Chapter 4. An organization-based cache mechanism for supporting PCS number
5.4 Alleviate COD and VHO overhead of using 3G/WLAN DMS
The development of WLAN/3G DMS is an implementation of terminal- and
service- level heterogeneous network convergence. With the ability to send and receive WLAN and 3G signals simultaneously, DMS combines the benefits of WLAN and ubiquitous 3G communication services in a single terminal. While 3G networks are expensive due to the high cost of spectrum acquisition, and the data rate is limited (up to 2Mbps), a DMS will first adopt WLAN for call origination in a dual- mode communication environment where a WLAN and a 3G network coexist to save communication cost. Besides, the energy for transmission in WLAN is half of that in a 3G network [43], adopt WLAN as the preferred network can extend the standby time of a DMS. When a user moves beyond the coverage of WLAN or when the visited WLAN is congested, the DMS adopts 3G system instead automatically. The determination and alternation of network adoption is transparent to users.
However, when a DMS originates a call via a congested WLAN, the fail of a try in WLAN brings extra effort to set up a call via 3G network and results in a long call-origination delay (COD). The traffic load of calls generated in a congested WLAN eventually offers to the 3G communication system. In addition, the increased signals and processes consume more battery energy. Dual- mode users who often originate calls via busy WLAN may spend a lot of time waiting on call setup and suffer from short standby time of DMS. To avoid the problems, 3G networks shall be given precedence over WLAN in congested hotspots.
For the sake of small and intermittent WLAN service regions, DMS needs to handoff between the 3G system and WLAN frequently. If a DMS moves out of WLAN coverage and enters a 3G cell, or vice versa, an ongoing connection shall be handed over between different network systems and causes a vertical handoff (VHO) process [44]. VHO is an expensive operation while it requires channel reallocations and information exchanges between heterogeneous network systems. If the dwell time of a DMS in a WLAN is short, the expense of VHO may nullify or exceed the communication profit from the connection in the WLAN. From the aspect of network systems, a dual- mode user who moves fast that the dwell time in a WLAN is too short to cover the cost of VHO shall keep on engaging to the visited 3G network rather than be handed over to the WLAN.
For eliminating the drawbacks of long COD, high VHO overhead, and high energy consumption, DMS shall be able to determine which network system to engage to when originating a call or when entering a dual- mode communicatio n environment. But, without the circumstance information of how busy a WLAN is, how large the coverage of a hotspot is, how fast the user moves, and how long a connection will last, a DMS can not make determination. Network systems did not provide the circumstance information to users, therefore network systems and communication protocols should be modified to come out and to share the
information with terminals. It is too expensive and too complicated to be a feasible approach.
Every dual- mode mobile user has his unique mobile and call patterns that can be induced from the history of the user’s itineraries and the behavior of call originations [45]. The history data can be utilized to infer the condition of the visited communication circumstance, which is helpful in determining the preferred network to avoid the drawbacks of conventional dual- mode service.
5.4.1 Issues and problem analysis
The drawback of long COD and heavy VHO overhead happens when users enter or reside in dual- mode communication environments where WLAN and 3G networks coexist. Our approach is to avoid unnecessary attempts on WLAN call-generations in congested WLANs. Here we assume the coverage of WLAN hotspots are a subset of the service areas of 3G networks. The WLAN and the 3G modes are both actuated on a DMS.
The major factor which influences the success of call originations is the traffic intensity (or offered traffic) of a communication environment. A WLAN hotspot can be established in public and strategic areas by cellular operators for extending network coverage, or be established by organizations to facilitate internal communications or save communication costs. The traffic intensity of a WLAN varies with the amount of users, the location of the WLAN, and the observation time epoch.
When the offered traffic load exceeds the carried load, new arrived calls are blocked or lost.
To prevent long COD, a mechanism to infer the communication condition of the visited WLAN before call setups is needed for a DMS. Currently user-ends cannot obtain information about the conditions of the visited communication circumstance.
Neither WLAN nor the 3G network system provides a mechanism for sharing the information with user-ends. A DMS can only infer the communication condition of a visited circumstance from the history of a user’s mobile and call patterns, including when a user used to visit a specific WLAN and how long he used to dwell in the WLAN; it can also calculate the frequency a user originated calls and the possibility of successful call-originations in a specific WLAN. By the gathered and calculated information, a DMS can determine whether it is proper to originate calls via the visited WLAN.
The possibility of successful call-originations decreases along the increase of the traffic intensity. Users experience the condition of the visited communication circumstance when originating calls. For representing the condition of the visited communication circumstance and how much a user relies on the WLAN, a DMS
needs to keep the IDs of the visited WLANs in which calls were originated, the frequencies of call-originations in every visited WLAN, and the probability calls were successfully originated. The probability of successful call-originations in the peak time and the off time can be very different, the observation epoch must also be considered as a parameter of communication conditions.
From another point of view, the drawback of high VHO overhead results from the small and intermittent WLAN service regions. A connecting DMS moves across the border of 3G and WLAN networks causes VHO process to hand over an ongoing connection to a heterogeneous network. The WLAN communication profit from a user depends on how long the communication holds in the WLAN. If a hotspot is small, or a dual- mode user moves fast, the dwell time in the WLAN could be too short to cover the VHO cost. A connection shall not be handed over to WLAN when the VHO overhead is expected to be heavy. For users who have routine itineraries, DMS can learn the average call- holding time in a specific WLAN, and determine if VHO overhead is heavy. DMS can learn the average VHO overhead from users’ moving patterns. A DMS notes VHO and the call lasting time in WLANs as information to denote the VHO overhead.
As a result, the parameters which represent the call and mobile patterns of dual- mode users, and the communication conditions of the visited WLAN can be concluded as the following:
1. The identifiers of visited WLANs (SSID). Every WLAN has a unique service set identifier (SSID). DMS keeps SSIDs of WLANs via which calls were originated.
2. The average call holding time in a WLAN (t_call).
3. The epoch to visit a WLAN (epochi). The grade of service (GoS) of a WLAN differs from the observation epoch. The time of a day can be divided into several epochs to distinguish the communication circumstances of every WLAN.
4. The frequency a DMS originated calls in the communication circumstance (calli).
5. The frequency of successful call-originations (scalli).
6. The last modified time of an entry (lmti) is aim for management purpose.
epoch1 call1 scall1 lmt1
WALN ID (SSIDi)
Call lasting
time (t_calli) epoch2 call2 scall2 lmt2
Table 5-3 An entry of circumstance record on a DMS
We propose to maintain these parameters in users’ DMS to log users’ call and mobile habits, and to estimate the condition of the visited communication circumstance. The parameters relate to every communication circumstance are listed in Table 5-3. Since the memory of a DMS is limited, the oldest entry of the record will
be removed when the memory runs out.
For avoiding long COD and frequent VHO, a set of rules to determine which network system to adopt in dual- mode environments is important.
5.4.2 Design of DMS training algorithm (DTA)
There are two reasons to give precedence of WLAN over 3G for dual- mode communication: The trials of call-originating in a congested WLAN must fail and must be regenerated via 3G network. That is, the traffic load generated by the calls happened in congested WLAN finally is offered to 3G networks, no matter WLAN or 3G is given precedence. Besides, the power consumption of DMS transmission in WLAN is half of that in 3G, WLAN is the preferred network of DMS in dual- mode communication environment.
Based on the concept of saving communication cost, a DMS adopts a WLAN for communication if the WLAN is first-time visited, and adds the circumstance information of the WLAN to DMS record. A modest communication circumstance represents high possibility of successful call originations, users are encouraged to utilize it for communication. In other respects, a user who often originates calls in a specific communication circumstance is considered as strongly relies on the circumstance. Eve n in the peak time, the user may like to originate calls via the WLAN to take chance. However, when users suffer from the frequent fails of the try, they are suggested adopting 3G network directly for shortening COD.
The coverage of WLAN is limited. While VHO is costly, frequent VHO is a notable drawback of dual- mode communication service. When the expected profit from a user will not exceed the operation cost of VHO, an ongoing call shall not be handed over to the visited WLAN. For a WLAN, the profit relates to the call holding time in the WLAN, which grows linearly as the total call lasting time. For users whose dwell time in WLAN is short shall adopt 3G network for communication. Thus the communication resource of WLAN and the 3G network can be utilized more efficiently.
The procedure of training a DMS to determine which network system to adopt is shown in Fig. 5-4.
Visited &
Fig. 5-4 The procedure for choosing a communication network l Step 1: initialization
A DMS checks the SSID of a WLAN after receiving the characteristics beacons of a WLAN and try to originate a call. The DMS increases the call-origination frequencies of the WLAN when a corresponding record exists; otherwise, it adds a new record for the WLAN.
l Step 2: determination:
The rate of successful call originations represents the communication condition of a WLAN. In a modest WLAN, users are encouraged to utilize WLAN for communication. If a user often originated calls via the WLAN, it implies the user strongly relies on the WLAN. Even if the WLAN is busy, users may like to try the WLAN. In the two cases, WLAN is given precedence of call originations.
From WLAN operators’ viewpoint, if the expected revenue from a user may exceed the cost of VHO, the user is welcome to use the WLAN. For a user whose dwell time in a WLAN is too short to generate profit, the user shall keep on engaging to the 3G network rather than be handed over to the WLAN.
The algorithm for determining which network system to adopt is depicted in the following:
If ((visiti == 1) && (callj >0)) {
// The first time the user visit WLAN in epoch j, and the user has stayed for a long time
Use WLAN to make a call;
callj ++;
} else if ((scallj/callj) > threshold_1) { // the rate of success-calls is high Use WLAN to make a call;
callj ++;
} else if (t_calli > threshold_2) {
// the call lasting time in this WLAN is long enough to cover VHO cost Use WLAN to make a call;
} else {
Use 3G to make a call;
}
l Step 3: adaptation:
After a call originated via WLAN is terminated, the DMS recalculate the average call holding time in the WLAN. Besides, an adaptation mechanism is invoked to update the record to present the condition of the communication circumstance.
If (success) { scallj ++;
} else {
// a chasten mechanism callj --;
scallj --;
}
While callj = scallj, according to the inequality (y/x) = (y-1)/(x-1), where x,y>0 and x>y, decrease the value of (scalli /calli) also decrease the opportunity of originating calls via WLAN.
For dual- mode users who have routine mobile and call patterns, the inferred possibility of successful call-originations and the call lasting time are close to the realities, thus the circumstance information will be really helpful. In some special cases such as parades or festivals, the traffic intensity increases suddenly that can not be inferred from the history data, and the DMS training algorithm (DTA) will not be helpful.