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U. Glaeser

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oth data partitioning and two-way decoding can be jointly employed to protect the error-sensitive motion<br />

data of the first video partition. The motion vectors and the administrative parameters contained in the<br />

first partition are all coded with reversible VLC words. The detection of a bit error in the forward direction<br />

triggers the decoder to stop its operation, regain synchronisation at the motion marker separating the<br />

two partitions in the corresponding VOP, and then decode backwards to salvage some of the correctly<br />

received bits that were initially skipped in the forward direction.<br />

New Generation Mobile Networks<br />

Packet-switched mobile access networks such as GPRS [13] and EGPRS [14] are intended to give subscribers<br />

access to a variety of mobile multimedia services that run on different networking platforms, let<br />

it be the core mobile network, i.e., UMTS, ATM, or even Internet. The packet-switched mobile access<br />

networks have a basic common feature in that they are all IP-based and allow time multi-slotting on a<br />

given radio interface. The multi-slotting capabilities enable the underlying networking platform to accommodate<br />

higher bit rates by providing the end-user with a larger physical layer capacity.<br />

The real-time interactive and conversational services are very much delay-critical, so the provision of<br />

these services over mobile networks can only be achieved by using a service class capable of guaranteeing<br />

the delay constraints with one-delays in the order of 200 msec being required. In order to achieve such<br />

delay requirements, it is necessary to avoid using any retransmissions or repeat-requests scenarios by operating<br />

the RLC layer of the GPRS protocol stack in the unacknowledged mode of operations. Similarly,<br />

the transport layer protocol that must be employed is the user datagram protocol (UDP), which operates<br />

over IP and does not make use of any repeat-request system.<br />

IP networks do not guarantee the delivery of packets and neither do they provide any mechanism to<br />

guarantee the orderly arrival of packets. This implies that not only does the inter-packet arrival time vary<br />

but it is also likely that packets may arrive out of order. Therefore, in order to transmit real-time video<br />

information, some transport-layer functionality must be overlaid on the network layer to provide timing<br />

information from which streaming video may be reconstructed. To offer this end-to-end network transport<br />

functionality, the IETF real-time transport protocol (RTP) [15] is used. RTP fulfills functions such<br />

as payload type identification, sequence numbering, timestamping, and delivery monitoring, and operates<br />

on top of IP and UDP for the provision of real-time services and video applications over the IP-based<br />

mobile networks.<br />

On the other hand, the mobile access networks employ channel protection schemes that provide error<br />

control capabilities against multipath fading and channel interferers. For instance, GPRS employs four<br />

channel protection schemes (CS-1 to CS-4), offering flexibility in the degree of protection and data traffic<br />

capacity available to the user. Varying the channel coding scheme allows for an optimization of the throughput<br />

across the radio interface as the channel quality varies. The data rates provided by GPRS with the<br />

channel coding schemes enabled are 8 kbit/s for CS-1, 12.35 kbit/s for CS-2, 14.55 kbit/s for CS-3, and<br />

20.35 kbit/s for CS-4; however, almost 15% of the bits in the payload of a radio block are used up by header<br />

information belonging to the overlying protocols. Therefore, the rates presented to the video source for<br />

each one of the channel coding schemes per time slot are 6.8 kbit/s for CS-1, 10.5 kbit/s for CS-2, 12.2<br />

kbit/s for CS-3, and 17.2 kbit/s for CS-4. It is, however, envisaged that the CS-1 and CS-2 schemes will be<br />

used for video applications. Obviously, the available throughput to a single terminal will be multiples of<br />

the given rates per slot, depending upon the multi-slotting capabilities of the terminal. Conversely, EGPRS<br />

provides 9 channel coding schemes of different protection rates and capabilities and the choice of a suitable<br />

scheme is again a trade-off between the throughput and the error protection potential.<br />

Provision of Video Services over Mobile Networks<br />

Taking into perspective the traffic characteristics of a coded video source employing a fixed quantiser,<br />

we observe that the output bit rate is highly variable with high peaks taking place each time an INTRAcoded<br />

frame is transmitted. INTRA frames require roughly three times on average the bandwidth required<br />

© 2002 by CRC Press LLC

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