516
13 Fiber Optic Communication Networks
In a ring topology, consecutive nodes are connected by point-to-point links that
are arranged to form a single closed path. Information in the form of data packets
is transmitted from node to node around the ring. The interface at each node is an
active device that has the ability to recognize its own address in a data packet in order
to accept messages. The active node forwards those messages that do not contain its
own address on to its next neighbor.
In a star architecture, all nodes are joined at a single point called the central node
or hub. The central node can be an active or a passive device. Using an active hub,
one can control all routing of messages in the network from the central node. This
is useful when most of the communications are between the central and the outlying
nodes, as opposed to information exchange between the attached stations. If there is
a great deal of message traffic between the outlying nodes, then a heavy switching
burden is placed on an active central node. In a star network with a passive central
node, a power splitter is used at the hub to divide the incoming optical signals among
all the outgoing lines to the attached stations.
In a mesh network, point-to-point links connect the nodes in an arbitrary fashion
that can vary greatly from one application to another. This topology allows significant
network configuration flexibility and offers connection protection in case there are
multiple link or node failures. Link protection in mesh networks is accomplished by
means of a mechanism that first determines where a failure has occurred and then
restores the interrupted services by redirecting the traffic from a failed link or node
to travel over another link in the mesh (see Sect. 3.3 for examples) [7, 8].
13.2.1 Performance of Passive Linear Buses
As shown in Fig. 13.5a, a linear bus uses a single cable, which connects all the
included nodes of the entire network. The main losses are the cable attenuations and
the optical power loss arising at each tap coupler. If a fraction F c of optical power is
extracted at each port of the coupler, then the connecting loss L c is
L c = −10 log(1 − F c )
(13.1)
For example, if this fraction F c is 20%, then L c is about 1 dB. That is, the optical
power is reduced by 1 dB at any coupling junction. The total loss of a linear bus
increases linearly with the number of network nodes. As a result of the high signal
reduction at each node, bus topologies typically are used only when a network
installation is small, simple, or temporary.
13 Fiber Optic Communication Networks
In a ring topology, consecutive nodes are connected by point-to-point links that
are arranged to form a single closed path. Information in the form of data packets
is transmitted from node to node around the ring. The interface at each node is an
active device that has the ability to recognize its own address in a data packet in order
to accept messages. The active node forwards those messages that do not contain its
own address on to its next neighbor.
In a star architecture, all nodes are joined at a single point called the central node
or hub. The central node can be an active or a passive device. Using an active hub,
one can control all routing of messages in the network from the central node. This
is useful when most of the communications are between the central and the outlying
nodes, as opposed to information exchange between the attached stations. If there is
a great deal of message traffic between the outlying nodes, then a heavy switching
burden is placed on an active central node. In a star network with a passive central
node, a power splitter is used at the hub to divide the incoming optical signals among
all the outgoing lines to the attached stations.
In a mesh network, point-to-point links connect the nodes in an arbitrary fashion
that can vary greatly from one application to another. This topology allows significant
network configuration flexibility and offers connection protection in case there are
multiple link or node failures. Link protection in mesh networks is accomplished by
means of a mechanism that first determines where a failure has occurred and then
restores the interrupted services by redirecting the traffic from a failed link or node
to travel over another link in the mesh (see Sect. 3.3 for examples) [7, 8].
13.2.1 Performance of Passive Linear Buses
As shown in Fig. 13.5a, a linear bus uses a single cable, which connects all the
included nodes of the entire network. The main losses are the cable attenuations and
the optical power loss arising at each tap coupler. If a fraction F c of optical power is
extracted at each port of the coupler, then the connecting loss L c is
L c = −10 log(1 − F c )
(13.1)
For example, if this fraction F c is 20%, then L c is about 1 dB. That is, the optical
power is reduced by 1 dB at any coupling junction. The total loss of a linear bus
increases linearly with the number of network nodes. As a result of the high signal
reduction at each node, bus topologies typically are used only when a network
installation is small, simple, or temporary.