IMPROVED CHANNEL SWITCHING FOR HYBRID

UNICAST/BROADCAST MOBILE TELEVISION

Stefan Diepolder and Jan Kritzner

Chair for Communication and Distributed System, RWTH Aachen University, Templergraben 55, Aachen, Germany

Keywords:

Mobile TV, Switching Algorithms and Multimedia Delivery.

Abstract:

Today’s 3G networks enable the delivery of mobile television services to the user. However, the distribution

mechanism differs from conventional television broadcasting. For an effective mobile TV system both unicast

and multicast transmission are combined.

One weak point of digital television is channel switching. Both the underlying network and the data structure

impose partially unavoidable delays. In this paper we present a mobile television architecture which supports

fast stream switching and present different techniques to reduce at least the perceived switching duration. The

different switching scenarios between unicast and multicast are discussed in detail.

This paper presents work in progress, so both already implemented and future extensions are described.

Though the implementation has been done with a 3G background the algorithms can be applied to every

digital television scenario.

1 INTRODUCTION AND

PROBLEM STATEMENT

One important problem of digital television is chan-

nel switching. During the standardisation of today’s

digital television architectures the focus had been on

deﬁning the channel characteristics similar to tradi-

tional analogue TV. However, this resulted in the

problem of slow switching between different chan-

nels. Especially the advent of conventional digi-

tal television via DVB-S and DVB-T (ETSI, 2004)

has confronted a large user base with this problem

(Knoche and McCarthy, 2005).

The next subsection will technically introduce the

problem of channel switching. Afterwards, the trade-

off between switching delay and video quality will be

discussed. In the next section ﬁrst ways to overcome

the problem with advanced techniques are shown,

which are reﬁned in section 3. First results are given

in the Conclusion.

Digital video compression technology is based on

reducing redundancy in the video stream. This redun-

dancy may either be spatial, i.e. within a frame, or

temporal, i.e. between different frames. This tempo-

ral correlation introduces dependencies between pic-

tures, i.e. some pictures can not be displayed without

others. I-frames are independent of other frames but

P-frames depend on previous frames. This backward

dependency chain ends with the previous I-frame. A

group of possibly dependent frames is called Group of

Frames (GOF). In case of a channel switch necessary

frames may not be available at the receiver, and de-

pending ones may be distorted during playback until

receiving the next I-frame of the new channel.

The next important factor is the time needed for

the access onto the data of the new channel. This

may either be a physical channel switch where a re-

ceiver has to be tuned to a new frequency, or some

logical switch where some transport connection has

to be opened.

Therefore, we see that there are two main causes

for the channel switch delay:

• physical and logical transport channel switch,

• video structure.

1.1 Mobile Television Switching

Architecture

For mobile television several purely broadcast-based

standards like DVB-H (Faria et al., 2006) have

been deﬁned. In this document an infrastructure

based on a 3GPP cellular network is proposed. We

combine a Packet Switched Streaming (PSS) server

(3GPP, 2005), which provides a unicast streaming ser-

vice, with a Multimedia Broadcast/ Multicast Service

213

Diepolder S. and Kritzner J. (2008).

IMPROVED CHANNEL SWITCHING FOR HYBRID UNICAST/BROADCAST MOBILE TELEVISION.

In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 213-218

DOI: 10.5220/0002021902130218

 SciTePress

(MBMS) (3GPP, 2004) extension.

This way we are able to transmit frequently re-

quired channels via multicast and some less fre-

quently requested channels directly to the particular

receivers via unicast. Unicast mobile TV must have

a back-channel for signaling channel selection, and

the server can do the switch without the need of re-

establishing the connection to the client. For broad-

cast channels no interaction with the server is neces-

sary. Different multicast channels may or may not

share one bearer. The temporal relation between uni-

cast and multicast is unknown.

To speed-up the process of switching channels in

a mobile TV environment it is necessary to add some

extra functionality to mobile clients and to the packet

switching server architecture. An algorithm for the

switch from unicast to multicast may require access

of the PSS server to the multicast data streams. There-

fore, a uniﬁed server for multicast and unicast is nec-

essary (see ﬁgure 1).

Figure 1: Proposed End-to-End Mobile TV Client/Server

Architecture.

The client’s architecture has to be improved, also.

In addition to an existing conventional unicast PSS

client an advanced multicast receiver is necessary. It

consists of a multicast receiver, an audio/video de-

multiplexer, and a buffer. The buffer consists of sep-

arated queues for video and audio. The usage is eas-

ier given that the queues of the different channels are

logically separated. However, they may dynamically

share the same memory space because they are not

ﬁlled simultaneously. The concepts shall be ready

to be deployed at the application level because the

ﬁrmware of today’s mobiles is closed. However, in

the long run it is possible to integrate new functions

into the ﬁrmware.

The buffer needs an advanced interface to the ap-

plication enabling the direct access to different parts

of the stream. The normal functionality like fetching

the next frame from the buffer or maybe ﬂushing is

not sufﬁcient. Special access to different Presentation

TimeStamps (PTSs) is needed for improved switching

by commands like ”Get PTS of next Frame”, ”Skip to

PTS”, ”Get ﬁrst PTS”, and ”Get last PTS”.

For our work we are assuming that the client is

capable of receiving more than one bearer at a time.

At the time of writing no such mobile user equipment

exists but it is planned to be available soon. Never-

theless, it can receive multicast and unicast simulta-

neously which is the basis of the enhanced channel

switching algorithms. In our scenario the MBMS ar-

chitecture may impose a delay of up to several sec-

onds onto the TV data. However, this is not important

for channel switching. There is no requirement for

temporal alignment between different channels.

As shown in ﬁgure 1 the proposed architecture is

combining PSS and MBMS into one. The dotted Con-

trol Channel from client application back to the PSS-

Server is necessary for some switching algorithms,

especially when switching from unicast to unicast and

from multicast to unicast.

1.2 Switching Delays and Problem

Statement

Several aspects affect the duration of a channel

switch. First there is the physical time needed to

switch a physical bearer. This delay occurs even when

the lower layers are technically able to receive more

channels simultaneously but the new channel the user

wants to switch to is unknown in advance. As an ex-

ample consider a switch between two multicast chan-

nels when the receiver is able to receive two multicast

bearers simultaneously. Given that both channels are

transmitted over one bearer the delay is zero. Given

that they are transmitted over different bearers, the

bearer carrying the new channel must be activated. If

the old bearer is received further on, the user can zap

back to the old channel with much less delay. When

unicast streaming is taken into account it is possible to

keep transport connections or sessions open to reduce

this kind of delay.

The ﬁrst part of the video decoding delay is buffer-

ing time. Due to the fact that usually the same time of

the old channel has been buffered as will be buffered

for the new channel, the buffering delay can be con-

cealed by playing out the last channel. Yet, the user

usually wants a swift reaction onto his channel switch

command – the old content is not what the user ex-

pects. Therefore, the buffering delay for mobile tele-

vision should be quite low. In case of multicast

streaming, the buffering delay only needs to accom-

modate the different transmission delays for frames

of different size. Unicast streaming protocols may

beneﬁt from higher buffering delays given that lost or

corrupted data frames can be corrected by retransmis-

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214

sions. This results in a trade-off between the quick-

ness of frame switching and the overall quality. The

last delay which has been mentioned consists of the

time between the scheduled playout of the ﬁrst re-

ceived video frame until the ﬁrst undistorted frame

can be shown. Given that after a channel switch the

receiver has to begin the decoding at a random posi-

tion within a GOF, the expected duration of the dis-

tortion is half the duration of a GOF. Again, there

is a trade-off between video quality and the speed of

switching: Long GOFs result in optimal encoding ef-

ﬁciency but slow switches.

2 CHANNEL-SWITCHING

ALGORITHMS

Now, we propose new algorithms to speed up chan-

nel switching which are more intelligent compared

to the conventional ”play out what you have when

you should” approach. Generally we are not trying

to speed up the time until data can be received from

the network but to reduce the perceived time until the

presentation of the new channel starts. We will take

any possible combination of switches between unicast

and multicast into account, and propose algorithms

tailored to the problem as much as possible. Further-

more, we will distinguish between audio and video

data.

2.1 Unicast to Unicast

Given that a user switches from channel 1 to channel

2, the unicast streaming system begins the transmis-

sion with the temporal nearest I-frame which is avail-

able in the transmission buffer. Therefore the client

sees some kind of virtual channel 2 which has some

temporal displacement. Therefore depending on the

time of their switching, two users viewing the same

TV channel which stand next to each other may have

a temporal displacement of up to one GOF-length in

the content they view. Usually this is acceptable and

people know this effect from the different delays for

analogue and digital conventional television. How-

ever, users may experience it as bad service if other

users see important information like goals in a soccer

game some seconds earlier than others. A more im-

portant problem is the strongly increased bandwidth

requirement given that a user zaps through multiple

channels. Then many large I-frames are transmitted,

and the bandwidth requirement exceeds normal val-

ues. Ideas to mitigate this problem are discussed in

section 3.

2.2 Multicast to Multicast

In the unicast-unicast scenario it is possible to reduce

the switching delay on the server side. In contrast to

the unicast-unicast scenario, in a pure multicast envi-

ronment the different multicast channels are simply

transmitted, possibly even without knowledge how

many subscribers use a certain service or how they

change channels. Therefore, a purely client-based so-

lution is necessary. A typical switching scenario de-

picting both the reception and playout over the time

is given in ﬁgure 2. A simpler graphic depicting the

playout process only can be found in the upper half of

ﬁgure 4. It is compared to the conventional approach

starting the video with the ﬁrst I-frame and skipping

undecodable frames from the ﬁrst GOF. Here d

GOF

and d

Bu f

denote the GOF and buffering duration re-

spectively.

The new algorithm works as follows: After the

switching command the receiver tunes to channel 2

Switch

). When the ﬁrst I-frame of the new channel

is received (after an average time of 0.5 · d

GOF

) it is

presented immediately. Until then either the playout

of the previous channel may continue or the old chan-

nel freezes to indicate a pending channel switch. Af-

ter the presentation of the ﬁrst I-frame it remains on

the screen until its normal presentation time elapses

(dashed area). Then consecutive P-frames are de-

coded normally.

The early presentation of that I-frame may result

in dropping the last P-frames of the previous channel,

and the associated audio-data will be dropped. Fur-

thermore, the ﬁrst undecodable P-frames of the new

channel will be dropped - but the audio data with

the according timestamps will be presented. There-

fore, the new channel begins with the presentation

of a still image, then audio is added, and afterwards

movement. This is the desired working scenario and

referred to as scenario 1.

However, it is not always possible to present the

ﬁrst I-frame early enough. The scenario as described

above only works with sufﬁcient buffer delay, i.e. the

I-frame has to be received before the playout of the

previous channel ends, as depicted in ﬁgure 2. Given

that the buffering delay is too low (ﬁgure 3) the old

channel ends and the picture has to freeze until the

ﬁrst I-frame of the new stream has been received. This

is referred to as scenario 2 which can be found in the

lower half of ﬁgure 4. For this scenario two cases are

possible: The I-frame is received sufﬁciently early,

or audio data of the new channel could be presented

before the I-frame is received. Whether to drop this

audio data or not depends on psychological factors.

Playing back audio without the video may irritate the

viewer. However, a switch-command taking too long

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215

Figure 2: Switch without buffer underruns (optimal case).

Figure 3: Switch resulting in an empty buffer due to late

I-frame.

Figure 4: Multicast Switching - Simpliﬁed Playout Only.

to show any effect is perceived as bad, also.

Whether the buffering delay is sufﬁcient for avoid-

ing a freeze or not depends on the time of the switch.

However, the probability of the desired and the sub-

optimal case depends on both the buffering delay and

the GOF length: If the buffering delay is larger than

the GOF-length plus the switching delay, the I-frame

will deﬁnitely be available (ﬁrst case). Otherwise the

probability for this optimal case is:

Buf

GOF

+ d

Switch

After the switch, it will take on average half a

GOF until an I-frame is received, hence the old chan-

nel will be shown for

E [d

Video

] =

GOF

+ d

Switch

Then the ﬁrst I-frame can be shown. Beginning with

the presentation of this ﬁrst I-frame there is a period

of silence until the presentation time for the ﬁrst audio

packet has been reached:

E [d

Audio

] = d

Switch

+ d

Buf

after which the normal audio playout begins, and the

channel switch is completed.

Compared to switching without an algorithm there

is no freeze of the old video, and audio can be pre-

sented on average half a GOF earlier than before.

The second scenario is more likely for short

buffering delays compared to the GOF-length:

= 1 −

buf

GOF

+ d

Switch

The expected duration of the old channel freeze

before the ﬁrst I-frame can be shown is

E [d

Freeze

] = d

GOF

+ d

Switch

− d

buf

2.3 Multicast to Unicast

The switch from multicast to unicast is done similar to

the switch from unicast to unicast. The transport con-

nection is kept open if possible, and the client requests

the new channel from the server. Then the server

starts the unicast transmission of the new channel with

an I-frame which can be presented directly, and au-

dio is added when the buffer is ﬁlled sufﬁciently. It

is possible that the reserved bandwidth of the uni-

cast bearer is not available immediately but only af-

ter some seconds. This problem can only be solved

by over-provisioning of the unicast channel. Then the

amount of buffered playout time is larger than the du-

ration of the buffering process.

2.4 Unicast to Multicast

In the ﬁrst step the switch from unicast to multicast

is identical to the switch between two multicast chan-

nels. The scenario with the freeze (scenario 2) ben-

eﬁts when one I-frame of the new channel is trans-

mitted before stopping the unicast bearer. Then the

playout of the old channel ends with the presentation

of a frame from the new channel.

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3 FURTHER IMPROVEMENTS

In the latter sections simple algorithms and a basic ar-

chitecture for mobile TV have been introduced. They

have already been implemented. However, this paper

presents work in progress. More advanced ideas and

extensions are discussed now. One general technique

to reduce the experienced switching delay is adaptive

playout; it could be used for any switching scenario.

In the following it is shown which further improve-

ments could be made to the presented algorithms.

3.1 Unicast to Unicast

For unicast-unicast switching two improvements

could be made: One on the server side, and another

one by replacing the client architecture. One prob-

lem of a switch is the peak in the required bandwidth.

This happens because a new channel always begins

with an I-frame. Especially when a user simply zaps

through different channels quickly, the reserved rate

of the bearer would not be sufﬁcient. This could

be solved by encoding specialised switching GOFs

with an I-frame of reduced quality, and consecutive

P-frames with better quantisers gradually improving

the video quality towards the end of the GOF. When

a new channel is requested the server transmits such a

switching GOF at the beginning, and afterwards con-

tinues with the presentation of the normal TV chan-

nel. The lower quality of the beginning of the ﬁrst

GOF reduces the necessary bandwidth of a switching

event, and mitigates the effects of channel hopping.

Another approach to achieve the same goal could

be the usage of switching P-frames for the channel

switch instead of I-frames. However, it is not guaran-

teed that there is enough correlation between the old

and the new channel to enable a substantial reduction

in the required bandwidth.

On the client side advanced control over the play-

out buffer would be beneﬁcial. Especially the de-

tection of the transition between one channel and

the other is important for advanced playout control,

e.g. a quick presentation of the ﬁrst I-frame of the

new channel as done by a multicast-multicast switch.

Up to now the switching duration is identical to the

buffering time.

3.2 Multicast to Multicast

Given that the client is able to receive more than one

multicast channel at a given time it could be beneﬁ-

cial to receive the data of both channels simultane-

ously. For example, conventional TV sets often have

a special button to go back to the previous channel.

Given that both of these channels are received such

a channel switch could be performed much faster. A

more complicated scenario would use a unicast con-

nection to support channel switches. When the user

changes from one multicast channel to the other the

server pushes one I-frame of the new channel to the

client. However, this technique has problems given

that the reserved bandwidth of the unicast bearer is

not available fast enough.

3.3 Multicast to Unicast

For multicast to unicast switches a good initial solu-

tion had already been found. The addition of adaptive

playout to this scenario would result in further im-

provements.

3.4 Unicast to Multicast

When the user switches from a unicast to a multi-

cast channel the server appends an I-frame of the new

channel to the old data stream which is eventually

(scenario 2) played out to give a ﬁrst impression of

the program. For a quicker reaction it would be good

to present this I-frame as soon as it is available at the

client and not after the buffer has been played out.

This is only possible if the advanced buffer control

with direct data access is available.

Even more interesting would be the transmission

of some kind of preﬁx data which would enable the

client to seamlessly switch to the new channel, e.g.

by making the ﬁrst received P-frames of the multicast

transmission decodable. However, due to the possibly

high difference in the forward trip times of the unicast

and the multicast system some kind of synchronisa-

tion technique is necessary determining which data of

the new multicast channel will be missing.

4 CONCLUSIONS

One of the main problems of today’s mobile TV sys-

tems, slow channel switching, has been analysed, and

a mobile TV system is developed to achieve a faster

channel switch. The different alternatives for the

transmission of mobile TV have been compared, and

the causes for slow channel switching have been anal-

ysed. To accelerate the switching process, algorithms

have been proposed which are implemented in a mo-

bile TV system using enhanced buffer management.

Slow channel switching is caused by the structure

of digital video, and this cannot be solved because up

to now it is the only way to achieve good compression

IMPROVED CHANNEL SWITCHING FOR HYBRID UNICAST/BROADCAST MOBILE TELEVISION

217

ratios. The aim was to attenuate the effect of the video

structure, and to improve the perceived quality.

The proposed algorithms for unicast and multicast

switching gain an improvement regarding the switch

delay (multicast) or resource utilisation (unicast) re-

spectively. Moreover, the switching algorithms guar-

antee a switch without distortions in the presented

video by avoiding decoding useless P-frames. The

search for the ﬁrst I-frame is done quickly when the

unicast switching algorithm is used, and it introduces

no relevant delay regarding the channel switch.

The proposed archicture and the algorithms have

been implemented on top of the Network-Integrated

Multimedia Middleware (NMM) (Lohse, 2005), and

will be evaluated. NMM is a framework for me-

dia processing building ﬂow graphs through special

nodes. Due to the object oriented design the nodes

are extensible. To build our system we have added

some functionality to the framework and especially to

the Switching Node, which implements the different

switching algorithms, the Media Source Node which

feeds the video content into the graph, and the dis-

play node, that shows the result of the delivered video

at the end of the switching process.

The delay of multicast-multicast switches is domi-

nated by waiting for the ﬁrst I-frame of the new chan-

nel. Therefore, a fast channel switch is possible if

the I-frame is received early. The proposed multicast

switching algorithm improves every case because I-

frames are presented as fast as possible. The gain due

to this immediate presentation roughly matches the

buffering delay, in initial simulations we measured an

average gain of 0.8 seconds, which is a noticeable im-

provement over the simple switching. Despite this im-

provement, a multicast switch takes at the average 2.8

seconds which is caused by the long GOF structure of

4 seconds for our simulations.

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