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Methods of Information Transmission
There are many different network protocols and several protocols can be
available even on a single layer. Especially with lower-layer protocols, we
distinguish between the types of transmission that they facilitate, whether
they provide connection-oriented or connection-less services, if the protocol
uses virtual circuits, and so on. We also distinguish between synchronous,
packet, and asynchronous transmission.
Synchronous Transmission
Synchronous transmission is needed when it is necessary to provide a stable
(guaranteed) bandwidth, for example, in audio and video. If the source does not
use the provided bandwidth it remains unused. Synchronous transmission uses
frames that are of fixed length and are transmitted at constant speeds.
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Book Excerpt: Introduction to Network Protocols
Chapter Contents
This excerpt from
Understanding TCP/IP: A clear and comprehensive guide to TCP/IP protocols by
Libor Dostálek, Alena Kabelová, is printed with permission from
Packt Publishing, Copyright 2007.
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In synchronous transmission, the guaranteed bandwidth is established by
dividing the transmitted frames into slots (see Figure 1.12). One or more slots
in any transmitted frame are reserved for a particular connection. Let's say
that each frame has slot 1 reserved for our connection. Since the frames follow
each other steadily in a network, our application has a guaranteed bandwidth
consisting of the number of slot 1s that can be transmitted through the network
in one second.
The concept becomes even clearer if we draw several frames under each other,
creating a 'superframe' (see Figure 1.13). The slots located directly under
each other belong to the same connection.
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Synchronous transmission is used to connect your company switchboard to the
phone company exchange. In this case, we use an E1(or T1 in United States) link
containing 32 slots of 64 Kbps each. A slot can be used for making a phone
call. Therefore, in theory, 32 calls are guaranteed at the same time (although
some slots are probably used for servicing).
The Internet does not use synchronous transmission, i.e., in general, does not
guarantee bandwidth. Quality audio or video transmission on the Internet is
usually achieved by overdimensioning the transmission lines. Recently, there
has been a steady increase in requests for audio and video transmission via the
Internet, so more and more often we come across systems that guarantee
bandwidth even on the Internet with the help of Quality of Service (QoS). In
order for us to reach the expected results, however, all appliances on route
from the source to the destination must support these services. Today, we are
more likely to get involved with only those areas on the Internet that
guarantee bandwidth such as within a particular Internet provider.
Packet Transmission
(From now onwards we will use the term packet to refer to 'packet', 'datagram',
'segment', 'protocol data unit'.) Packet transmission is especially valuable
for transferring data. Packets usually carry data of variable size.
One packet always carries data of one particular application (of one
connection). It is not possible to guarantee bandwidth, because the packets are
of various lengths. On the other hand, we can use the bandwidth more
effectively because if one application does not transmit data, then other
applications can use the bandwidth instead.
Asynchronous Transmission
Asynchronous transmission is used in the ATM protocol. This transmission type
combines features of packet transmission with features of synchronous
transmission.
Similarly to synchronous transmission, in asynchronous transmission, the data
are transmitted in packets that are rather small, but are all of the same size;
these packets are called cells. Similarly to packet transmission, data for one
application (one connection) is transmitted in one cell. All cells have the
same length; so if we guarantee that the nth cell will be available for a
certain application (a particular connection), the bandwidth will be guaranteed
by this as well. Additionally, it doesn't really matter if the application does
not send the cell since a different application's cell might be sent instead.
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