Rerouting for Hando in a Wireless ATM Network Bora A. Akyol Donald C. Cox Stanford University Stanford, CA 94040
[email protected]
Abstract
Hando is the procedure by which a user's radio link is transferred between radio ports in the network without an interruption of the user connection. In this paper, we discuss how a Wireless Asynchronous Transfer Mode (ATM) network may reroute a user connection during a hando . We propose a novel procedure called \Nearest Common Node Rerouting (NCNR)." NCNR is designed to perform the rerouting of user connections due to a hando event in a fast and ecient manner. We conclude by comparing NCNR to other rerouting schemes discussed in the literature.
1 Introduction
The rise of the wireless communications paired with the rapid developments in Asynchronous Transfer Mode (ATM) networking technology signals the start of a new era in which the users will not only need higher bandwidth but they will also demand mobility. A future wireless user may be carrying a hand held computer with conferencing capabilities and may demand data communication speeds of 10 Mbits/sec[8]. A wireless ATM network (WATM-N), which is designed to provide high speed isochronous and asynchronous communications for wireless users, is a good match for these demands[1, 8, 10]. Hando is implemented by the network to give the users freedom of motion beyond a limited wireless coverage area while they are communicating. The hando is the procedure by which a user's radio link is transferred from one radio port to another through the network without an interruption of the user connection[2]. In this paper, we rst summarize the hando procedure in the wireless ATM network; then propose a novel procedure (Nearest Common Node Rerouting or NCNR) for rerouting connections in the wireless ATM network to support a hando event. We conclude by comparing the proposed hando scheme to Research supported by Motorola Inc., Schaumburg, IL. and Paci c Bell, San Ramon, CA.
the existing schemes in the literature and discussing the bene ts of NCNR[9, 10, 11, 12].
2 Hando Procedure
The hando procedure is performed to assure the integrity of a radio connection and to minimize interference to the users in the coverage area of neighboring cells[2, 3]. The wireless ATM network consists of radio ports, user terminals and network interface equipment. A user terminal might have a few simultaneous connections in the wireless ATM network. When a hando occurs these connections may need to be rerouted. In this paper, we assume that a group of radio ports is connected to the same wireless ATM network interface equipment. This collection of ports is called a \Zone"[1]. The zone architecture is illustrated in Figure 1. The zone is managed by the \Zone Manager" process. There are two levels in a hando event: Network Level and Radio Level. The radio level hando is the actual transfer of the radio link between two ports. The network level hando supports the radio level hando by performing rerouting and bu ering. The radio level hando determines some of the procedures used in network level hando . We also assume that these zones are interconnected by wireless ATM network switching nodes. Based on the \zone" concept, a few di erent situations for the rerouting may be investigated:
2.1 Intra-zone Hando
In the intra-zone hando the user is moving within the zone. The only rerouting that is performed in this case is in the wireless ATM network interface equipment within the zone. This type of rerouting does not involve ATM network switching; hence, is not discussed any further.
2.2 Inter-zone Hando
The inter-zone hando occurs when the radio ports involved in the hando belong to di erent zones. In this case the rerouting involves the wireless ATM network. An inter-zone hando might require rerouting at one or more wireless ATM switches depending on
Measure
Nh Nr Nn Nc B
W/L Rerouting Scheme Robustness Estimated by the author
Direct Link 7 2 2 1 1 W DY
NCNR No Direct Link 9 4 4+D 1 1 W DY
5
5
Yuan
BAHAMA
Acampora
12 x 4 2 (est.) 2+D 2+D (est.) 1 1 1 1 W L CF / D Y CF
2 (est.) 2N N +D
2
SRMC
Toh
x 10 Centralized 6 (*) N +D 3+D N N 1 N 1 1 W W L T T DY
3
4
2
3
Table 1: Comparing Rerouting Algorithms. Symbols: x: Not described in literature. N: Number of leaves in the tree structure in virtual tree based schemes. D: Number dependent on network topology. (*) Nr includes partial tear-down; equals 4 without partial tear-down.. Radio Port
Backbone ATM Network
Radio Port Radio Port Radio Port
Radio Port
Radio Port Controller To the ATM Network
Radio Port
Radio Port Controller Wireless-to-ATM Network Interface Database
Wireless ATM Network Node(s)
NCN for A&C
NCN for A&B Zone A
Zone B
Zone C
Figure 1: A Zone in the Wireless ATM Network
Figure 2: Depiction of Nearest Common Node
the location of hando and topology of the network. This type of rerouting is discussed in the next section.
users of the WATM-N may subscribe to services ranging from time-sensitive trac types (audio, video) to throughput dependent trac types (data, le transfers, etc.). The trac type of the connection involved in the hando is known by the zone managers. The two kinds of trac types impose di ering constraints on the network and the hando process. For example, time-sensitive (TS) voice trac will not be easy to bu er due to constant cell generation rate and strict time delay constraints; however, it can tolerate occasional loss of cells. On the other hand, data trac will not tolerate cell loss, but may tolerate delays on the order of few hundred milliseconds. Based on this fact, the NCNR procedure for these two types of traf c is di erent. Note also that due to the nature of the xed network, the transmission delay and latency of the links from the NCN to the zones involved in
3 Rerouting for Inter-zone Hando
In this section we propose a novel inter-zone hando procedure referred to as the Nearest Common Node Rerouting (NCNR). The NCNR attempts to perform the rerouting for a hando at the closest ATM network node that is common to both zones involved in the hando transaction. The term \common" is used to denote a network node that is hierarchically above both of the zones in question or a parent of both zones in the network topology tree (See Figure 2). The nearest common node rerouting minimizes the resources required for re-routing and conserves network bandwidth by eliminating unnecessary connections (See Section 5). As mentioned previously, the
the hando are assumed to be negligible compared to the radio transmission medium. In this section we rst explain the NCNR procedure for time(delay)-sensitive trac. We then conclude by explaining the NCNR procedure for throughput dependent trac based on the NCNR for time-sensitive trac.
3.1 NCNR for TS Trac (NCNR-TS)
Endpoint
WATM Node A
WATM Node
2. The zone manager of A rst checks to see if a direct physical link (not involving any other network nodes) between A and B exists. There are two possible cases if this condition is satis ed:
WATM Node
WATM Node
Deleted User Connection Previous Port Candidate Port
Handoff
The NCNR for TS trac is performed as follows:
1. A hando session between zones A and B is started. Let B be the candidate zone for the hando . Let A be the present zone.
WATM Node B
Figure 5: Case 2.b in a Flat Network Wireless ATM Network Candidate Radio Port WATM Node B
WATM Node D
Handoff Deleted User Conn.
Portable
Endpoint
WATM Node C WATM Node
B
A
WATM Node
Figure 6: Case 2.b in a Hierarchical Network Previous Port
Handoff
Figure 3: Case 2.a in a Flat Network
Wireless ATM Network Previous Radio Port WATM Node A
WATM Node D
Portable Add-on User Conn. Handoff
WATM Node C
Previous Radio Port
WATM Node
Add-on User Connection
Candidate Port
WATM Node A
WATM Node B
Candidate Radio Port
Figure 4: Case 2.a in Hierarchical Network
(a) If A is a parent1 of B, then A noti es B and the new connection is established without any further network involvement (See Figures 3 and 4). After the connection is established, A acts an anchor for the connection. Until the stability of the hando is established, both A and B act as network connection points for the user connection. This process is explained at step 6 of this procedure. Once the radio level hando is completed, then A acts only as a wireless ATM switch in the connection path. (b) If B is a parent of A then A sends a message to B relaying the hando request. B then acts as an anchor for the hando procedure. Until the stability of the hando is established, both A and B may be used for information transfer from/to the terminal to/from the network (See step 6 of this 1 The parent is determined by means of either the network topology in a hierarchical network or by means of closeness in terms of hops to the end point. The end point is de ned as the terminating point for the user connection in the network.
procedure). Once the hando is stable, B deletes the user connection from itself to A. The rerouting is thus completed. See Figures 5 and 6. 3. If A and B are not connected by a direct physical link then the zone manager of A (ZMA) contacts the end point for the user connection by sending a hando start message. The hando start2 message contains the ATM addresses of zones A, B and the endpoint for the user connection. 4. The hando start message traverses the network from A to the endpoint for the user connection. The network switching nodes on this path upon receiving this message check to see whether all three ATM addresses are routed on di erent egress ports of the switch (See Figure 2). When such a node is found it is designated as the nearest common node (NCN). The NCN sets the NCN bit in the hando start message. The rest of the switches on this path do not perform the egress port test3 . 5. The NCN then forwards a reroute message to all of the switches located between B and itself. The nodes that receive the reroute message check for resource availability; if the resources required by the connection are available, the necessary connections are established and circuit translation tables are set up. If the resources are not available, the hando attempt fails and the involved parties are noti ed. 6. When the reroute message is received by B, a reroute acknowledgment message is sent from B to A. This message completes the rerouting process. The radio level hando is attempted at this point by employing the procedures discussed in [1, 6, 7]. As the radio level hando is started, the NCN starts to forward the user information to both A and B in a point to multi-point manner. This multi-party connection is necessary until the radio level hando is stabilized. The radio level hando may extend beyond one radio burst in most radio systems as the user terminal tries to select the optimal link. Especially in a fading environment, a small motion of the terminal may The signaling messages will be denoted by bold lettering in the text. 3 Another possible implementation will be to not forward this message after the NCN is found. By forwarding the message to the end point, we allow the user applications to adjust to the hando process. 2
cause the radio link to switch back and forth between the two radio ports involved in the hando ; hence, a point to multi-point link from the NCN to both A and B ensures the timely delivery of time-sensitive information. Note that the user information may be discarded at the zone that is not in contact with the user terminal and transmitted from the zone that is in contact with the terminal. In the uplink direction, the information may be transmitted through either zone involved in the hando and correctly routed to the end point by the NCN. 7. If the radio level hando is successful4 , the connection between A and the NCN is cleared by A by sending a clear connection message to the NCN. The data bu ered at B is then transmitted in sequence to the user terminal. If the radio level hando is not successful, then the information bu ered at A is transmitted to the user terminal.
3.2 NCNR for Throughput Dependent Trac (NCNR-TD)
NCNR for throughput dependent (TD) trac is very similar to the procedure employed for TS traf c. TD trac is not sensitive to small (approximately few hundred milliseconds) delays; however, the loss of information is not tolerated by this trac type. We can take advantage of the delay tolerating nature of this trac in the rerouting process. The NCNR-TD di ers from the procedure for NCNR-TS as follows: 1. As the radio level hando is started, the downlink user information is bu ered at both A and B. No user information is transmitted in the downlink direction until the radio level hando is completed. Once the radio level hando is completed, the information is transmitted in a rst in rst out manner. 2. If A's bu er is non-empty before the hando is started, then A's bu er is transmitted to the user terminal if possible; otherwise, these data are transmitted to B and go in front of all other cells bu ered for transmission. This preserves the cell sequence. 3. In the uplink direction, before the radio level hando is started, the trac is transmitted through A if possible; otherwise it is bu ered 4 The radio level hando is successful only when the stability of the new radio link is established. The determination of the stability of the new link is beyond the scope of this paper.
at the terminal. As the radio level hando is started, the user terminal starts bu ering the user information. Once the hando is stabilized the bu ered information is transmitted. These di erences ensure the integrity of user data as well as the cell sequence (See Section 4). Multiple connections may be rerouted in the network using the virtual path concept. By assigning a virtual path identi er for connections between a user and multiple end points and performing the rerouting on the virtual path instead of on a virtual circuit basis, ecient rerouting of multiple connections may be achieved.
4 Preserving the Cell Sequence in NCNR
In any ATM network, cell sequence of individual ATM cells must be preserved for correct re-assembly of encapsulated user information[4]. Therefore, during a hando , the preservation of cell sequence is a primary concern of the network. In a hando transaction, there are two parties involved: the hando terminal (HT) and the end point. For TS trac, the cell sequence in NCNR is preserved by discarding the cells, duplicated at the NCN and transmitted to both zones involved in the hando , at the zone that is not currently in contact with the HT. This reduces the number of active paths to one in the downlink direction, thereby preserving the cell sequence. In the uplink, data from only one zone are forwarded to the endpoint by the NCN. This preserves the cell sequence. If soft hando , such as in CDMA, is being used where the uplink direction data is received by two zones simultaneously, then NCN is responsible for combining the two streams into one. Techniques for soft hando are beyond the scope of this paper. For TD trac, during a hando , the involved zone managers start bu ering information from the start of radio level hando to the completion (successful or unsuccessful); moreover, until the radio level hando is started, the user connection is assumed to be active and all incoming cells are transmitted to the HT through the previous port. If that is not possible, the cells are forwarded to the candidate port through the NCN for later transmission. In the reverse direction, the HT bu ers the user information as soon as the radio hando is initiated. The cells that are in transmission in the network from the HT to the end point are delivered normally. By employing the procedure discussed above for TD trac, the cell sequence during the rerouting is preserved due to the fact that at any given time during
a hando , there is only one active path between the two parties involved in the connection. Since there is only one active path at any given time, all the cells transmitted through the network take this path and arrive in sequence to their destination.
5 A Comparison of Existing Rerouting Schemes for Wireless ATM Networks
The NCNR enhances the current ATM signaling protocols by adding support for rerouting an active connection. In the current ATM Signaling Protocols there is no provision for rerouting a connection once it is established[4]. By using NCNR, multiple connections between a mobile portable and end points may be rerouted without re-establishing connections, while the hando procedure is being performed. In this section, we compare NCNR with some other rerouting for hando schemes proposed for wireless ATM networks. These are Yuan([10]),BAHAMA([11]), Acampora ([9]), SRMC([12]) and Toh([13]). The existing rerouting algorithms may be classi ed under three categories: Cell Forwarding, Virtual Tree Based and Dynamic Rerouting. Yuan and BAHAMA algorithms use cell forwarding. In cell forwarding based rerouting, an add-on connection is created to assist hando . The bene t of such a simple approach is the preservation of cell sequence as the previous switch acts as an anchor and all cells follow the same path. However, the cell forwarding based rerouting is only ecient in a at or a ring network with a fully meshed topology. In a hierarchical network, the NCN is automatically involved in cell forwarding and when the NCN is involved, it is more ecient to reroute instead of forward[5]; moreover, with cell forwarding for a fast moving user in a system that has small radio coverage areas, there will be a network trail left behind the user consisting of cell forwarding among multiple zones. The NCNR always performs the rerouting at the nearest common node thereby minimizing the amount of bandwidth used and minimizing the amount of rerouting. The disadvantages of virtual tree based rerouting schemes when compared to dynamic rerouting schemes such as NCNR or Toh are given in [5, 13]. Brie y, virtual tree based rerouting creates multiple connections for a single user connection to all possible hando candidate zones and performs an immediate rerouting. Acampora and SRMC use virtual tree based rerouting. This type of rerouting is fast; however, it predicts hando events and pre-establishes routing and/or connections. This may result in significant wasted overhead for hando s that never occur.
The dynamic rerouting algorithms are based on the principle of partial re-establishment of the user connection. NCNR and Toh are dynamic rerouting algorithms. These algorithms are ecient and reasonably fast[5, 13]. Another point that must be noted is the support for di erent kinds of trac in NCNR. NCNR is unique in its design for handling both time-sensitive and throughput-dependent trac types; none of the other algorithms proposed in the literature provide support for di erent trac types. One approach to quantifying a network management algorithm is the simulation of a wide area network either on a hardware level or by using network simulators. A second approach is to characterize the algorithm and simplify the measurement approach. We chose the second approach for characterizing and quantifying various hando and associated rerouting algorithms. The following are of interest for comparing hando and rerouting algorithms: Signaling Bandwidth used for Hando Signaling, Number of signaling messages exchanged during hando (Nh ), Number of signaling messages exchanged for rerouting during a hando (Nr ), Number of network nodes involved in the rerouting (Nn ), Number of user connections established for rerouting (Nc ), User Bandwidth allocated for hando (B ), Whether the protocol is proposed for wide (W) or local area networks (L), Rerouting Scheme used: Cell Forwarding (CF), Dynamic as in NCNR (DY), Tree such as in Acampora or SRMC (T), Robustness to Instabilities in Hando , We use the criteria listed above to compare the various hando and rerouting procedures in our paper (See Table 1). This table may be used as a rst step in choosing a hando and rerouting algorithm for a given wireless network.
6 Conclusion
In this paper we proposed a novel rerouting scheme for a hando transaction in a wireless ATM network. This rerouting scheme is Nearest Common Node Rerouting (NCNR). We have utilized the zone
concept to illustrate the NCNR algorithm. NCNR is based on nding the ATM node that is a root of or common to both of the zones involved in hando . The rerouting is then performed starting from this common node. A comparison of NCNR with related work is also presented. Another contribution of this paper is the recognition of di erent constraints associated with the hando of di ering trac types. From the comparisons, NCNR is seen as a promising scheme for rerouting a wireless ATM connection for hando .
References
[1] Bora A. Akyol and Donald C. Cox, \Handling Mobility in a Wireless ATM Network," INFOCOM' 96, MAR. 24-28, 1996, San Francisco, CA. [2] D. C. Cox, \Wireless Network Access for Personal Communications," IEEE Comm. Mag., Vol. 30, No. 12, pp. 96-115, Dec. 1992. [3] D. C. Cox, \A Radio System Proposal for Widespread Low-Power Tetherless Communications," IEEE Trans. on Comm., Vol. 39, No. 2, pp. 324-335, Feb. 1991. [4] ATM Forum User Network Interface speci cation version 3.1, ATM Forum, May 1994. [5] Bora Akyol and Donald C. Cox, \Rerouting for Hando in a Wireless ATM Network," submitted to IEEE Personal Comm. Mag. for publication. [6] Bora Akyol and Donald C. Cox, \Signaling Alternatives in a Wireless ATM Network, Part 1: The Overlay Signaling Approach," submitted to IEEE JSAC for publication. [7] Bora A. Akyol and Donald C. Cox, \Signaling Alternatives in a Wireless ATM Network, Part 2: The Migratory Signaling Approach," Submitted to IEEE JSAC for publication. [8] D. Raychaudhuri and N. Wilson, \ATM Based Transport Architecture for Multiservices Wireless Personal Communication Networks," IEEE JSAC Oct. 1994, pp 1401-1414. [9] A. S. Acampora and M. Naghshineh, \An architecture and methodology for mobile-executed cell hand-o in wireless ATM networks," Proceedings of International Zurich Seminar on Digital Comm., Zurich, Switzerland, 8-11 March 1994. [10] R. Yuan et al., \Mobility Support in a Wireless ATM Network," 5th Workshop on Third Generation Wireless Information Networks, 1995, Kluwer Publishers, pp. 335-345. [11] K. Y. Eng et al., \A Wireless Broadband ad-hoc ATM local-area Network," Wireless Networks, vol. 1 , pp. 161174, 1995. [12] O. T. W. Yu and V. C. M. Leung, \B-ISDN Architectures and Protocols to Support Wireless Personal Communications Internetworking," PIMRC' 95, Sep. 27-29, 1995, Toronto, Canada. [13] C. Toh, \The Design and Implementation of a Hybrid Handover Protocol for Multimedia Wireless LANs," Mobicom 95, Nov. 13-15, 1995, Berkeley, CA., U.S.A.
[14] K. Keeton et al., \Providing Connection-oriented Network Services to Mobile Hosts," USENIX Symposium on Mobile & Location-Independent Computing , Aug. 2-3, 1993, Cambridge, MA., USA.
