. Using the newly defined MIH primitives and parameters, we can improve the handover performance in the MIPv6 and the FMIPv6. This paper is organized as follows. Following this introduction, in Section 2, we describe the basic
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operation of handover and components of the handover latency in the MIPv6/FMIPv6. In Section 3, we overview the MIH services defined in the IEEE 802.21. In Section 4, we propose a new fast handover mechanism using the MIH services with new primitives and parameters. Then, we evaluate the performance of our mechanism, compared with the MIPv6 and the FMIPv6. Additionally, we investigate it is analyzed that the relationship among the handover initiation time, cell coverage overlap, and network capacity. Finally, in Section 5, we present concluding remarks.
2. Related Work
the configured addresses must be verified on the new link according to the DAD operation. Once the MN has detected that it has moved to a new network, it obtained a new CoA that and has been granted during the access to the new network. Then MN must perform the Binding Update operation by informing its Home Agents (HAs) and Correspondent Nodes (CNs) of its new location, thus new CoA. The total handover latency in the MIPv6 can be expressed as a sum of L2 handover latency and L3 handover latency. L2 handover latency is about 100 to 300 ms. And L3 handover is about 2,000 to 3,000 ms. This handover latency is so long that MN suffer from packet loss and service disruption.
2.1. Handover Latency in MIPv6
2.2. Fast Handovers for the MIPv6
The MIPv6 supports handover that changes its point of attachment to the network when a MN moves to a new IP subnet. The basic handover procedure for the MIP consists of two components, L2 handover and L3 handover. The term L2 handover denotes its support for roaming at the link layer level, while the L3 handover occurs at the network layer level. Usually, the L3 handover is independent of the L2 handover, although it must precede the L3 handover. In Fig. 1, the MIPv6 handover procedure is illustrated. The MIPv6 consists of three operations. These operations may overlap each another. Movement detection which includes L2 handover is a prerequisite procedure for other handover operation. L2 handover that must precede the L3 handover performs channel scanning, authentication, association. After L2 handover, MN can detect movement to a new IP subnet by the operation of movement detection. In the base MIPv6 specification, during the movement detection, MN performs the unreachability detection and then is accomplished through finding a new and different AR available. Router discovery is achieved through the reception of a router advertisement (RA) sent from the new AR (nAR). The RA message contains the information of a router such as its prefix, link layer address (MAC), MTU, and so on. The MN must configure a new IPv6 address to be used on the new network in CoA configuration procedure. This will be the MN’s new CoA (nCoA). The uniqueness of
The FMIPv6 addresses the following problem of handover latency reduction: how to allow the MN to send packets as soon as it detects a new subnet link, and how to deliver packets to a mobile node as soon as its attachment is detected by the new access router [4][5]. In the FMIPv6, the MN is informed of new AR’s advertised prefix and validates the duplication of nCoA on the new link prior to the MN’s movement. So, the MN is already configured with nCoA before it is attached to the new link. In other words, the FMIPv6 is designed to eliminate the delays associated with movement detection and CoA testing, the time introduced by the CoA configuration procedure. The FMIPv6 is designed to allow MN to anticipate in its IP layer mobility. Link layer triggers are required for anticipation and handover initiation. They are delivered to network layer modules as events for reporting changes in respect to the link and physical layer conditions. In Table 1, link layer triggers for the FMIPv6 are described. Fig. 2 depicts the predictive FMIPv6 procedure utilizing the link layer triggers in Table 1. However, the FMIPv6 protocol has several problems such as the followings: ˍ AR may have the protocol to exchange information about their neighbors for constructing the mapping table between AP’s MAC addresses and their corresponding AR. But it is not defined in the IETF. ˍ Link layer triggers should be specified by the standards organizations such as the IETF and IEEE. ˍ During the handover initiation, the MN could lose its connectivity to the oAR due to a sudden degradation in the link. The MN processes normal handover in the MIPv6 or the reactive FMIPv6. Therefore, the handover latency increases.
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Fig. 1. The MIPv6 handover procedure.
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Table 1. Link layer triggers for the FMIPv6. Primitive
Description This trigger may be used to get a list of available link by AP scan. This trigger specifies that a new available link is detected A link down event will be fired in the near future, so the network layer must initiate the handover procedure This indicates that the link cannot be used for data transmission any more This is provided to L3 when a new link is connected
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3. Media Independent Handover Function Recently, IEEE 802 is developing standards to enable handover and interoperability between heterogeneous networks. IEEE 802.21 specification, MIH, defines method to provide the link layer intelligence and other related network information to the upper layers to optimize handovers between heterogeneous media [6]. In the FMIPv6, the network layer must detect the indication of a handover from the link layer in advance to achieve seamless handovers. So, we need to introduce the MIH services for link layer trigger in the FMIPv6. MIH defines asynchronous and synchronous services that enhance handover between heterogeneous access links and service access points (SAPs) that is an interface between lower layers and upper layers. In the case of a system with multiple network interfaces of arbitrary type, the MIPv6 can use the event service, command service, and information service provided by MIH to manage, determine, and control the state of the underlying interfaces. These services provided by MIH help the MIPv6 and other protocols.
2.3. Handover Latency in the FMIPv6
3.1. Information Service
In this session, we compare the handover latency of the MIPv6 with the FMIPv6 as shown in Fig. 3. The total handover latency for the MIPv6 can be expressed as sum of delay for L2 handover, delay for DAD, delay for router discovery and delay for binding update procedure. Here, the delay for the new CoA creation can be neglected. The FMIPv6 has the handover initiation time to perform the CoA configuration prior to the L2 handover. After the L2 handover, the MN transmits an F-NA to inform the nAR of its presence and performs the procedure of binding update. Therefore, the FMIPv6 can reduce the total handover latency by preconfiguring the CoA in the handover initiation phase.
The information service provides a framework and corresponding mechanisms by which a MIH function entity can discover and obtain network information existing within a geographical area to facilitate the handovers. The information service primarily provides a query/response type of mechanism for information transfer. The information may be stored within the MIH layer or maybe presented to some information server from where the MIH layer can access. The information service provides access to static information such as neighbor maps, helping in network discovery. Also, the service may provide access to dynamic information which may optimize link layer connectivity with different networks. This could include link layer parameters such as channel information, MAC addresses, security information, etc.
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Fig. 3. Handover Latency in the MIPv6 and the FMIPv6.
The event service is used to facilitate handover detection within the MIP. Events inform the condition of the present network and transmission behavior of the L2 data links, such as MAC, Radio resource management, etc. The MIP makes a registration to receive events from MIH layer using a request/confirm primitives.
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The defined events includes Pre-trigger (L2 Handoff Imminent), Link Available, Link Up, Link Parameters Change, Link Going Up, Link Down, Link Going Down, L2SDU Transmission Status, Event Rollback, etc.
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The command service refers to the commands sent from the higher layers to the lower layers. It includes the commands from upper layer to MIH (e.g. upper layer mobility protocol to MIH, or policy engine to MIH, etc), and from MIH to lower layer (e.g. MIH to MAC, or MIH to PHY). These commands mainly carry the upper layer decisions to the lower layer, and control the behavior of lower layer entities.
4. The proposed Handover Mechanism Generally, the IP layer in the FMIPv6 needs the L2 trigger to perform handover initiation for nCoA configuration before L2 handover. In this case, MIH services are very useful for link layer triggers. But, with the existing MIH primitives, the handover performance of the MIPv6 is very limited because MIH service only is used for detecting L2 layer information. Especially, it hardly improves the performance of the FMIPv6. In this session, we propose an enhanced handover mechanism with new additional primitives and parameters to the MIH services.
4.1. Proposed Handover Scheme The proposed handover scheme defines new MIH primitives and parameters. As shown in Table 2, the new primitive “MIH-PrefixInfo” contains the prefix information of AR. The L3 module of AR sends “MIH-PrefixInfo” to link layer to inform the subnet information when a new AP is attached or the prefix is exchanged. Therefore, the link-layer of AR and AP Table 2. A new primitive and new parameters for the proposed mechanism. Primitives MIH-PrefixInfo
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Parameters Interface ID, Prefix Interface ID, Prefix, MAC Addr, BW, Quality Level Interface ID, Prefix, MAC Addr, BW, Quality Level Interface ID, MAC Addr, BW, Quality Level Interface ID, MAC Addr Interface ID, MAC Addr
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can obtain the prefix information of its AR. L3 module in MN gathers the information of neighbor APs using “MIH-LinkList” and “MIH-LinkAvailable”. In this paper, we suggest that “prefix” parameter is added to these primitives and thereby the MN can get the prefix information of the new AP. Fig. 4 shows the proposed mechanism using the new primitives and parameters defined in Table 2. In the proposed mechanism, MN can make the mapping table between AP’s MAC and AR’s prefix, as it receives both AP and AR information from “MIHLinkList” and “MIH-LinkAvailable” primitive with AR’s prefix parameter. In the original FMIPv6, ARs exchange the information about their neighbors and have to reconstruct the mapping table for proxy advertisements with information on the neighboring subnets. In the proposed mechanism, since MN has the mapping table regarding new primitive and parameter in advance, AR need not exchange information about their neighbor. Also, MN need not exchange RtSolPr/PrRtAdv messages or RS/RA messages for Router Discovery, because the router information is already contained in MIH primitives. So, the CoA configuration procedure that is related with router discovery or RtSolPr/PrRtAdv can be decreased. Namely, with the proposed handover scheme, the MIPv6 can reduce the handover latency. And the FMIPv6 can reduce the handover initiation by reducing the Router Discovery of handover procedure.
4.2. Performance Evaluation Table 3 shows the handover latency and the handover initiation time for the MIPv6 and the FMIPv6 with/without the proposed scheme. In the MIPv6, the proposed mechanism can remove DRD, the time for router discovery, from the handover latency, handover delay after L2 handover. In case of the
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FMIPv6, the handover initiation time is reduced by removing the time for Proxy Router Discovery, DPrRD. Table 3. Handover latency and handover initiation time for the MIPv6/FMIPv6 with/without the proposed scheme. Handover Mechanism
Handover Latency
Handover Initiation Time
MIPv6
DL2 + DRD + DDAD + DBU
Proposed MIPv6
DL2 + DDAD + DBU
FMIPv6
DL2 + 2DMN-nAR + DBU
DPrRD + DFMIP
Proposed FMIPv6
DL2 + 2DMN-nAR + DBU
DFMIP
The handover latency shown in Table 3 can be expressed as follows: DHO-MIPv6 = DL2+W+G+3RTTMN-nAR+RTTnAR-HA DHO-ProMIPv6 = DL2+W+2RTTMN-nAR+RTTnAR-HA DHO-FMIPv6 = DL2+2RTTMN-nAR+RTTnAR-HA DHO-ProFMIPv6 = DL2+2RTTMN-nAR+RTTnAR-HA where DL2 is the L2 handover latency, W is the time for DAD, G is DRD for periodic RA, RTTMN-nAR is the round trip time between MN and nAR, and RTTnAR-HA is the round trip time between nAR and HA. 3.5 MIPv6 Proposed MIPv6 FMIPv6 Proposed FMIPv6
Handover Latency (sec)
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As shown in Fig. 6, coverage overlap is required for MN to perform handover initiation and link layer handover in the FMIPv6. If the coverage overlap is too large, the coverage area is reduced, resulting in capacity reduction of coverage. On the other hand, if it is too small, the MN does not have enough time for the handover initiation which the MN predicts its movement between coverages. It results in handover latency reduction [7]. We can express the handover initiation time as follows: =DPrRD + DFMIP =2RTTMN-oAR+(2RTTMN-oAR+2RTToAR-nAR) =4RTTMN-oAR +2ǻ TProFMIPv6=2RTTMN-oAR +2ǻ
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Fig. 5. Comparison of handover latency the MIPv6/FMIPv6 with/without the proposed scheme. Fig. 5 compares the handover latency among the MIPv6, the FMIPv6, and the proposed mechanism. We assume that W is 1000 ms for RetransTimer (default 1000ms) and DupAddrDetectTransmits (default 1000ms), G is 500ms, rtAdvInterval/2, and RTTnAR-HA are 10ms. In case of the MIPv6, the MIPv6 with the proposed mechanism can obtain better performance than that of the original MIPv6. In case of the FMIPv6,
Fig. 7 shows the coverage overlap length according to the moving speed of MN for the FMIPv6 and the proposed mechanism. Here, we can see that the handover initiation time has decreased and the size of the coverage overlap area has decreased. Thus, the whole coverage capacity has increased. Consequently, the proposed mechanism is cost-effective for network design. In other hand, the proposed mechanism increases the probability that the FMIPv6 is performed in predictive mode.
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12 FMIPv6 Proposed FMIPv6
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Fig. 7. Relationship between the coverage overlap length for handover initiation time and the moving speed of MN.
[2] Douglas Howie, “Consequences of using MIPv6 to achieve mobile ubiquitous multimedia,” MUM, pp.34-39, Dec. 2002. [3] Yoon-Young An, Chang-Min Park and Sung Back Hong, “The Method for Reducing Packet Loss in Handover Service of Mobile IPv6,” AIC 29th Conference, Dec. 2003. [4] R. Koodli(Ed.), “Fast Handover for Mobile IPv6,” IETF RFC 4068, Jul. 2005. [5] Lila Dimopoulou, Georgios Leoleis, and Iakovos S. Venieris, “Fast Handover Support in a WLAN Environment: Challenges and Perspectives,” IEEE Networks, pp.14-20, May/Jun. 2005. [6] Vivek Gupta, “IEEE P802.21 Media Independent Handover Services Joint Harmonized Contribution,” Draft 802.21 21-05-0240-00-0000, Mar. 2005. [7] Hesham Soliman, “Mobile IPv6 : Mobility in a Wireless Internet,” Addison-Wesley, 2004.
5. Conclusion In this paper, we proposed an enhanced handover mechanism with improved performance, with MIH services defined in IEEE 802.21. To do so, we defined a new primitive, “MIH-PrefixInfo”, and added a parameter ‘prefix’ to the existing MIH primitives. Using MIH services with the new primitive, MN can obtain the nAR’s information without router discovery or RtSolPr/PrRtAdv messages. Therefore, the proposed scheme can reduce the handover latency in the MIPv6 by removing the router discovery time. Moreover, when the proposed mechanism is applied to the FMIPv6, we can increase the probability that the FMIPv6 can be performed in predictive mode by reducing the handover initiation time, thereby we can reduce the expected handover latency in the FMIPv6. In addition, with the proposed scheme, we can design the network cost-effectively by reducing coverage overlap between adjacent cells because the handover initiation time in the FMIPv6 is decreased.
Acknowledgements This work was supported in part by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) program, the DGIST, and the BK21 project.
References [1] Jian Ma and Zhigang Kan, “Role of Mobile IPv6 for mobile networks and its remaining issues,” APOC, pp.119-133, Oct. 2001.
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