reduced cell switching in a mobile computing environment- · reduced cell switching in a mobile...

Reduced Cell Switching in a Mobile Computing Environment-

Tracy Camp Dept. of Math. and Computer

Sciences Colorado School of Mines

Golden, CO 80401 [email protected]

John Lusth Dept. of Math. and Computer

Sciences Boise State University

Boise, Idaho 83725 lusth @cs.BoiseState.edu

Jeff Matocha Computer Science

Department Xavier University of LA New Orleans, LA 70125 jlmatoch @xula.edu

ABSTRACT With the huge growth and the market for laptop and palm- top computer purchases, a rapid increase of mobile usage in the Internet is expected. As mobile nodes move in a wireless computer network, a mobile node must determine when to switch its link-level point of attachment to the wired network. In this paper, we present six cell switching techniques and discuss their attributes. Specifically, we present the Late, Early, and Strong cell switching techniques and three variations of them. We then investigate the performance of these six techniques to discover the best method a mobile node should use to determine when to perform its re-attachment to the wired network.

1. INTRODUCTION There is expected to be one billion wireless telephone and Internet users by 2002 [11]. With the rapid development and growth of the Internet, the World Wide Web, and multime- dia applications on the wired networks, it is reasonable to expect that a strong user demand for these applications on wireless networks will exist. In fact, according to Huitema [4], "many experts are convinced that tomorrow's computers will all be mobile!"

In 1994, a two-day workshop was held in Virginia to identify major research issues in networking and communications [8]. The Wireless Networks and Access portion of the report, developed by the members of the workshop, listed a funda- mental research priority for "algorithms and criteria used to determine when handoff from one access point to another is required." [8]. Partially due to the 1994 workshop rec- ommendations, another workshop was held in March 1997 to identify the major research issues in wireless and mobile communications and networking [9]. Part of this 1997 research report on location management confirms that inves-

*This work supported in part by NSF Grant ANI-9996156.

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed fbr profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on selwers or to redistribute to lists, requires prior specific permission and/or a fee. MOBICOM 2000 Boston MA USA Copyright ACM 2000 1-58113-197-6/00/08...$5.00

tigation of algorithms for when a Mobile Node (MN) should switch its link-level point of at tachment (i.e., when to switch cells) continues to be a research priority. In this paper, we consider the movement of a mobile node in a wireless computer network. As the mobile node moves, the mobile node must determine when to switch cells in order to maintain its connection to the Internet.

home network

Wired Network

wireless cells

Figure 1: A Mobi le Env ironment .

For simplicity, we define the following terms (see Figure 1). An MN is a node that has the ability to switch its link- level point of at tachment to a wired network due to movement. Each MN is assigned an Internet Protocol (IP) address. Since IP addresses are location dependent, including the addressing scheme in IP version 6 (IPv6), the network prefix in the MN's IP address associates the MN with a home network. An MN is said to be away from home if its current link-level point of at tachment is not its home network.

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Each network with MNs has a Home Agent (HA) to han- dle mobility concerns. As in a cellular telephone network, a wireless computer network consists of connected, adjacent, or overlapping cells. The current diameter of a cell in the cellular telephone network is 1-2 miles [5]. Due to the expected increase in volume of da ta traffic, and the high percentage of in-building use, the diameter of a cell in a wireless computer network is expected to be much smaller (a picocell), covering only a room or a building floor [18]. 1 Each wireless cell has a base station (Base) that is connected to a static network. Each Base provides a link-level point of a t tachment to the wired network for the MNs in its cell; all communication for an MN in a cell is completed through the cell's Base.

There are many challenges in solving the mobility problem [5, 10, 17]. Forman and Zahorjan categorize the challenges into three areas: wireless issues (lower bandwidths, higher error rates, more frequent disconnections, and reduced se- curity than wired lines), portabi l i ty issues (lower power and smaller storage capacity than desktop computers), and mo- bil i ty issues (location management) [2]. This paper studies the last of these three challenges from the MN's perspective, i.e., how the MN determines when it should switch its link- level point of a t tachment to a new Base when it is in the presence of competing Bases.

If an MN has the tools necessary to determine either signal strength or signal quality, then the MN can use this information to determine when it should switch its link-level point of a t tachment from its current Base to a new Base. We present six techniques tha t an MN may use to determine when to switch cells in this paper; four of these techniques assume that an MN has signal strength or signal quality information available. We discuss the advantages and disadvantages of the six cell switching techniques, and we investigate their performance for four movement patterns.

In Section 2, we briefly discuss Mobile IP and cell switching according to the s tandard adopted by the Internet Engi- neering Task Force (IETF). In Section 3 and Section 4, we present six cell switching techniques. We investigate the performance of these six techniques in Section 5. Lastly, we state conclusions and avenues for future work in Section 6.

2. BACKGROUND Location management consists of handling two procedures: move and correspondence. A move procedure occurs when an MN switches its current link-level point of at tachment. A correspondence occurs when a node, either mobile or static, initiates communication with an MN. The IETF has adopted a s tandard for the Internet tha t defines the move and correspondence procedures for IPv4 [12]. This s tandard is called Mobile IPv4 and we briefly describe it in Section 2.1. In Section 2.2, we discuss Mobile IP 's suggestions (IPv4 and IPv6) on how an MN may determine when to switch its link- level point of at tachment.

2.1 Move/Correspondence in MIPv4 Every Base in a wireless computer network has an agent that handles mobility concerns [12]; we use the term Base to in-

1 An efficient cell switching algorithm is therefore extremely important .

clude the function performed by a foreign agent. A Base advertises its availability in the cell for which it provides service. These advertisements are periodically t ransmit ted; RFC 2002 recommends tha t an advertisement be sent every second [12]. Thus, an MN is able to discover the existence of a Base shortly after the MN enters a cell managed by the Base. When an MN receives an advertisement and first determines tha t it is away from home, the MN regis- ters itself with the advertising base [12]. Furthermore, as the MN roams through the wireless computer cells, the MN may decide to switch its link-level point of at tachment to a new Base. In the registration process of both cases, the Base offers the MN a temporary care-of-address and a life- t ime associated with this care-of-address. Furthermore, the MN transmits the care-of-address and lifetime to the MN's HA. When the MN determines tha t it is no longer away from home, the MN instructs the HA to remove its care-of- address. The above operations define the move procedure in Mobile IP.

When a node, either stat ic or mobile, initiates communication with an MN, a Mobile IP correspondence procedure occurs. A sending node t ransmits packets to the home address of an MN. If the MN is away from home, then the HA inter- cepts each packet for the MN and encapsulates, or tunnels, the packet to the MN's care-of-address. The network prefix of the MN's care-of-address is the same as the network prefix of the Base where the MN resides. According to Mobile IP, every home agent must support IP-within-IP encapsulation, which adds a second IP header to route the packet to an MN's care-of-address [13]. Generic Routing Encapsula- tion (GRE) [3] and minimal encapsulation [14], which have less overhead than IP-within-IP, are optional encapsulation techniques.

When an MN decides to switch cells to a new Base, an update on the MN's care-of-address is sent to the MN's HA. If the MN moves outside the range of its previous Base's coverage, then the MN may experience an interruption in packet reception while the HA updates the MN's new location. During this interruption, packets sent to the MN's previous Base may need re-transmission. A cell switching technique tha t minimizes packet re-transmission is, obviously, preferred.

2.2 Cell Switching in MIPv4/MIPv6 Mobile IPv4 suggests two algorithms (Algorithm 1 and Al- gorithm 2) tha t MNs may use to determine when to switch their link-level point of a t tachment [12]. As s ta ted in the s tandard, other techniques may be used; no standard technique exists for determining when an MN should switch cells.

In Mobile IPv4 's Algori thm 1, the MN determines it has moved to a new cell if the MN misses some advertisements from the old cell. In other words, the MN has lost contact with its Base. At this point, the MN should a t tempt to discover a new Base with which to register. In Algorithm 2, the MN determines it has moved to a new cell if the network prefixes of received advert isements are different from the network prefix of the MN's current registration. Thus, the MN has moved into a new subnet and should register with this new Base that is sending advertisements.

1 4 4

When an MN is away from home and is registered with a Base, it is important that the MN detects if the Base can not reach the MN. Otherwise, nodes attempting to communicate with the MN can not do so. Both Algorithm 1 and Algo- ri thm 2 in [12] allow the MN to detect when its current Base can not reach the MN; neither algorithm, however, does this detection quickly. In addition to the MN detecting whether the Base can reach the MN, it is important for an MN to detect when it can no longer reach its current Base. (Wireless links often do not work symmetrically.) Otherwise, when the MN attempts to communicate with another node (mobile or static), it can not do so. In the working draft of Mobile IPv6 [6], unlike the Mobile IPv4 standard [12], the Mobile IP Working Group of the IETF considers techniques that allow the MN to quickly detect if its upward or downward communication link between the MN and the Base is unavailable.

As in Mobile IPv4, Mobile IPv6 suggests techniques that MNs may use to determine when to switch their link-level point of attachment [6, 16]. There are three possibilities in the Mobile IPv6 working draft to verify that an MN's current Base can reach the MN (i.e., that the downward link between the MN and the Base is available). An MN determines its Base can reach the MN if

1. the MN receives IPv6 packets, such as Base advertisements, directed to the MN from its Base,

2. the MN receives IPv6 packets from its Base that are directed to other MNs in the cell managed by the Base, or

3. the MN asks its Base whether it is available and the MN receives an acknowledgment.

The third possibility is an expensive last resort if the first two possibilities fail.

In order for an MN to verify that it can reach its current Base (i.e., that the upward link between the MN and the Base is available), the Mobile IPv6 working draft discusses two possibilities. An MN determines it can reach its Base if

1. the MN is receiving acknowledgments on data previ- ously transferred or

2. the MN asks its Base whether it is available and the MN receives an acknowledgment.

Again, the second possibility is an expensive last resort.

In the above suggestions, an MN can detect when the upward or downward communication link between the MN and its current Base becomes unavailable. An MN determines a communication link is unavailable when multiple packets to or from the MN are not received, as one or two unreceived packets may only indicate a temporarily blocked signal. Once an MN determines that the upward or downward communication link between the MN and its current Base no longer exists, the MN must switch to a new cell. Pack- ets sent to or from an MN while the communication link is unavailable may require re-transmission.

If an MN has access to lower-layer information, then the MN may decide to switch to a new cell before a communication link becomes unavailable. As we discuss in Section 1, if an MN has the tools necessary to determine either signal strength or signal quality, then the MN may use this information to determine when to switch to a new cell, thus providing a better connection between the MN and its Base. Although the Mobile IPv6 Working Group mention the possibility of using lower-layer information to determine when to switch cells in [6], the working draft does not investigate such switching techniques. We provide this missing investigation in this paper.

Before we present six cell switching techniques, we consider an MN that has multiple care-of-addresses registered concurrently. In Mobile IPv6, an MN may have multiple care- of-addresses through which the MN may be reached [6, 16]; in an overlapping wireless cell environment, an MN may be registered with more than one Base concurrently, thus ob- taining multiple link-level points of attachment. The MN maintains, however, a single primary care-of-address which is registered with the MN's HA. Thus, our investigation into cell switching techniques in this paper concerns when an MN should switch its primary care-of-address. When an MN decides to switch its primary care-of-address, it should maintain its previous primary care-of-address as a secondary or non-primary care-of-address. Furthermore, the MN should continue to receive packets from its previous address. Thus, when an MN decides to switch its primary care-of-address, a smooth handoff between the old address and the new address will occur.

3. SIMPLE CELL SWITCHING In this section, we discuss three simple cell switching techniques that an MN may use to determine when to switch its link-level point of at tachment to the wired network:

• Late Cell Switching (Late),

• Early Cell Switching (Early), and

• Strong Cell Switching (Strong).

Figure 2 illustrates when an MN determines to switch cells, when the MN is traveling from Base A's cell coverage to Base B's cell coverage, for each of these three simple cell switching techniques. This figure, as well as many of the following figures, magnifies the center portion of two wireless cells, not the entire overlapping area as shown in Figure 1. Hence the boundaries appear as straight lines. The line with the large arrowhead indicates the path of the MN. The circle along the path illustrates when the MN determines to switch cells for each of these three simple cell switching techniques.

As illustrated in Figure 2, if an MN uses the Late technique, the MN decides to switch to a new cell when the upward and downward communication links of the old cell are no longer available. In Late, an MN may experience an interruption in packet reception; these packets may require re-transmission. If an MN uses Early, the MN switches to a new cell as soon as the MN receives an advertisement from a new Base. Thus, as soon as the MN detects the existence of a new cell, the

145

toA

MN

\ Early t

Line of equal strength

D tuB

Westernmost Easternmost extent of B extent of A

toA -.~

A

B (under Early) A (under Late) ff

\


~- toB

F i g u r e 2: Late , Early , and S t r o n g Ce l l S w i t c h i n g T e c h n i q u e s .

F i g u r e 3: W e a k S igna l for Late; S t r o n g S igna l for E a r l y .

MN decides to switch its pr imary care-of-address to the Base in this new cell.

Unlike Late and Early, Strong relies on lower-layer information such as signal strength or signal quality. An MN can determine signal strength of a part icular Base, if the MN has special hardware, by measuring the average power of the signal t ransmi t ted by the Base [7]. Note tha t the received signal measured by the MN can be either an advertisement from the Base (see Section 2.1) or any packet t ransmi t ted by the Base, regardless of the packet 's address. In a similar manner, an MN can assess signal quality of a part icular Base, if the MN uses an error detection method, by mon- itoring packets t ransmi t ted by the Base and determining the percentage of these packets containing bit errors. Note again that the monitored packets need not be addressed to the MN. If an MN uses Strong, the MN switches to a new cell when the signal s trength (or quality) of the new cell becomes stronger than the signal s trength (or quality) of the current cell.

In the rest of this section, we discuss advantages and disadvantages of these three simple techniques. Figure 3 illus- t rates an example of movement by an MN in the overlapping area of two wireless cells. As before, the figure magnifies the center portion of two wireless cells. The letters A and B, when near the pa th of the MN, indicate the Base with which the MN is registered (or registering) for tha t portion of the path. If the MN uses Late, then the MN switches cells when it crosses the easternmost extent of Base A's coverage. Thus, the MN travels near the edge of A's cell without switching, although it has a weak signal (or low signal quality) from the cell where the MN is registered. This weak signal may result in extensive packet re-transmission. Although this example is unacceptable for Late, Early works well. In other words, if the MN uses Early, then the MN decides to switch cells as soon as it crosses the westernmost extent of Base B's coverage. Analogously, if the MN crosses the westernmost extent of Base B, but spends a significant amount of t ime near tha t border, Late performs well in terms of received sig-

hal s trength from the cell where the MN is registered, while Early performs poorly. Strong, on the other hand, maintains a strong received signal in both movement examples.

Wi th respect to registration frequency, Figures 4 and 5 illus- t ra te an example of movement by an MN in the overlapping area of two wireless cells where Late works well but Early is unacceptable. If the MN uses Late, then the MN decides to switch cells when it crosses the easternmost extent of Base A's coverage the first time. (See Figure 4.) Even as the MN moves back and forth across the easternmost extent of Base A's coverage, the MN continues to maintain a primary care-of-address from Base B. If the MN uses Early, on the other hand, then the MN switches its pr imary care-of- address each t ime it crosses the easternmost extent of Base A's coverage. (See Figure 5.) Thus, the overhead of Early, in this movement example, is unacceptable.

toA

A

B(

B

) B toB

B


F i g u r e 4: Low S w i t c h i n g O v e r h e a d for Late .

To explain the increase in registration activity of Early over Late, we introduce the concept of an "oscillating region."

1 4 6

t o A

---.-_..._ A

, (

, (

B

)B toB

B


Figure 5: High Switching O v e r h e a d for Ear ly .

4. CELL SWITCHING W/HYSTERESIS Cellular telephone systems employ a modified version of Strong. Instead of switching cells immediately when signal strength (or an estimate of signal strength via signal quality) from the new cell exceeds signal strength from the old cell, the MN waits until the difference in signal strength exceeds a preset value. This differential, which is expressed in dB if signal strength is used or bit error rate if signal quality is used, creates a hysteresis effect [7]. Instead of a single line of equal strength shown in Figure 6, consider the two light gray lines of signal strength shown in Figure 7. One line represents the boundary where the signal from Base B is, for example, X dB stronger than the signal from Base A, and the other line represents the boundary where the signal from Base A is X dB stronger than the signal from Base B. The hysteresis region (the area between the two boundaries) needs to be large enough to prevent excessive cell switching due to movement of the MN or fading of the radio signal, yet the boundaries should be sufficiently far from the outer extent of each cell so that reliable communication exists.

A region is an oscillating region if traversing the region in either direction induces a switch. For Late, the oscillating region is the area between the two extent lines. In Early, there are two oscillating regions; both oscillating regions are at the extent lines. We define an oscillating region as "degenerate" if the region has no width. Thus, both oscillating regions in Early are degenerate.

Furthermore, both Figure 4 and Figure 5 illustrate peri- ods where packets sent to or from an MN may require re- transmission. If Late is used, as shown in Figure 4, then Base A's coverage becomes unavailable when the MN first crosses into Base B's cell. Until the primary care-of-address for the MN is updated to Base B, packets transmitted for the MN by Base A may require re-transmission. If Early is used, as shown in Figure 5, then Base A is unavailable each time the MN re-crosses the easternmost extent of A. Again, packets transmitted for the MN by Base A may require re- transmission. Strong overcomes this disadvantage of Late and Early, as the MN switches cells when communication links to both Base A and Base B remain available. Thus, even though the MN switches its primary care-of-address, it can continue to receive data from its secondary care-of- address. Figure 6 illustrates, however, an example of movement by an MN in the overlapping area of two wireless cells where the overhead of Strong switching is unacceptable. In the figure, the MN is repeatedly traversing the degenerate oscillating region located at the line of equal strength.

Although Strong is the preferred technique with respect to signal strength, the potential for excessive switching renders the technique unacceptable. By removing the degenerate oscillating region in Strong, however, it is possible to reduce the number of switches while preserving the excellent signal strength characteristics. In Section 4, we show how to remove the degenerate oscillating region in Strong through the addition of hysteresis. We introduce the hysteresis concept by summarizing how it is used in cellular telephone systems.

toA

Line of equal strength

Westernmost extent of B

Easternmost extent of A

toB

Figure 6: High Switching Overhead for Strong.

t o A

A jB


toB

Figure 7: Late-SH Example.

147

While the above technique, which we call Late at static hysteresis lines or Late-SH, works well for cellular telephone environments with relatively large-sized cells and voice communication, L a te -SH will no t per form as well in mobile com- pu t ing env i ronments wi th small-s ized cells and data communication. As we mention above, the size of the hysteresis region is a tradeoff between limiting excessive switching (i.e., making the region as wide as possible) and guaranteeing reliable communication for either cell at each boundary (i.e., making the boundaries close to the line of equal strength). Such requirements are difficult to simultaneously satisfy if cell diameter is small and if accuracy requirements become stringent, as necessitated by the t ranspor t of da ta communication compared to the t ranspor t of voice communication. We, therefore, propose a new switching technique to address the inadequacies of applying Late-SH in a mobile computing environment.

toA 4

B


toB

As with Strong and Late-SH, our switching technique, which we call Strong with hysteresis or Strong-H, uses lower-layer information to determine when an MN should switch cells. In Strong-H, an MN always decides to switch to a new cell before its old cell becomes unavailable. In other words, an MN will switch its pr imary care-of-address before it loses contact with the Base of its previous pr imary care-of-address; therefore, smooth handoffs should occur. Unlike Strong and Late-SH, however, Strong-H provides sufficient hysteresis to eliminate excessive switching, even in microcellular systems, while still guaranteeing accurate communication from both cells at the point of handoff.

In Strong-H, an MN determines when to switch cells by using three signal strength (or quality) lines in the overlapping area of two wireless cells. One line, the middle line, represents a line of equal strength. The other two lines represent the boundary lines as we describe above for Late-SH. We call the area between a boundary line and the line representing the extent of a cell's coverage a cell's zone. First , as with Late-SH, an MN in a cell's zone is always registered (or registering) with tha t cell's Base. Thus, after an MN determines to switch its link-level point of a t tachment and before the primary care-of-address for the MN is updated, the MN can continue to receive messages from a previous pr imary care-of-address. Second, if an MN roams from a cell's zone and crosses the line of equal strength, then the MN switches cells. Thus, the MN switches to a new pr imary care-of-address provided by the Base with the stronger signal.

Figure 8 illustrates an example of movement for an MN tha t uses Strong-H. The example illustrates, as in Strong, that when an MN moves from a cell's zone and crosses the line of equal strength, the MN switches cells. Whenever the MN roams into a cell's zone, the MN updates its registration, if needed, to the Base representing tha t cell's zone. In other words, as in Late-SH, an MN outside the hysteresis region is always registered with the Base closest to the MN.

In Figure 6, we illustrate an example of movement by an MN where the switching overhead of Strong is unacceptable; Strong-H avoids this disadvantage of Strong. In the following performance investigation, we consider two versions of Strong-H. The first version is Strong-H when the two bound-

Figure 8: Strong-H Example .

ary lines remain static. We call this technique Strong with static hysteresis or Strong-SH. The second version is Strong- H when the two boundary lines change dynamically, based on the MN's movement pat tern. We call this technique Strong with dynamic hysteresis or Strong-DH. In Strong- DH, the boundary lines expand if the MN switches a second t ime within some given t ime period; the boundary lines contract if the MN does not switch cells within some given t ime period. Thus, Strong-DH tries to provide strong signal strength (or quality) when the MN is not roaming within the overlapping area of two wireless cells and low switching overhead when the MN is roaming in this area.

5. PERFORMANCE INVESTIGATION We investigate the performance of six switching strategies via simulation: Late, Early, Strong, Late-SH, Strong-SH, and Strong-DH. In Section 5.1 we describe the simulation model. We present the results from the simulation in Sec- tion 5.2. We have developed the simulation model in Java. Thus, any interested reader may execute the simulation, watch the different movement patterns, and view the results: h t tp : / /www.xu la . edu /~ jmatocha/swi tch / .

5.1 Methodology Our simulation models a rectangular region of the overlapping area of two wireless cells with Bases A and B. Base A is located at the middle of the westernmost edge of its cell's coverage; Base B is located at the middle easternmost edge of its cell's coverage. We model the extent of Base A's coverage by a north-south line 75% of the distance from Base A to Base B; we model the extent of Base B's coverage by a north-south line 75% of the distance from Base B to Base A, giving a one-third overlap in coverage with respect to cell diameters and a one-half overlap in the simulation region. We model the movement of an MN with a walk constrained to eight directions: north, south, east, west, northeast, south- east, northwest, and southwest. We add randomness to the simulation via a Lehmer linear congruential random number generator.

We investigate four types of walks by manipulat ing the fre-

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Figure 9: R a n d o m W a l k .

Figure 10: Aimless Walk.

quency the simulation shifts a preferred direction of movement to some other direction. The four types of walks corresponded to four different shift frequencies, 1.00, 0.05, 0.0025, and 0.0008. Lower shift frequencies translate to longer straight-line runs, while a shift frequency of 1.00 translates to a pure random walk. Based upon the movement pat terns that these four frequencies generate, we name the four walks "random", "aimless", "headstrong", and "arrow". The average distance an MN travels before changing directions for random, aimless, headstrong, and arrow is 0.00375, 0.075, 1.5, and 3.5 cell diameters, respectively. Fig- ures 9 to 12 illustrate example walks for random, aimless, headstrong, and arrow respectively. We perform five simulation replications, where each walk consists of 1,000,000 steps, for each of the four movement patterns. We collect statistics for number of switches and average signal strength of each walk for the six switching techniques.

We estimate relative instantaneous signal strength ( i .s .s . ) by the following formula:

i .s .s . = 1 /d 2,

where d is the distance from the MN to its pr imary Base. The square in the denominator of the formula directly re- flects the fact that signal strength falls off with the square

J

f Figure 11: Headstrong W a l k .

Figure 12: Arrow Walk.

of the distance from the source of the signal [1]. The simulation calculates the average signal strength for an MN, which is the average of the instantaneous values over the entire simulation.

In our simulation, the stat ic lines of hysteresis, which run north and south, par t i t ion the region into three equally sized zones. The following rules for Strong-SH and Strong-DH describe how an MN behaves in the presence of the middle and two boundary lines:

1. Suppose the MN is registered with the eastern Base and it is heading west. If it crosses the western boundary line, then it switches cells.

2. Suppose the MN is registered with the western Base and it is heading east. If it crosses the eastern boundary line, then it switches cells.

3. Otherwise, if the MN crosses a boundary line and then a middle line, then it switches cells.

For Strong-DH, the simulation requires several parameters. We initialize the dynamic hysteresis lines near the center of the simulation region. The hysteresis lines expand 10% of

149

Z

2,500

2,000

1,500

1,000

500

. . . . . ' I ~ I I Random Aimless Headstrong Arrow

Movement pattern

I Late

Early [----] Strong

F i g u r e 13: S w i t c h i n g B e h a v i o r o f Late , Ear ly , a n d S t r o n g .

the width of the simulation region, a shift width, each t ime the MN switches cells twice within t t ime units or steps. We contract the hysteresis region by 1/2 i the shift width if the MN does not switch cells within (s - t) t ime units or steps, where i is the number of contractions since the last expansion. The algorithm for expanding and contracting the hysteresis region is given in the appendix. The algorithm ensures tha t the hysteresis region may not be collapsed more than the previous expansion. The maximum the hysteresis region may expand is to the stat ic hysteresis lines. In the simulation, we set s much larger than t, s > > t, as expansion should be at a faster rate than contraction. In the following simulation results, t = 1,000 and s = 13,000.

5 . 2 R e s u l t s Figure 13 illustrates the switching behavior from the simulation for the three simple cell switching techniques (Late, Early, and Strong) and four movement pat terns (random, aimless, headstrong, and arrow). As we mention in Sec- t ion 5.1, we average the results of five simulation replications and each replication consists of 1,000,000 steps. Wi th the random movement pat tern, the number of switches for the Early technique is double the number of switches for the Strong technique; the number of switches the Late technique incurs for this pat tern, on the other hand, is insignificant. This result is due to the fact that Early has two degenerate oscillating regions, Strong has one degenerate oscillating region, and Late has no degenerate oscillating regions. As the movement pa t te rn progresses from a random pat te rn to an arrow pattern, the number of switches for Late increases, the number of switches for Early decreases, and the number of switches for Strong is not significantly affected. The arrow movement pa t te rn acts as an equalizer in the number of switches for the different switching techniques. In fact,

if we had a movement pa t te rn with the average distance an MN travels before changing directions at, say, 10.0 cell diameters, then the number of switches for all of the switching techniques would be essentially identical.

Similar to Figure 13, Figure 14 illustrates the switching behavior from the simulation for the three hysteresis switching techniques (Late-SH, Strong-SH, and Strong-DH) and four movement pat terns (random, aimless, headstrong, and arrow). All three of the hysteresis cell switching techniques are similar to the simple cell switching technique tha t performs the best in terms of minimizing the number of switches, i.e., the Late technique. This similarity is due to the fact that the Late technique and the three hysteresis techniques have wide oscillating regions. The number of switches for the hysteresis techniques are slightly higher than the number of switches for the Late technique, since the oscillating regions for the hysteresis techniques are narrower than the oscillating region in Late. Fhrthermore, the oscillating regions in the Strong-H techniques (Strong-SH and Strong-DH) are narrower than the oscillating region in Late-SH; thus, the number of switches for the Strong-H techniques are slightly higher than the number of switches for the Late-SH technique. Again, the arrow movement pa t te rn acts as an equalizer in the number of switches.

The average signal s trength from the simulation is not meant to be interpreted in a quanti tat ive way. Rather, it should be used to quali tat ively discriminate between the various switching strategies. In order to i l lustrate these qualitative differences, the repor ted values for average signal strength have been normalized between the theoretical best and theoretical worst values. The best possible technique is Strong, as an MN always receives its packets from the strongest

150

X

2,500

2,000

1,500

1,000

500

Random Aimless Headstrong Arrow Movement pattern

/ Late-SH

Strong-SH

Strong-DH

Figure 14: Switching Behav ior o f Late-SH, Strong-SH, and S t r o n g - D H .

base among competing bases. The worst possible technique, which we call Weak Cell Switching (Weak), has the MN receiving packets from the weakest base among competing bases. All cell switching techniques must fall between these two extremes. Figures 15 and 16 illustrate the signal strength from the simulation for the six switching techniques and four movement patterns. Notice that each cell switching technique is normalized between Weak (0.0000) and Strong (1.0000).

Figure 15 illustrates that the signal strength of Strong re- mains at 1.0 for all four movement patterns. As the movement pattern progresses from a random pattern to an arrow pattern, the signal strength for Late decreases while the signal strength for Early increases. Similar to switching behavior, the arrow movement pattern acts as an equalizer in the received signal strength for the Late and Early techniques. In other words, the normalized signal strengths of the Late and Early techniques converge as the average distance an MN travels before changing directions approaches infinity. The normalized signal strengths of the Late and Early techniques, however, are far from the desired signal strength.

As shown in Figure 16, all three of the hysteresis cell switching techniques are similar to the simple cell switching technique that performs the best in terms of received signal strength, i.e., the Strong technique. Since the hysteresis techniques far outperform the Late technique in terms of signal strength, and since the hysteresis techniques have a relatively minimal increase in the number of switches compared to the Late technique, the improved performance of the hysteresis techniques seem worth the overhead.

Of the three hysteresis techniques, Strong-DH has the best signal strength performance, regardless of movement pattern, but the worst switching overhead. Conversely, Late-

SH has the best switching performance, but the worst signal strength performance. Since the signal strength of Late-SH deteriorates as the MN progresses from a random pattern to an arrow pattern (which seems the most realistic of the four simulated), and since mobile computing environments have small-sized cells compared to cellular telephone environments, we conclude that the cellular telephone switching technique (Late-SH) is not appropriate for cell switching in Mobile IP. Strong-SH and Strong-DH, on the other hand, offers strong signal strength, without large switching overhead, for cell switching in a mobile computing environment.

6. CONCLUSIONS/FUTURE WORK To decrease switching overhead, it is best to reduce the number of oscillating regions, to avoid degenerate oscillating regions, and to maximize the size of the oscillating regions that exist. Both Late and Late-SH have one non-degenerate oscillating region; furthermore, the oscillating region for Late is the maximum of the non-hysteresis techniques and the oscillating region for Late-SH is the maximum of the hysteresis techniques. Thus, Late and Late-SH have the minimal switching overhead of the non-hysteresis and hysteresis techniques, respectively. Late has a wider oscillating region than Late-SH, thus Late has the minimal switching overhead of all six cell switching techniques presented.

To increase signal strength, it is best to avoid switching cells at or near the extent of a cell's coverage. Since Strong switches cells at the line of equal strength, Strong has the best possible signal strength behavior. Both Late and Early, on the other hand, have low signal strength just before or just after, respectively, the time they determine to switch cells.

While Late has the minimal switching overhead, it has low signal strength for some movement patterns. While Strong

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has the maximum signal strength, it has high switching overhead for some movement patterns. The switching overhead of the hysteresis techniques are close to the low switching overhead of Late and the signal strength behavior of the hysteresis techniques are close to the maximum signal strength behavior of Strong. Thus, the hysteresis techniques are preferred over the non-hysteresis techniques.

Of the three hysteresis techniques, Strong-DH is the preferred technique. Strong-DH is robust, simple, and offers good performance. The signal strength in Strong-DH is higher than in Strong-SH, while the overhead in switching for this improved signal s trength is relatively minimal. Fur- thermore, the signal s trength in Strong-DH is not dependent on the movement pat tern, which is a desired quality in a cell switching technique. The signal strength of Strong-SH, on the other hand, deteriorates as the movement pa t te rn progresses from an arrow pa t te rn to a random pattern. Lastly, although the switching overhead in Strong-DH is higher than in Late-SH, the improved signal s trength justifies the extra cost.

We did not test different values for t and s and we did not consider other expansion and contraction algorithms for the dynamic hysteresis lines in Strong-DH. If we use different values for these two parameters, or if we use a different algori thm to manipulate the size of the hysteresis region, then perhaps we can reduce the switching cost, increase the signal strength, or both. We plan to test other parameter values and consider other algorithms for Strong-DH in the future.

There is a large delay associated with cell switching in Mo- bile IP. This delay is based on two factors. First , there is a delay as an MN determines tha t it needs to switch cells,

as one or two unreceived packets may only indicate a temporarily blocked signal. Second, there is a delay as the HA updates the MN's new location. All of the results presented in Section 5.2, however, assume tha t the delay to update the pr imary care-of-address is negligible compared to the movement speed. In other words, our simulation environment assumes tha t the HA updates the pr imary care-of-address for an MN before the MN moves more than a simulation step or two. Since this assumption is only realistic when an MN is barely moving, further work in this area is needed. (We note tha t a longer delay puts even more importance on maximizing the average signal strength.) We plan to extend our simulation environment and investigate the effect of the delay associated with cell switching in Mobile IP in the performance of the six switching techniques.

We predict tha t this extension to our simulation environment will have a large effect on the performance of Late and Early for all movement patterns; for arrow-type movement patterns, we believe the performance of Late will deteriorate while the performance of Early will improve. The effect on the performance of Strong, Late-SH, Strong-SH, and Strong- DH should be minimal for random and aimless movement patterns, since these four techniques do not switch cells at extent lines. For arrow-type movement patterns, we believe the performance of Late-SH and Strong-SH will slightly deteriorate. On the other hand, with proper tuning, an expansion and contraction algori thm for the dynamic hysteresis technique should be able to hinder a deterioration in the performance of Strong-DH for arrow-type movement patterns.

A second extension concerns the cellular environment in the simulation. In our simulation, we only consider the over-

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lapping area of two wireless cells. In addition, we do not consider obstacles that can block an MN's signal or interfer- ence. We plan to extend the simulation model in our future work.

8. ADDITIONAL AUTHORS Additional author: Charles E. Perkins (Communications Research Group, Nokia Research Center, 313 Fairchild Drive, Mountain View, CA 94043 email: char l iep~iprg .nokia .com).

In four of the six techniques discussed in this paper, we assume that an MN has the tools necessary to determine either signal strength or signal quality, and that the MN can use this information to determine when it should switch its link-level point of attachment from its current Base to a new Base. Both Late and Early do not assume a tool exists to determine either signal strength or signal quality. Thus, in some situations, Late and Early may be the only options.

We are currently investigating the performance of extensions to the Late and Early techniques. RFC 2002 proposes an algorithm similar to Late, i.e., Lazy Cell Switching [12]. As a counterproposal, a technique similar to Early, i.e., Eager Cell Switching, is also proposed [15]. Although the difference between Early and Eager, or Late and Lazy, is small, the difference affects the performance of these two techniques dramatically. For example, the Eager Cell Switching technique includes an improvement to the Early technique in order to remove the degenerate oscillating region, thus dramatically reducing the number of switches. An investigation into the performance of Lazy and Eager, in order to compare these two techniques, is needed.

9. REFERENCES

[1] J.J. Egli, Radio Propagation Above 40MC Over Irregular Terrain, Proc. of the Institute of Radio Engineers, October 1957, pp. 1383-1391.

[2] G. Forman and J. Zahorjan, The Challenges of Mobile Computing, IEEE Computer (April 1994) 38-46.

[3] S. Hanks, T. Li, D. Farinacci, and P. Tralna, Generic Routing Encapsulation (GRE), Request for Comments 1701, October 1994.

[4] C. Huitema, Routing in the Internet, Prentice Hall, Inc., New Jersey, 1995.

[5] T. Imielinski and B.R. Badrinath, Mobile Wireless Computing, Communications of the ACM 37 (10) (1994) 19-28.

[6] D. Johnson and C. Perkins, Mobility Support in IPv6, Internet Draft: draft-ietf-mobileip-ipv6-12.txt, April 2000.

7. ACKNOWLEDGMENTS We thank Harold Stern, and the anonymous referees, for providing helpful suggestions that improved the quality of this paper.

[7] A. Mehrotra, Cellular Radio: Analog and Digital Systems, Artech House Publishers, New York, 1994.

[81 NSF, Report to the NSF Division of Networking and Communications Research and Infrastructure: Research Priorities in Networking and Communications, Airlie House, Virginia, April 1994.

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[9] NSF, Draft Report to the NSF Division of Networking and Communications Research and Infrastructure: Research Priorities in Wireless and Mobile Communications and Networking, Airlie House, Virginia, March 1997.

[10] K. Pahlavan, T. Probert, and M. Chase, Trends in Local Wireless Networks, IEEE Communications Magazine (March 1995) 88-95.

[11] C. Perkins, Personal Communication, May 23rd, 2000.

[12] C. Perkins, IP Mobility Support, Request for Comments 2002, October 1996.

[13] C. Perkins, IP Encapsulation within IP, Request for Comments 2003, October 1996.

[14] C. Perkins, Minimal Encapsulation within IP, Request for Comments 2004, October 1996.

[15] C. Perkins, Mobile Networking within the IETF, Tutorial at the Second Annual International Conference on Mobile Computing and Networking (MOBICOM '96), November 1996.

[16] C. Perkins and D. Johnson, Mobility Support in IPv6, Proc. Second Annual International Conference on Mobile Computing and Networking (MOBICOM '96) Rye, New York, November 1996, pp. 27-37.

[17] E. Pitoura and B. Bhaxgava, Dealing with Mobility: Issues and Research Challenges, Technical Report CSD-TR-93-070, Department of Computer Science, Purdue University, 1993.

[18] F.J. Ricci, Personal Communication Systems Applications, Prentice Hall, Inc., New Jersey, 1997.

APPENDIX A. THE STRONG-DH ALGORITHM SwitchP = False; ShiftWidth = 0.1 * MaxHysteresis; StepsSinceAction = 0; InPercent=l/2;

while (Step < MaxSteps) { Step++;

/ /Decis ion of where to move and check for switch.

if (StepsSinceAction < s) StepsSinceAction++;

else { / / c o n t r a c t hysteresis region HysteresisWest + = InPercent*ShiftWidth; HysteresisEast - = InPercent*ShiftWidth; InPercent / = 2.0; StepsSinceAction = t-t-l;

/ / R e s e t , but avoid problems with expand. }

if (SwitchP) { if (StepsSinceAction < t) {

/ / e x p a n d hysteresis region HysteresisWest - = ShiftWidth; HysteresisEast + = ShiftWidth; InPercent = 1.0/2.0; / /Rese t the retraction percentage. SwitchP = False;

} StepsSinceAction = 0;

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