Synchronization and handoff management schemes for wireless multimedia systems

Synchronization and handoff management schemesfor wireless multimedia systems q

Azzedine Boukerche *, Sungbum Hong, Tom Jacob

Department of Computer Science, University of North of Texas, P.O. Box 311366, Denton, TX 76203, USA

Abstract

This paper presents handoff management schemes for synchronization algorithms for wireless multimedia systems.

The synchronization and handoff management schemes allow mobile hosts to receive time-dependant multimedia

streams without delivery interruption while moving from one cell to another. They also maintain the correct ordering of

the media components, through the execution of the wireless multimedia application by a means of timestamped

messages passed among mobile hosts, base stations and servers. The timestamp values are used to compute the delay for

each multimedia unit for each server. Furthermore, the proposed schemes always search for a quasi-receiver among the

base stations with which the mobile hosts can communicate to synchronize multimedia units. We discuss the algorithms

and present a set of simulation experiments that evaluate the performance of our schemes, using message complexity

and buffer usage at each frame arrival time. Our results indicate that our schemes exhibit no underflow or overflow

within the bounded delivery time.

� 2002 Published by Elsevier Science B.V.

Keywords: Distributed algorithm; Synchronization; Mobile multimedia; Wireless communications; End-to-end protocol

1. Introduction

Advances in wireless technology that allow

mobile clients to carry multimedia traffic have

served as an impetus for the emergence of newapplications like wireless digital news services and

video on demand. The multimedia units can be

time dependent or non-time-dependent objects.

Video and voice streams are examples of time-

dependent objects. In this paper, we examine

synchronization and handoff management schemes

for time-dependent multimedia streams.

The basic abstraction for a time-constrained

media element is a timed stream of media com-ponents. The term ‘‘media component’’ means a

video frame or audio sample. These components

normally must be kept in temporal order when

they are being played. The process of maintaining

this ordering of the media components is referred

to as multimedia synchronization [2]. There are

two timing aspects for time-constrained elements:

(i) intra-media continuity is subject to a real timeconstraint in handling media elements; and (ii)

inter-media synchronization is subject to temporal

q This work was supported by Texas Advanced Research

Program Grant ARP/ATP-003594-0092-2001.* Corresponding author. Fax: +1-940-565-2799.

E-mail addresses: [email protected] (A. Boukerche),

[email protected] (S. Hong), [email protected] (T. Jacob).

1389-1286/03/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S1389-1286 (02 )00422-X

Computer Networks 41 (2003) 347–362

www.elsevier.com/locate/comnet

mail to: [email protected]

correlation during a playback of media elements

[13].

The synchronization problem under a combi-

nation of wireless/wired networks is more com-

plicated than the problem for wired networks

because wireless networks must use a base stationto deliver packets and have much less resources

[5]. However, studying a combination of wired/

wireless networks is valuable because they can

provide a richer multimedia environment for

mobile users than can pure (cellular) wireless sys-

tems.

Recall that a cellular wireless communication

network [7] has its coverage area partitionedinto cells, where each cell is served by an antenna

or base station (BS). Cell size and shape depend on

signal strength and the presence of obstacles to

signal propagation. As shown in Fig. 1, a set of

base stations is controlled by a base station con-

troller (BSC). Several BSCs are connected to a

mobile switching center (MSC), which manages all

calls in a large geographical area, doing call setupand handoff. The links between the base stations

and mobile hosts are wireless, whereas those be-

tween the base stations and their MSCs are wired.

This architecture allows a base station to com-

municate with many mobile hosts concurrently.

Each mobile host can move from one base station

to another. Even though the synchronization

problem for distributed stored multimedia streamsbased on fixed networks has been well studied

[1,2], to the best of our knowledge, little has been

done for wireless multimedia systems [3].

1.1. Related work

Most of the research work to synchronize play-

out and delivery of distributed stored multimedia

streams has been done for wired networks. In thispaper, we review research work related to the

multimedia synchronization problem. Interested

readers may wish to consult [1,2,8,16] for multi-

media synchronization work done in a wired net-

work.

Most wireless multimedia services still need

more network resources than are currently avail-

able on a wireless network. Depending on resourcesufficiency for wireless networks, access speed

and mobility can vary, so that synchronization

schemes must be developed differently to fit vary-

ing network conditions. In this section, we shall

review the synchronization schemes for several

wireless network types.

A wireless local area network (LAN) provides

wide bandwidth within a geographical area andsupports low speed mobility. In [14], four lip

synchronization schemes are studied with single

transport streams. In [15], a set of lip synchroni-

zation schemes is studied with the single-stream

approach, where the performance of the schemes is

measured on a wireless LAN. In all of these

schemes, a single server is used to store multimedia

units for all models.A personal handy phone system (PHS) is a kind

of micro-cellular digital cordless telephone system

that covers a slightly wider geographical area with

moderately better mobility than a wireless LAN,

but covers much less area than a personal com-

munication system (PCS). Kato et al. [9,10] pro-

posed applications of a slide control scheme for

wireless systems. The first paper [9] showed the slidecontrol scheme to be effective when audio and video

streams are transmitted over two wireless channels

for mobile hosts. In the second paper [10], the au-

thors studied the interleaved transmission of audio

and video for wireless multimedia synchronization

as well as a quality of service control scheme.

A bluetooth link connects mobile terminals with

their peripheral equipment over the ISM (Indus-trial, Scientific and Medical) band. Using this

technology, a synchronization scheme has been

studied under interference from other systems [12].Fig. 1. Network with soft handoff.

348 A. Boukerche et al. / Computer Networks 41 (2003) 347–362

1.2. Contributions of this paper

Although distributed synchronizations for

wired multimedia systems have been an area of

extensive research [1,2], to the best of our know-ledge, there are few reported results on the imple-

mentation of synchronization schemes for wireless

(or a combination of wired/wireless) multimedia

systems.

A combination of wired/wireless networks for

multimedia servers differs significantly from wired

networks: (i) in delivering packets for mobile

hosts, all packets must pass through at least onebase station (BS) to reach a mobile host; (ii) a

mobile host (MH) has small memory and small

screen size; (iii) there is low network bandwidth

between a base station and a mobile host; and (iv)

a MH can get several communication channels

from different BSs. Because of such differences,

conventional synchronization strategies for deliv-

ering multimedia units (MMUs) to mobile hostswithin a certain time cannot be applied in the

mixed environment. A single base station must

pass many MMUs and much control information

to mobile hosts, which can cause congestion at the

base station. Because of the small memory and low

bandwidth at a mobile host, this traffic may also

cause mobile host memory underflow or overflow.

A mobile host can move from one cell to another,which means the communication channel can

change. A mobile host can also hold more than

one communication channel in certain areas [6].

In this paper, we propose handoff management

schemes to support the synchronization of multi-

media units (MMU) for wireless and mobile cli-

ents 1 that use base stations as quasi-receivers to

control and ensure the MMUs correct ordering.The schemes are proposed for managing MMUs

so as to deliver them to mobile hosts on time.

Furthermore, our proposed synchronization and

handoff management schemes cope with network

jitter, end-system jitter, clock drift, and changing

network conditions during soft handoff. Hence, our

schemes are suitable both for synchronizing inter-

and intra-streams during playback video and for

re-synchronizing streams in a wireless network. We

also present a set of simulation experiments to

study the performance of our algorithms.

The remainder of the paper is organized as

follows. Section 2 contains a description of thecommunication model we use in this paper. Sec-

tion 3 presents an outline of our multimedia units

synchronization algorithm for wireless multimedia

systems. Section 4 describes our handoff manage-

ment schemes for multimedia units synchroniza-

tion, which we refer to as one-phase and two-phase

schemes. Section 5 provides their proof of cor-

rectness. Section 6 reports simulation experimentsevaluating the performance of our algorithms for

different arrival patterns. Section 7 presents our

conclusions and future research topics.

2. Communication model

In our design, we assume that our system con-

sists of K scalable server nodes, N base station

nodes and L wireless clients (mobile hosts). Gen-

erally, the system contains a variety of servers,

depending on the needs of the mobile hosts. Forinstance, in a video-on-demand system, a video

consists of many video frames that can be divided

into K equal-size parts, called subframes. These

subframes are equally located at K different server

nodes using a technique called subframe stripping

[11]. During a playback, wireless clients conti-

nually receive subframes from a server. Servers

have admission control capability and also canchange the data transfer rate at the mobile host�srequest.

In our model, all mobile hosts get services from

the same set of servers. Their communication is

through base stations, where many servers connect

to each base station, and each base station con-

nects to many mobile hosts. Also at certain times,

two or three base stations can communicate with amobile host simultaneously (see Fig. 1). Because of

this, we need to assign special roles to the base

stations to represent mobile hosts within their

cells. During the handoff period, there should be a

management function that can handle ordering of

MMU arrivals at the base stations.

1 In the sequel, mobile host and mobile client are taken to

mean the same things.

A. Boukerche et al. / Computer Networks 41 (2003) 347–362 349

3. Description of the wireless multimedia synchro-

nization algorithm

In this section, we present the basic ideas of

MoSync, our distributed synchronization algo-rithm for wireless multimedia systems. Unlike

other types of distributed schedulers, the MoSync

algorithm uses three types of nodes: servers, Quasi-

receivers, and receivers. The servers have type

server, the base stations have type quasi-receiver,

and the mobile clients have type receiver. The

mobile hosts (wireless multimedia clients) have the

responsibility to synchronize the multimedia ob-jects received from multimedia servers. Mobile

hosts report the amount of buffer consumed to the

base station that controls the area where they are

located, and they let the base station knows the

differences among multimedia objects� arrival

times. Once a base station collects all information

about the arrival time of each multimedia object, it

calculates the synchronized time for the nextpackets. A scheduler in the server manages the

transfer of the subframes as part of the frames to

the wireless clients on time.

We now describe the basic protocol of MoSync

for each type of hosts to facilitate understanding

the basic ideas of our multimedia synchronization

algorithm.

As illustrated in Fig. 2, MHl requests multime-dia service through BSm from the server Sk. The

base station has three roles: a messenger role, a

filter role, and a quasi-receiver role. The base sta-

tion, acting as a messenger, passes multimedia

packets to mobile clients. As a filter, it sends a

request for packets to servers, and as a quasi-

receiver, it receives only the first packets from the

servers. After that, the multimedia objects are sentdirectly from the senders to the mobile hosts. It

also calculates the synchronization point for each

server, i.e., Tmax � Ti, where Tmax is the largest ob-

served delay round trip time (RTT) from all

servers and Ti is the observed RTT from the server

Si. When a base station requests the first group of

multimedia objects, it sends the synchronization

point information to all servers.As shown in Fig. 3, the base station receives

messages from two sources, neighboring base sta-

tions and mobile hosts. The messages from mobile

hosts consist of multimedia ‘‘Request’’ messages,

information ‘‘Update’’ messages and ‘‘Done’’

messages. The messages from neighboring base

stations consist of ‘‘On’’ and ‘‘Off’’ messages. On

receiving the ‘‘Request’’ message from a mobile

client, if a base station�s own current synchroni-

zation point is less than the minimum delay time

d, 2 it requests multimedia objects from serverswithout a setup synchronization step. If it is big-

ger than d, then the base station requests dummy

packets 3 to make a fresh synchronization point,

and then requests the multimedia objects as

shown in Figs. 2–4. On receiving the ‘‘Update’’

message from a mobile client as shown in Fig. 3, a

base station gets a message including the differ-

ence between the arrival time and the expected

Fig. 2. Basic protocol for a mobile host.

2 Minimum delay time is a constant value chosen as a

parameter of our algorithm.3 The packet contains only the size of the packet, source and

destination.


arrival time. After the base station gets the mes-

sage, it subtracts the last time difference from the

new time difference. If the result is smaller than

the minimum delay, then it increases the credit for

the server that sent the message. If not, the credit

will not be changed. Note that the credit reflects

the status of the network; i.e., delivery multimedia

units. Accordingly, the base station will send a‘‘Request’’ message or an ‘‘N-Request’’ message.

The ‘‘Request’’ message requests only one multi-

media unit but ‘‘N-Request’’ asks for multiple

multimedia units. If an N-Request message is

being served already, then another ‘‘N-Request’’

can be sent to the server only after either Nmultimedia units have arrived or the time differ-

ence is bigger than d. On receiving the ‘‘Done’’message from a mobile host, a base station will

terminate the multimedia service. On receiving the

‘‘On’’ message from a neighboring base station, a

base station will know that the setup delay time is

available at the neighboring base station. If the

base station receives an ‘‘Off’’ message, then it will

know that the setup delay time is not available on

the neighboring BS.As illustrated in Fig. 4, servers will receive four

types of messages: (i) a message to setup initial

synchronization; (ii) a message to request one

single multimedia object; (iii) a message to request

N multimedia objects; and (iv) a message to in-

terrupt delivery of N multimedia objects. When a

server receives an interrupt (INT) message from a

base station, it terminates the delivery of themultiple multimedia objects and also eliminates all

future deliveries scheduled for the base station.

Then, with a new delay time, it sends out a multi-

media object to the base station. When a server

gets an ‘‘N-Request’’ message from a base station,

the server makes the future delivery schedule for

the mobile hosts and sends the first multimedia

object with it. Upon receipt of the first delivery,the server delivers multimedia objects every RTitime.

When a mobile host receives its first multimedia

object, it calculates the latest arrival time, the

differences between all multimedia object arrival

times, and the buffer usage as follows:

buffer usage ¼ ðNumMMUsÞ � ðPoutTimeÞ� CurTime þ ReqTime;

Fig. 4. Basic protocol for a server.

Fig. 3. Basic protocol for a base station.


where NumMMUs is the number of multimedia

units, PoutTime is the play-out time for an MMU,

CurTime is the current time at the mobile host,

and ReqTime is the time at which the mobile host

requested the MMUs.

4. Description of the handoff management schemes

Handoff management for synchronizing multi-

media units is one of the central issues in a wireless

environment. As illustrated in Fig. 1, there are

basically four types of handoffs: (i) Inter-sectorhandoff where MHs communicate with two sectors

of the same cell; (ii) Inter-cell handoff where MHs

communicate with two or three sectors of different

cells; (iii) three-way soft handoff where MHs

communicate with two sectors of one cell and a

sector of another cell; and (iv) soft-softer handoff

where MHs communicate using two BSs that be-

long to different code division multiple access(CDMA) carriers.

Recall that the basic idea of MoSync is that the

wireless base stations act as intermediaries for the

multimedia streams on behalf of the mobile hosts

and between the hosts and the multimedia servers.

MoSync was designed for time division multiple

access (TDMA) and frequency division multiple

access (FDMA) systems. Those systems use hardhandoff, which offers a single base station to a

mobile host at any moment. On the other hand,

the CDMA system uses a soft handoff, which of-

fers multiple base stations to a mobile host. Hav-

ing multiple base stations available to a mobile

host at a certain location has several advantages.

One of the advantages is that channel capacity can

be increased significantly. Data can also be deliv-ered through multiple base stations.

In this section, we wish to investigate further

the handoff management problem for MoSync and

propose schemes to be able to route messages

when mobile hosts cross the inter-cell area without

the delay found in the MoSync algorithm. In

the course of our experiments, we have inves-

tigated several handoff management schemes.In this paper, we propose two handoff manage-

ment schemes, which we refer to as one-phase and

two-phase schemes, to manage soft handoff for

MoSync. To facilitate understanding our handoff

management schemes for MoSync, we first outline

the basic ideas of the proposed algorithms using

the communication model adopted in Section 2,

then formally describe them by means of pseudo-

code.

4.1. Description of the one-phase scheme

As in MoSync, the one-phase scheme consists of

three parts, one for each type of host, and MHl

requests multimedia service through BSm from the

server Sk. The one-phase scheme uses two types of

base station, primary base station and non-primarybase station. When a mobile host moves into an

area where multiple base stations can see the mo-

bile host, the mobile host asks the primary base

station for a new base station. After the primary

base station gets the message requesting a new

base station from the mobile host, the primary

inserts the name tag of the new base station into its

own BS list. The number of base stations in anarea is no more than three. If there is any base

station already in the BS list, then the primary base

station informs the existing base station about the

new base station. The existing base station sends

‘‘new BS’’ to the mobile host. The base station

may hold MMUs for a very short time until the

primary base station is chosen. The MMUs will be

forwarded to the new primary base station after ithas been chosen. After all the changes are made,

the primary base station sends a short message to

the servers to inform them of the new primary base

station. After the servers have received the mes-

sage ‘‘New Primary BS’’, then the servers send new

MMUs through the new primary base station.

4.2. Description of the two-phase scheme

The two-phase scheme consists of two parts: (i)

Setup Handoff; and (ii) End Handoff. In the first

phase, Setup Handoff performs two major tasks:

updating new arrival base stations and maintain-

ing the synchronization for newly arrived mobile

hosts. If a mobile host can reach another base

station, then the mobile host reports ‘‘new BS ar-rived’’ to its primary base station. If a mobile host

requests MMUs from the new base station and the


base station does not have information aboutdelay times for each server, then the base station

sends a ‘‘Req’’ message to all servers. When a

mobile host receives the first dummy object group,

it calculates the latest arrival time and the differ-

ences between multimedia object arrival times using

MoSync. In the second phase, the End Handoff

procedure deals with the ordering of MMUs and

with the flow of MMUs for a new mobile host.Any base station can be the new primary base

station. The algorithm notifies mobile hosts, base

stations and servers, and then chooses the closest

common node (CCN) from the current primary

base station and new base stations. The new CCN

must be a node within the wireless network area.

If there is not such a node, the MSC of the cur-

rent primary base station will be the CCN untila new primary base station can be found. The

CCN reroutes the MMUs using the new pri-

mary base station and also deliver MMUs in

timestamp order. As soon as mobile hosts move

out from the inter-cell area, the End Handoff will

be terminated. As an MMU handler, the algorithm

handles MMU flows during both soft and hard

handoffs.

4.3. Formal description of the handoff management

schemes

We now formally describe the handoff schemes

by means of pseudo-code (see Table 1 for the no-

tation):

4.3.1. Pseudo-code for one-phase scheme

BaseStationm:

B1: Upon Received ‘‘New base station BSn’’ from

MH

B2: Send ‘‘New base station BSn’’ to BSn

B3: Add BSn in the list of base stations

B4: Upon Received ‘‘New base station BSn’’ from

BSmB5: Send ‘‘New base station BSn’’ to MH

B6: Upon Found MMUs in buffer

B7: Send all MMUs to the new primary BSnB8: Drop BSm as a primary base station

B9: Upon Received MMUs from BSmB10: Send MMUs to MH

B11: Upon Transferred Quasi-receiver role

B12: Send ‘‘New primary base station BSn’’ toserver

B13: Upon Received ‘‘New base station BSn &BSk’’ from MH

B14: Send ‘‘New base station BSn and BSk’’ toBSn and BSk

B15: Add BSn & BSk to the current base sta-

tion list

B16: Upon Received ‘‘New base station BSn &BSk’’ from BSm

B17: Send ‘‘New base station BSn and BSk’’ to

BSkB18: Add BSn and BSk to the current base sta-

tion list

B19: Upon Received ‘‘Dummy Packet’’ from SkB20: If (MH is under BSm) Do ‘‘MoSync’’ al-

gorithm

B21: Else Foreword ‘‘DummyPacket’’ to new

primary BS BSm

B22: Upon Received ‘‘Dummy Packet’’ from BSmB23: Do ‘‘MoSync’’ algorithm like ‘‘Dummy

Packets’’ from Server

B24: Upon Received ‘‘Update MSG’’ from MHl

B25: Send ‘‘New MH with D0k ’’ to Sk

Mobile Hosts:

M1: Upon Received a pilot signal from BSnM2: Send MSG ‘‘new base station’’ to primary

BSm

M3: Upon Received ‘‘New base station BSn’’ from

BSnM4: Update the information that BSm, BSn are

involved in handoff

Table 1

Notations

Symbols Meaning

DSl Play-out rate for MHl

TSik Timestamp for server k

TSil Timestamp for mobile host l

DT 0k Delay time for server k

ST 0k Arrival time from server k

MSik Multimedia substream i


4.3.2. Pseudo-code for two-phase scheme

Setup Handoff // Mobile host notifies current pri-

mary BS about the new assigned BS

Serverk: all the functions in MoSync Algorithm areshared here

Base Stationm:

B1: Upon Received a MSG ‘‘New BSn’’ from

MHl

B2: Add BSn to primary BS Candidate Set //

prepare to move

MobileHostl:

M1: Upon Received a pilot signal from BSnM2: Send MSG ‘‘New BSn’’ to Primary BSmM3: Send Request Dummy MMU to BSn //

setup new synchronization through BSnM4: if (Within a MSC control) Initialize agency

on CCN to both BSn and BSm

End Handoff

Serverk:

S1: Upon Received a MSG ‘‘New Primary BS’’

from BSnS2: Change routing path of MMUs for MHl

S3: Send a MSG ‘‘New Primary BS’’ to BSm //

reply message

BaseStationm: // MSC chooses New Primary BS

B0: Upon Received a MSG ‘‘New Primary BS’’

from MSC

B1: Send a MSG ‘‘New Primary BS’’ to MHl,

and ServerkB2: ‘‘New PBS_Server’’ is set to ‘‘True’’ // Ser-

ver holds old pathB3: Send a MSG ‘‘New Primary BS, Request

MMUs’’ to BSnB4: Activate routing agency on CCN to re-

route MMUs

B10: Upon Received a MSG ‘‘New Primary BS,

Request MMUs’’ from BSnB11: Do Forward MMUs to BSn for MHl

B12: While (‘‘New PBS_Server’’)


from Serverk

B21: ‘‘New PBS_Server’’ is set ‘‘False’’ // we

know Server is changed

B22: Stop Forwarding MMUs to New Pri-

mary BS for MHl

B23: Deactivate the routing agency on CCN toreroute MMUs


from BSnB31: ‘‘PBS_All’’ is set ‘‘TRUE’’ // found a new

Primary BS for MHl

B32: If (MMUs on Buffer) then Forward MMUs

to MHl

B50: Upon Received MMUs from Serverk for a

new Primary BS

B51: If (‘‘PBS_All’’) Send MMU to MHl

through new Primary BSB52: Else Save MMUs on BSm // could not find a

new Primary BS

B60: Upon Received MMUs from BSn for MHl

B61: Send MMUs to MHl

MobileHostl:

M1: Upon Received a MSG ‘‘new Primary BSfrom the new BS’’

M2: Update Delay Information for BSm

5. Proof of correctness

The movement of mobile hosts can be divided

into four types: (i) no moving during the com-munication; (ii) moving from one cell into an inter-

cell area; (iii) moving from an inter-cell area to

another cell; and (iv) moving back to the original

cell shown in Fig. 5. There is no moving mobile

host in the case (i). Therefore, it is considered a

hard handoff, which our previous work has shown

to be correct [3]. Although the other cases are

similar to case (i), we still need to consider theproblem related to ‘‘this case happened before that

case’’. As we can see, we need only to consider

‘‘case (ii) happened before case (iii)’’.

Each mobile host MHl has a message buffer

of finite size b. Suppose that MHl has a constant

play-out rate of r multimedia units per second. If


its servers are not able to keep the buffer suffi-

ciently full, then MHl will not be able to play-out

smoothly. Similarly, if the servers send multimedia

units too fast, the buffer will overflow. Suppose

that the buffer is partially filled. Let t be the point

in time at which the buffer will be empty if no moremultimedia units arrive; let T1 be the time by which

more multimedia units must arrive to guarantee

smooth play-out, given the bounded jitter; and let

T2 be the earliest time at which the buffer could

overflow, given the jitter. Suppose further that

there are K servers Sk sending multimedia streams

to MHl, that each stream has length I , and that dik

is the delay of the ith unit sent by Sk. Let dmaxk ¼

maxfdikj16 i6 Ig and dmin

k ¼ minfdikj16 i6 Ig. Let

ftmk be the time taken for forwarding an MMU

from one base station to another.

Theorem 1. Suppose that the servers Sk are servingthe single mobile host MHl that is moving from onecell to another cell and there is bounded jitter. Themobile host plays-out its multimedia units smoothlywithout buffer overflow whenever the servers Sk obeythe following two constraints:

ðaÞ tP T1 P 2dmaxk þ ftmk and ðbÞ t6 T2 þ 2dmin

k :

Proof. As explained earlier, only case (iii) must be

considered for the delay time dik because of for-

warding delay time ftmk . MHl plays-out its multi-

media units with constant rate r. The server Sksends multimedia units to MHl only as a result of

an initial ‘‘Request’’ message issued from BSm.

Consider the worst case delivery time. Assume that

MHl keeps moving from one cell to another cell.

Each delay time for each server is increased by the

delay time ftmk . If MHl requests MMUs and then

moves, there are two messages, a ‘‘Request’’ and a

‘‘Reply’’, taking 2dmaxk þ ftmk total time for delivery.

Therefore, t cannot be allowed to fall below this

figure, i.e., tP T1 P 2dmaxk þ ftmk . In the best case

where it takes 2dmink for both the ‘‘Request’’ and

‘‘Reply’’ messages to be sent, overflow may occur

if MHl does not play out its current units by the

time T2 plus the minimum time for the next unit to

arrive, i.e., t6 T2 þ 2dmink .

6. Simulation experiments

We have developed a discrete-event model to

simulate a cellular wireless multimedia system on a

combination of wireless and wired networks. We

have assumed that wired networks are used for

communication between base stations and multi-media servers, and that cellular wireless networks

are used for communicating between multimedia

mobile hosts and base stations. In our model, there

are 300 channels available for 60 cells, with system

load equally distributed over all cells. Each multi-

media host has buffer space at least three times as

large as the largest MMU delivered from any

server and at most six times as large.Experimental evaluation of our handoff man-

agement and synchronization schemes requires the

examination of several test cases, comprising many

types of jitter and resynchronization. We evaluate

the performance of MoSync and the two handoff

management schemes for both uniform and non-uniform MMU arrival distributions. Table 2 dis-

plays the simulation parameters we have used inour experiments.

Fig. 5. Location and movements of MH.


In the uniform case, we evaluate the perfor-mance of our scheme assuming uniform MMU

delay under jitter. All cells have the same mul-

timedia unit demand, and the requests from the

mobile host have inter-arrival time k and mean

service time l. All of our schemes are simulated

over four uniform MMU delay ranges: (A) 0–20

ms, (B) 20–40 ms, (C) 40–60 ms, (D) 60–80 ms.

MMU delay times for all of the multimediarequests are evenly distributed between the mini-

mum and maximum delay times, with the mini-

mum delay time for delivery assumed to be 50 ms

in each case and with the delay time for routing

between base stations assumed to be 20 ms. As we

shall see, the different jitter ranges have different

effects. For example, jitter ranges A and B will

cause overflow after some point while range D willcause underflow.

In the second, non-uniform case, an exponential

distribution is assumed. For this model, we simu-

late the algorithm over three MMU delay ranges:

(A) 0–200 ms, (B) 0–400 ms, (C) 0–600 ms. The

ranges are exponentially distributed with mean

values of 20, 40 and 60 ms, respectively. We set an

upper bound for each non-uniform distribution toavoid failure caused by very large delays in deli-

very from MMUs to mobile hosts. The mean

communication session is set to 20 MMUs, and

mean buffer size of a mobile host is set to three

times the size of an MMU. These ranges will also

exhibit different behaviors, as we shall see later.

We evaluate the performance of our algorithms

using variable-sized buffer space for each mobile

host and a variable number of requests from the

mobile hosts. We also choose the following per-

formance metrics to evaluate our schemes:

ii(i) message complexity, which measures the

overhead of our scheme in terms of the num-ber of messages needed to satisfy the user�smultimedia request

i(ii) buffer usage, which measures the synchroni-

zation behavior for each mobile host

(iii) underflow, which measures the average defi-

ciency in the client�s buffer of the number

of MMUs currently needed for smooth

play-out(iv) overflow, which measures the average num-

ber of MMUs that overflow a mobile client�sbuffer.

6.1. Experimental results

In this section, we divide our set of simulation

experiments into two parts. In the first part, wesimulate the one-phase algorithm with MoSync. In

the second part, we implement the two-phase

scheme with MoSync. In both parts, we evaluate

both algorithms using the above metrics. The fol-

lowing experimental data was obtained by aver-

aging several runs.

6.1.1. Part A for the one-phase scheme

In this section, we implement a one-phase

scheme that controls buffer usage and reduces the

number of messages between source and quasi-

receiver. In both parts, we evaluate the one-phase

scheme using the above metrics. The results ob-

tained for buffer usage are shown in Figs. 6 and 7in both uniform and non-uniform cases during

handoff.

In Fig. 6, buffer usage for the ranges A1, A2 and

A3 is always greater than 0 and less than 18

MMUs, which means that all MMUs can continue

to play-out smoothly over the full range of 50 re-

quests. With range A4, underflow occurs before the

request is received. Note that the rate of claimedbuffer space is the number of frames divided by the

number of requests. For example, with range A4, a

mobile host needs buffer space for 38 MMUs on

Table 2

Simulation parameters

Number of cells 60

Number of servers 4

Buffer size of a MH Three times an MMU

Ployout time/MMU 100 ms

Mean service time lArrival rate kForward time to BS 20 ms

Rate of handoff 5% of MMUs

RTT to request/deliver

an MMU

50 ms

Jitter (uniform) A1[0–20], A2[20–40], A3[40–60],

A4[60–80] ms

Jitter (non-uniform) B1[0–200], B2[0–400], B3[0–600] ms


average over the full 50 requests, as we shall see inPart B but in Part A, A4 needs buffer space for only

22 MMUs on the same requests. The claimed

MMUs are 42% less than in the case of Part B, as

we shall see later. In Fig. 7, buffer usage for the

range B1 is always greater than 0 and less than 5

MMUs, which means that all MMUs can continue


quests. With ranges B2 and B3, underflow occursbefore the request is received. Note that the rate of

claimed buffer space is the number of frames di-

vided by the number of requests. Both Figs. 8 and

9 show that our scheme performs quite well in

all ranges of jitter, because the average number

of messages per MMU is bounded by 12. Inthe special case of ranges A1, A2 and B1, message

complexity is close to 11 after two MMU requests

in the uniform case. These results are worse than

those obtained in Part B, as we shall see later.

Both Figs. 10 and 11 show the results of our

schemes in both uniform and non-uniform cases of

underflow. In Fig. 10, only the data for range A4

produces underflow after 78 MMU requests. Also,almost 100% of new requests cannot receive en-

ough MMUs to play-out smoothly. The ranges A1,

and A2 do not produce any underflow until request

100 and range A3 cause a little overflow. As we

can see in Fig. 11, the data for ranges B2 and B3

Fig. 7. Buffer usage for one-phase algorithm: non-uniform.

Fig. 8. Message complexity for one-phase algorithm: uniform.

Fig. 9. Message complexity for one-phase algorithm: non-

uniform.

Fig. 6. Buffer usage for one-phase algorithm: uniform.


produces underflow after 62 MMU requests.After 110 MMU requests, almost 100% of new

requests cannot receive enough MMUs to play-out

smoothly. The range B1 does not produce any

underflow until request 150. In Figs. 12 and 13, we

portray the overflow rate for both uniform and

non-uniform cases. Recall that the overflow rate

represents the average number of MMUs that

overflow a mobile client�s buffer, while the under-flow rate represents the average deficiency in the

client�s buffer of the number of multimedia units

currently needed for smooth play-out. As we can

see, our scheme exhibits a very low overflow and

underflow rate in all ranges of jitter and in both

uniform and non-uniform cases.

6.1.2. Part B for two-phase scheme

Figs. 14 and 15 portray the buffer usage of the

two-phase algorithm according to the number ofmultimedia requests. As we can see in Fig. 14,

ranges A1, A2 and A3 are always greater than 0 and

less than 20 which means that all MMUs can play-

out smoothly as long as they hold proper buffer

size. Range A4 requires about 38 MMUs more to

play-out smoothly at the same number of requests

at 15 requests. In Fig. 15, buffer usage for the

range B1 is always greater than 0 and less than 5MMUs, which means that all MMUs can continue


quests. With ranges B2 and B3, underflow occurs

Fig. 12. Overflow rate for one-phase algorithm: uniform.

Fig. 13. Overflow rate for one-phase algorithm: non-uniform.

Fig. 10. Underflow rate for one-phase algorithm: uniform.

Fig. 11. Underflow rate for one-phase algorithm: non-uniform.


before the request is received. Figs. 16 and 17

portray the message overhead of our algorithm for

the uniform. As we can see in Fig. 16, ranges A1

and A2 need a message complexity that is 8 after 2

requests while range A3 and range A4 need a mes-sage complexity of only 9 after 15 requests. Fig. 17

portrays the message overhead of our algorithm

for the non-uniform case. As we can see in Fig. 17,

range B1 needs a message complexity that is 8 after

2 requests while range B2 and range B3 need a

message complexity of only 9.5 after 2 requests. In

Figs. 18 and 19 we have a high rate of underflow

with range A4 in the uniform case while ranges A1,A2 and A3 do not produce underflow before 100

MMU requests. The ranges A2 and A3 show around10% underflow rate after 100 MMU requests. In

Fig. 19, our results indicate that the data in range

B1 do not cause underflow. In our simulations, the

rate of underflow for the ranges B2 and B3 reaches

100% at 62 requests. In Figs. 20 and 21, we portray

the overflow rate for the uniform and non-uniform

cases. As we can see, our scheme performs quite

well for all ranges of jitter.Based upon the results above, we can conclude

that the one-phase scheme in the case of reason-

ably bounded jitter supports inter-stream syn-

chronization with an acceptable delay. Note that

the one-phase scheme uses a delay time to allow

Fig. 15. Buffer usage for two-phase algorithm: non-uniform.

Fig. 16. Message complexity for two-phase algorithm: uniform.Fig. 14. Buffer usage for two-phase algorithm: uniform.

Fig. 17. Message complexity for two-phase algorithm: non-

uniform.


intra-synchronization and thereby guarantees

inter-stream synchronization as well. Our results

indicate that the two-phase algorithm exhibits amuch better message complexity than the one-

phase scheme.

7. Conclusion and future research

Synchronization among mobile clients and

multimedia units has proven to be a very chal-lenging problem in a wireless environment when

compared to a wired one. In this paper, we have

presented MoSync, a distributed synchronization

mechanism for wireless multimedia systems. We

have proposed two handoff management schemes

to support MoSync and the synchronization of

multimedia units (MMU) for mobile clients in a

distributed and wireless multimedia system thatuses base stations as quasi-receivers to control the

MMUs synchronization. The proposed solutions

with MoSync cope with network jitter, end-systemjitter, clock drift, and changing network conditionsduring a soft handoff. Hence, our schemes are

suitable both for synchronizing inter- and intra-streams during playback video and also for re-synchronizing streams in a wireless network. Theschemes are proposed for managing MMUs and

delivering them to mobile hosts on time. We have

also presented a set of simulation experiments that

study the performance of our algorithms. We haveFig. 20. Overflow rate for two-phase algorithm: uniform.

Fig. 21. Overflow rate for two-phase algorithm: non-uniform.

Fig. 19. Underflow rate for two-phase algorithm: non-uniform.

Fig. 18. Underflow rate for two-phase algorithm: uniform.


simulated the proposed schemes evaluating their

performance in several cases using different arrival

patterns. First, we have measured their message

complexity and buffer usage, then we have re-

ported our algorithm�s performance under varieties

of jitter in terms of the amount of underflow andoverflow at each mobile host.

As future work of this research, we plan to

enhance our synchronization protocols by includ-

ing the QoS provisioning and guarantees between

the end systems, and introduce analytical and

simulation models to investigate and obtain crucial

performance characteristics of our schemes. Next,

we intend to examine the potential importance ofpacket scheduling on the performance of MoSync.

Future work is needed to determine the impact of

poor and good packets scheduling on a wider

range of wireless multimedia applications [4,7].

Acknowledgements

Heartfelt thanks to Ioanis Nikolaidis, and

Harry Rudin, the Joint Editor-in-Chief of Com-

puter Networks, for reviewing the manuscript and

providing criticism. Thanks are in order to the

anonymous referees for their valuable comments

and suggestions that helped us improve the quality

of this paper.

References

[1] E. Biersack, W. Geyer, Synchronization delivery and play-

out of distributed stored multimedia streams, in: Multi-

media Systems, vol. 7, ACM/Springer, New York/Berlin,

1999, pp. 70–90.

[2] G. Blakowski, R. Steinmetz, A media synchronization

survey: reference model, specification, case studies, IEEE

Journal on Selected Areas in Communications 14 (1) (1996)

5–35.

[3] A. Boukerche, S. Hong, T. Jacob, A synchronization

scheme of multimedia streams in mobile systems: Proof

and correctness, Proc. 26th IEEE International Conference

on Local Networking, Tampa, FL, 2001, pp. 638–645.

[4] A. Boukerche, S. Hong, T. Jacob, Synchronization proto-

cols for wireless multimedia systems, Proc. IEEE Interna-

tional Workshop on Mobility Management and Wireless

Access. Fort Worth, TX, 2001, pp. 25–30.

[5] G.H. Forman, J. Zahorjan, The challenges of mobile

computing, IEEE Computer 27 (6) (1994).

[6] V.K. Garg, IS-95 CDMA and CDMA2000, Prentice Hall,

Englewood Cliffs, NJ, 1999.

[7] Y. Ishibashi, S. Tasaka, T. Takeo, A performance

comparison of media synchronization schemes for collab-

orative systems in an interconnected ATM-wireless LAN,

Proc., IEEE PIMRC�98, 1998, pp. 265–271.

[8] T.V. Johnson, A. Zhang, Dynamic play-out scheduling

algorithms for continuous multimedia streams, in: Multi-

media Systems, vol. 7, ACM/Springer, New York/Berlin,

1999, pp. 313–325.

[9] M. Kato, Y. Kawai, S. Tasaka, Performance evaluation of

media synchronization in PHS with the H.223 Annes

multiplexing protocol, Proc. IEEE ICUPC�98, 1998, pp.

183–190.

[10] M. Kato, S. Tasaka, K. Nakamura, A comparison of

media synchronization quality between TCP and UDP in

PHS Internet access, Proc. IEEE PIMRC99, 1999, pp.

1092–1097.

[11] L. Lamont, L. Li, R. Brimont, N.D. Georganas, Synchro-

nization of multimedia data for a multimedia news-on-

demand application, IEEE Journal of Selected Areas in

Communications 14 (1) (1996) 264–278.

[12] H. Okura, M. Kato, S. Tasaka, A media synchronization

experiment on continuous media transmission in Bluetooth

LAN access, Proc. IEEE PIMRC 2001.

[13] M.J. Perez-Luque, T.D.C. Little, A temporal reference

framework for multimedia synchronization, IEEE Journal

of Selected Areas in Communications 14 (1) (1996) 36–51.

[14] S. Tasaka, Y. Ishibashi, Stored media synchronization

schemes in ATM and wireless networks: A performance

comparison, Conf. Rec IEEE ICUPC�97, October 1997,

pp. 766–772.

[15] S. Tasaka, H. Nakanishi, Y. Ishibashi, Dynamic resolution

control and media synchronization of MPEG in wireless

LANs, Conf. Rec IEEE GLOBECOM�97, November 1997,

pp. 138–144.

[16] T. Znati, R. Simon, B. Field, A network-based scheme for

synchronization of multimedia streams, TR-95-02, Dept.

of Computer Science, Univ. of Pittsburgh, Pittsburgh, PA,

1995.

Azzedine Boukerche is an AssistantProfessor of Computer Sciences at theUniversity of North Texas, and Di-rector of the Parallel, Distributed andMobile Systems (PARADISE) Re-search Laboratory at UNT. Prior tothis, he was working as a Senior Sci-entist at the Simulation Sciences Divi-sion, Metron Corporation located inSan Diego. He was employed as aFaculty at the School of ComputerScience McGill University, and alsotaught at Polytechnic of Montreal. Hespent a year at the JPL-California In-

stitute of Technology where he contributed to a project centeredabout the specification and verification of the software used tocontrol interplanetary spacecraft operated by JPL/NASALaboratory.

His current research interests include multimedia system, wire-less networks and mobile computing, distributed computing,


distributed interactive simulation, and VLSI designs. Dr.Boukerche has published several research papers in these areas.He was the recipient of the best research paper award atPADS�97, and the recipient of the 3rd National Award forTelecommunication Software 1999 for his work on a distributedsecurity systems on mobile phone operations, and has beennominated for the best paper award at the IEEE/ACMPADS�99 and ACM MSWiM2001. He was the Program Chairof the IEEE International workshop on Distributed Simulationand Real Time Applications (DS-RT�99), the Program Chair ofthe ACM Workshop on Modeling, Analysis and Simulation ofWireless and Mobile Systems (MSWiM�99), the General Co-Chair of the principle Symposium on Modeling Analysis, andSimulation of Computer and Telecommunication Systems(MASCOTS), in 1998, the General Co-Chair of the ACMworkshop on modeling Analysis, and Simulation of mobile andwireless Systems, (MSWiM 2000), and the General Chair of theIEEE International workshop on Distributed Simulation andReal Time Applications in 2000, a Guest Editor for VLSI Design,the Journal of Parallel and Distributed Computing (JPDC),ACM Wireless Networks (WINET), and ACM Mobile Networksand Applications (MONET). Boukerche serves as the ProgramCo-Chair of the 35th Annual Simulation Symposium, ProgramCo-Chair of 10th IEEE/ACM MASCOTS 2002, Program Co-Chair of IEEE MobiWAC 2002, Program Vice Chair ofWWW�02, and General Co-Chair of the Int�l InformationTechnology Symposium (I2TS�2002). He serves as a Vice pro-gram Co-Chair for EuroPar 2003. He has been a member of theProgram Committee of Globecom, ICPP, MASCOTS, ICC,ICCI, MSWiM, PADS, VTC and WoWMoM conferences. Dr.Boukerche is a member of IEEE and ACM, and is an AssociateEditor of TRANSACTIONS of the SCS.

Sungbum Hong received his BS inElectronics Engineering from Kook-mim University, Seoul, Korea, in 1985.He had worked at the Oriental Tele-communication Co., Sungnam, Korea,as a software engineer from 1984 to1987.

He received a MS degree in Com-puter Science from University of Ten-nessee, Chattanooga, TN., in 1993. Heis currently a Ph.D. candidate in theInterdisciplinary Program in Informa-tion Science at University of NorthTexas, Denton, TX. He have been in-

volved in research in wireless computing since 1997. His re-search interests are in system performance and evaluation ofparallel and distributed simulation systems and the design ofdistributed algorithms for wireless communication systems.

Tom Jacob received his Ph.D. inMathematics from Emory Universityin 1974. Currently Associate Professorof Computer Science at the Universityof North Texas, he has also been onthe faculty at the University of NewOrleans and the University of Dallas.His research interests are in distributedcomputing, networking and telecom-munications.


Documents

Synchronization and handoff management schemes for wireless multimedia systems