Answer to Question #213812 in Computer Networks for Aroosha ch

Question

As another example, consider a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices. Figure shows the situation.

Answer 1

• Even though a switch has a fixed number of inputs and outputs, which limits the number of hosts that can be connected to a single switch, large networks can be built by interconnecting a number of switches.
• We can connect switches to each other and to hosts using point-to-point links, which typically means that we can build networks of large geographic scope.
• Adding a new host to the network by connecting it to a switch does not necessarily reduce the performance of the network for other hosts already connected.

Figure 56. A switch provides a star topology.

This last claim cannot be made for the shared-media networks discussed in the last chapter. For example, it is impossible for two hosts on the same 10-Mbps Ethernet segment to transmit continuously at 10 Mbps because they share the same transmission medium. Every host on a switched network has its own link to the switch, so it may be entirely possible for many hosts to transmit at the full link speed (bandwidth), provided that the switch is designed with enough aggregate capacity. Providing high aggregate throughput is one of the design goals for a switch; we return to this topic later. In general, switched networks are considered more scalable (i.e., more capable of growing to large numbers of nodes) than shared-media networks because of this ability to support many hosts at full speed.

Dense Wavelength Division Multiplexing

Our focus on packet-switched networks obsures the fact that, especially in wide-area networks, the underlying physical transport is all-optical: there are no packets. At this layer, commercially available DWDM (Dense Wavelength Division Multiplexing) equipment is able to transmit a large numbers of optical wavelengths (colors) down a single fiber. For example, one might send data on 100 or more different wavelengths, and each wavelength might carry as much as 100 Gbps of data.

Connecting these fibers is an optical device called a ROADM (Reconfigurable Optical Add/Drop Multiplexers). A collection of ROADMs (nodes) and fibers (links) form an optical transport network, where each ROADM is able to forward individual wavelengths along a multi-hop path, creating a logical end-to-end circuit. From the perspective of a packet-switched network that might be constructed on top of this optical transport, one wavelength, even if it crosses multiple ROADMs, appears to be a single point-to-point link between two switches, over which one might elect to run SONET or 100-Gbps Ethernet as the framing protocol. The reconfigurability feature of ROADMs means that it is possible to change these underlying end-to-end wavelengths, effectively creating a new topology at the packet-switching layer.