Biomedical Engineering Reference
In-Depth Information
Transmissions Technology
One of the major technological innovations initiated by pioneers of the Internet was the development
of alternatives to the traditional switched network model first used by telephone switchboard
operators at the turn of the 19th century. Even though some communications networks, including
many of the cell phone networks, still follow the switched model, computer networks such as the
Internet are based on a more complex but bandwidth-sparing packet model. The major disadvantage
of the much simpler switching model is that it can't provide more communications channels than
there are switches.
As shown in
Figure 3-5
, in a switched communications network, once a connection is established, it
monopolizes the circuit until the switch is released, even though the data has been transferred from
source to destination and the connection may remain idle. As such, the switching circuit in the figure
is capable of supporting only six simultaneous bi-directional communications channels. That is, the
communications path through the network is identical, regardless of which party originates the
message.
Figure 3-5. Switched Communications. In switched communications
networks, a fixed, continuous bi-directional connection is established
between the message source and recipient.
Packet communications makes use of the pauses and breaks in typical communications allowing a
single physical communications circuit to establish multiple, virtual channels. In the packet paradigm,
messages are parsed into small segments and packaged into labeled packets by the message
disassembler and packet generator (see
Figure 3-6
). These packets travel via various routes through
the network to the destination, as determined by the traffic density at nodes along the way. Because
packets are dynamically routed to different nodes as a function of the momentary demand for virtual
communications channels, some packets may arrive ahead of others, out of the original sequence.
Figure 3-6. Packet Communications. Multiple, virtual communications
channels are established by breaking up messages into small packets and
reassembling them at the destination.
