An Efficient DECT-Mobile IP Interworking for Mobile Computing Anthony Lo *, Winston Seah * and Edwin Schreuder + * Centre for Wireless Communications 1, National University of Singapore, 20 Science Park Road, #02-34/37 TeleTech Park, Singapore Science Park II, Singapore 117674 Email: [cwcloa, winston]@leonis.nus.edu.sg + Ericsson Business Mobile Networks, Institutenweg 25, P. O. Box 645, 7500 AP Enschede, The Netherlands Email: Edwin.Schreuder@emn.ericsson.se Abstract Digital Enhanced Cordless Telecommunications (DECT) [1] is a general radio access technology, which supports local-area mobility only. The interconnection of DECT to Internet is inadequate to support wide-area mobility because IP [2] does not support host mobility between different subnetworks (subnets). This paper describes the interworking of DECT with Mobile IP [3] to support subnet-level mobility, while DECT is employed for user mobility within a subnet. To achieve an efficient interworking, we exploit and take advantage of the mobility features of DECT, which offers faster convergence and far fewer overheads. Hence, the Mobile IP handover latency can be reduced to improve the performance of transport protocols (e.g., TCP). We define the necessary protocol signalling for the DECT-Mobile IP interworking mobility management procedures, which require no modification to both DECT and Mobile IP. I. Introduction Digital Enhanced Cordless Telecommunications (DECT) [1] is a second generation, pico-cellular system standardized by the European Standards Institute (ETSI). DECT is a generic radio access technology for short range wireless communications so that it can be interfaced to a large number of local and public networks such as Internet, PSTN, X.25, etc. As such, DECT does not support wide-area mobility due to a lack of a dedicated backbone network, in contrast to, for example GSM. Thus, user mobility is limited to within a home DECT network, e.g., handover between base stations. Once the user roams to a visited DECT network, the user is unable to maintain on-going connections or receive incoming calls. Hence, DECT requires an external infrastructure with wide-area mobility management features. DECT can be interfaced to the Internet to provide seamless wireless access. The interconnection of DECT to Internet is, however, inadequate to support wide-area mobility because the Internet is based on the Internet Protocol (IP) [2], which does not facilitate host mobility between different subnetworks. In this paper, we investigate the use of Mobile IP to facilitate wide-area mobility across cooperating DECT networks. Mobile IP was used because it is a standard developed by the Internet Engineering Task Force (IETF) to support mobile computing. In order to obtain an efficient interworking, we exploit the mobility management features of both DECT and Mobile IP. In addition, we look at how to optimize and minimize the number of Mobile IP signalling messages sent over the radio link to cope with scarce radio resources by taking advantage of DECT mobility features. Also, the interworking should not modify both DECT and Mobile IP. The remainder of the paper is organized as follows. Section II describes the interconnection architecture of DECT and the Internet. Section III discusses and compares the mobility management features of both DECT and Mobile IP, and takes advantage of the mobility features of DECT. Finally, Section IV concludes the paper. II. DECT-Internet Interworking Architecture and Protocols Figure 1 depicts the interworking configuration between DECT and Internet. Each of the DECT networks constitutes a wireless Local Area Network (LAN) segment, which is connected to an existing wired LAN (such as the Ethernet) [4]. The DECT network consists of a Fixed Part (FP) and one or several Portable Parts (s). The FP in turn encompasses three functional entities: an Interworking Unit (IWU), a Common Control Fixed Part (), and Radio Fixed Parts (s). The is a base transceiver station, which covers one cell in the pico-cellular system. The cell radius is typically ranging from 50m to 300m. The supports the Physical () and Medium Access Control () layers, and functions as a data link relay. It retrieves Data Link Control () frames and relays them between the and the. The is a central system, which handles the and Network (NWK) layers for one or a cluster of s. It interoperates with the via a conventional data networking infrastructure for carriage of the frames. The is the peer entity to the data link; the and the operate the end-points of the data connection. The is connected to the Internet via an IWU. The service area of a mobility agent (i.e., a Home Agent or a Foreign Agent) is called a subnetwork (or subnet). Thus, the DECT wireless LAN is a subnet. Since DECT 1 CWC is a national R & D Centre funded by the Singapore National Science and Technology Board.
Picocell Subnet 1 IWU HA/FA DECT FP-1 Picocell Internet Remote Host IWU HA/FA DECT FP-2 Subnet 2 Figure 1: DECT-Internet Interworking Internal Handover External Handover Application TCP Mobile IP Mobile IP Application TCP IP Relay Internet + IWU + HA/FA Remote Host Figure 2: DECT Protocol Architecture U-plane is based on micro-cellular concept, the subnet is further partitioned into one or more cells. Each cell is covered by an. The is identified by two addresses: a home IP address and a DECT International Portable User Identity (IPUI). The binding between the two addresses is static and location independent. The IP address is used for routing purposes in the Internet, while the IPUI is used by to identify itself to the DECT FP. The DECT protocol architecture is structured into two planes: Control plane (C-plane) and User plane (Uplane). Figure 2 depicts the architecture of the U-Plane, which comprises the, and layers. The U-plane is used to transfer user data (e.g., IP packets). The IWU functionality is network specific and is not defined in the DECT standards. Hence, the IWU can be realized as a sublayer rather than as a separate physical entity. We adopted the OSI terminology by calling this sublayer a Dependent Convergence NWK Relay NWK Figure 3: DECT Protocol Architecture C-plane
Function () [5], which is used to bridge the gap between the requirements of Mobile IP and the services provided by the underlying DECT protocols. The mobility agent, i.e., a Home Agent (HA) or a Foreign Agent (FA) can also be integrated into to form a single entity as illustrated in Figure 2. The protocols associated with the C-Plane protocol, see Figure 3, are for transfer of signalling information for control of user services. The functionality of, and is common to both planes. The NWK layer is the main signalling layer of the protocol. The basic Call Control service of the NWK layer contains all the functions necessary to establish, maintain, and terminate connections. Once the connection has been established, the U-Plane is ready to transmit and receive IP packets. The NWK layer also provides control functions to support additional services such as Mobility Management (MM). III. Mobility Management in DECT- Mobile IP Interworking In this section, we will discuss two mobility management procedures, location management and handover of DECT and Mobile IP. A. Location Management When a mobile node is not engaged in a communication context, the network must be able to determine its current location in order to set-up and route properly an incoming connection. Location management is concerned with the issues of tracking and finding the mobile hosts in order to allow roaming within the network [6]. DECT Location Management In DECT, four location management operations are defined: location registration, location update, detach and paging. Location registration is used by to inform FP about its current location in terms of Location Areas (LAs). In our configuration, an LA is overlaid on to a subnet. To enable the to detect a change of LA, each keeps broadcasting, on a periodic basis, the cell identity; that is Radio Fixed Part Identity (I) on the DECT Medium Access Control N-channel. The I consists of a Primary Access Right Identity (PARI) field and a Radio Fixed Part Number (RPN) field. Within the same LA, all cells have a similar PARI and a unique RPN. The powered-on listens to the I being broadcast and compares it to the one stored in the cache. If the PARI field of the two Is is different, then the interprets this as a change of LA, and the location registration is invoked. Location update is used by the FP to inform the of a modification of the location area. Detach refers to the process whereby a informs the FP that it is not ready to receive incoming connections. Paging refers to that procedure whereby the network searches for the exact through which a can be reached. This is done by sending paging messages through all the candidate s. If paging is successful, then the will send a paging response to the through which the -terminated connection can be set-up. Mobile IP Location Management Mobile IP location management procedure consists of only one operation, registration, which is basically similar to the location registration of DECT. That is, the informs the HA of its current care-of address once it has moved to another subnet. FAs transmit periodic agent advertisement messages, which allow a to identify a change in subnet. The can use two movement detection methods. First, the can record the lifetime field of the agent advertisement message. When the lifetime expires, the must perform location registration using a valid advertisement message that it received from a new FA. Alternatively, the compares the subnetwork prefix in the agent advertisement message with the subnetwork prefix of its care-of address. If the two prefixes are different, the can assume it has moved and initiates the registration process. DECT-Mobile IP Location Management From the above, it is clear that both DECT and Mobile IP provide a similar mechanism for movement detection. When DECT is interworked with Mobile IP, the movement detection of DECT is utilized because it offers faster convergence than Mobile IP. Moreover, Mobile IP agent advertisement messages are relatively large and it is rather inefficient to broadcast them periodically because the wireless link has limited bandwidth. The sublayer of the can be used to intercept and store the latest agent advertisement message broadcast by the mobility agent. In this way, no modification to the Mobile IP is necessary as the same Mobile IP maybe serving other network segments (e.g., Ethernet), which do not have any move detection capability. The will forward a copy of the stored agent advertisement message to any, which has successfully completed the DECT location registration process due to a change of LA (subnet). The completion of the location registration is easily perceived by the because the responsibility of the is to accept or reject the requested location registration by the. Figure 4 illustrates the signalling flow for the DECT- Mobile IP interwork location registration over time. When the determines that the PARI has changed, it establishes a physical connection (events 1 2) with the new. The location registration process is started by sending a Locate Request message (event 3) by the. The replies with a response location registration service primitive indicating accept. The in turn sends the corresponding location registration accept message, Locate Accept (event 4) to the. Then, the sends an Agent Advertisement message to the (event 5). Upon receiving the agent advertisement, the sends a Mobile IP Registration Request message to the FA via the (event 6). In
+FA HA 1. Bearer Request 2. Bearer Confirm 3. Locate Request 4. Locate Accept 5. Agent Advertisement 6. Registration Request 9. Registration Reply Mobile IP Registration 7. Registration Request 8. Registration Reply Time 10. Release 11. Release event 7, the FA then determines the s HA and relays the Registration Request message to the HA, which performs a set of validity checks on the message. If the Registration Request is valid then the HA updates the s binding entry. If the has just moved from its home subnet, the HA sends a Gratuitous ARP (Address Resolution Protocol) associating HA s hardware address with the s IP address, so that IP packets destined for the maybe intercepted by the HA. The HA sends a Mobile IP Registration Reply to the FA, which in turn forwards to the via the (events 8 9). Finally, the physical connection is released in events 10 and 11. The interaction between the and the FA can further be optimized to utilize the limited bandwidth efficiently. In the case of re-registration (where the current registration is due to expire), it is not necessary to send the complete registration request and reply messages over the wireless link. The sublayer can be used to examine and transmit only parameters that are different from the previous registration; the peer then reconstructs the complete registration messages. B. Handover Figure 4: DECT-Mobile IP Location Registration Signalling Flow Handover is the mechanism that automatically transfers a connection in progress with minimal interruption from one cell to another to avoid adverse effects of user mobility. In DECT systems, handover is initiated by. Two different types of handover are defined: internal handover and external handover. Internal handover can involve a change of physical channels (frequency or timeslot) within a cell during a call or a change of s belonging to the same FP (or subnet), i.e., only changes in RPN. On the other hand, external handover refers to the movement of from a cell in FP-1 to a different cell in FP-2, i.e., changes in PARI. Internal handover is completely handled by the DECT protocols. External handover is more complicated because it involves two independent FPs, where each FP is located in a different subnet, see Figure 1. In addition, DECT does not handle switching of IP packets from FP-1 to FP-2. Thus, to make external handover possible, a network layer protocol signalling, i.e., Mobile IP can be employed to re-route IP packets. Figure 5 depicts the steps involved in the procedure. Upon detection of a better wireless link and a change of PARI, the establishes a physical connection (events 1 2) with a target. Note that the old link is remained active while the new link is being established to achieve seamless handover. In event 3, the requests a handover to a target by sending a NWK layer message, CC-Setup, indicating a handover. Upon receipt of this message, as shown in event 4, the target replies with a CC-Connect message to the, to indicate confirmation of the external handover. If the connection to the serving was ciphered, then the connection to target shall also be ciphered. The ciphering procedure (event 5) is initiated by the soon after the receipt of the CC-Connect message. The then acknowledges the external handover confirmation by replying with a CC-Connect-Ack message, which is shown in event 6. The receipt of this message will trigger the of the target to send the Agent Advertisement message to (event 7). Then the initiates the Mobile IP registration process (events 8 11). If the registration is successful, the HA begins tunnelling packets to the new care-of address. Also, the releases the old link and associated NWK resources (events 12 15) with the serving. Once the old
Serving Serving +FA Target Target +FA HA IP Packet Tunnelled IP Packet 1. Bearer Request 2. Bearer Confirm 3. CC-Setup 4. CC-Connect 5. Layer 2 Ciphering 6. CC-Connect-Ack 7. Agent Advertisement 8. Registration Request Mobile IP Registration 9. Registration Request 10. Registration Reply 11. Registration Reply Time IP Packet Tunnelled IP Packet 12. CC-Release 13. CC-Release-COM 14. Release 15. Release connection has been terminated, any packets in transit to the s previous FA can be forwarded to the target FA using the Mobile IP Route Optimization [7], which provides a means for the previous FA to be notified of the s new mobility binding. IV. Summary Figure 5: External Handover Protocol Signalling Flow DECT can be interfaced to the Internet to provide seamless wireless access. The DECT network, however, supports limited user mobility (i.e., mobility within its subnetwork). In this paper, we investigated the utilization of Mobile IP to support wide-area mobility across cooperating DECT subnetworks, which are connected to the Internet. The interworking between DECT and Mobile IP is defined in such a way that neither DECT nor Mobile IP need to be modified. In order to achieve an efficient interworking arrangement, the mobility management features of DECT and Mobile IP were exploited. We identified that both DECT and Mobile IP provide a similar movement detection mechanism. The former is utilized because it offers faster convergence and fewer overheads, which reduces Mobile IP handover latency; hence, it improves the performance of transport protocols (e.g., TCP). We discussed how to optimize Mobile IP control messages to efficiently utilize the bandwidth-limited wireless links. The DECT-Mobile IP interwork protocol signalling for location registration and external handover was defined. V. References 1. ETSI: Digital Enhanced Cordless Telecommunications (DECT); Common Interface (CI); Part 1 to 9, ETS 300 175-1 to ETS 300 175-9, 1996. 2. W. Steven, TCP/IP Illustrated, Vol. 1, Addison Wesley, Reading, MA, 1994. 3. C. E. Perkins, Mobile IP: Design Principles and Practices, Addison-Wesley, Reading, MA, 1998. 4. A. Lo, Wireless LANs: The DECT Approach, Asia-Pacific Conference on Communications (APCC), Sydney, Australia, 1997. 5. B. N. Jain and A. K. Agrawala, Open Systems Interconnection, McGraw-Hill Inc., 1993. 6. N. E. Kruijt, D. Sparreboom, F. C. Schoute and R. Prasad, Location Management Strategies for Cellular Mobile Networks, Electronics & Communication Engineering Journal, Apr. 1998. 7. C. E. Perkins and D. B. Johnson, Route Optimization, Cluster Computing, Vol.1, No.2, Dec. 1998.