Digital Signal Processing Reference
In-Depth Information
a sequence identifier. TCP can detect if a packet is missing at the
destination, and keep packets in order, even if some packets get
delayed across the network. If a packet is corrupted, or fails to arrive,
the destination can request a retransmission from the source.
This is extremely valuable for many types of service using the
internet. For example, when sending an email with a file attach-
ment, every byte must arrive and be reassembled in the correct
sequence in order to be useful. Commerce over the internet
demands error-free communication.
However, this protocol is ill-suited for some types of service,
including streaming video and audio: if an IP packet is corrupted or
substantially delayed, it makes little sense to request a retrans-
mission, as the data is needed in a timely manner or not at all.
Requesting retransmissions needlessly uses up more network
resources and bandwidth. Also, TCP has provisions to reduce data
rates if there are toomany corrupted packets-known as flow control-
and reducing the data rate can be very disruptive to a streaming
application. For example, in a voice over IP phone call, having a few
clicks due to lost data is preferable to pauses of silence while waiting
for all the audio data to arrive correctly. TCP also incurs more latency
or delay due to its error-free connection features.
A simpler alternative protocol is user datagramprotocol (UDP).
There is no handshaking between source and destination; the data
is just sent. There are no attempts at tracking missing IP packets or
retransmitting, or flow control. It is low latency, simple and low
overhead. It is more suited for streaming data applications. If the
IP packets contain data that has its own error correction built in,
like transmit streams, then corrupted data may be corrected.
An extension of UDP is real-time protocol (RTP). RTP is
specifically designed for real-time data-streaming applications
which cannot tolerate interruption in data flow, and need
minimum latency. RTP does provide some additional features
compared to UDP: a time-stamping feature to allow multiple
streams from a given source to be synchronized, such as video
and audio; multi-casting support, so one source can send the
same data to many destinations simultaneously; and packet
sequencing, so lost packets can be detected, which can allow
a video decoder, for example, to use previous or nearby video data
as a best guess for the lost data. Note the actual protocol used
with RTP is real-time streaming protocol (RTSP).
18.5 IP Transport
Video over IP can be physically transmitted using many
methods, some of which are listed below.
