Today, network designers are moving away from using bridges and hubs and are primarily using switches and routers to build networks. Technology advances are producing faster and more intelligent desktop computers and workstations. The combination of more powerful computers/workstations and network-intensive applications has created a need for network capacity, or bandwidth, that is much greater than the 10 Mbps that is available on shared Ethernet/802.3 LANS. Today's networks are experiencing an increase in the transmission of large graphics files, images, full-motion video, and multimedia applications, as well as an increase in the number of users on a network.
All these factors place an even greater strain on Ethernet's 10-Mbps bandwidth. As more people utilize a network to share large files, access file servers, and connect to the Internet, network congestion occurs. This can result in slower response times, longer file transfers, and network users becoming less productive due to network delays. To relieve network congestion, more bandwidth is needed or the available bandwidth must be used more efficiently.
Transmission time equals the number of bits being sent times the bit time for a given technology. Another way to think about transmission time is as the time it takes a frame to actually be transmitted (small frames take a shorter amount of time, large frames take a longer amount of time to be transmitted). Each 10 Mbps Ethernet bit has a 100 ns window for transmitting (the bit time). A byte equals 8 bits. Therfore, 1 byte takes a minimum of 800 ns to transmit. A 64-byte frame (the smallest allowable 10BASE-T frame, so that CSMA/CD will work properly) takes 51, 200 ns, or 51.2 microseconds, to transmit (64 bytes at 800 ns per byte equals 51, 200 ns and 51, 200 ns divided by 1000 equals 51.2 microseconds). Transmission time of an entire 1000-byte frame from the source station requires 800 microseconds just to complete the frame. The time at which the frame actually arrives at the destination station depends on the additional latency (delay) introduced by the network.
Full-duplex
Ethernet allows the transmission of a packet and the reception of a
different packet at the same time. This simultaneous transmission and
reception requires the use of two pairs of wires in the cable and a
switched connection between each node. This connection is considered
point-to-point and is collision free. Because both nodes can
transmit and receive at the same time, there are no negotiations for
bandwidth. Full-duplex Ethernet can use an existing shared medium as long
as the medium meets minimum Ethernet standards.
To transmit and receive simultaneously, a dedicated port is required for
each node. Full-duplex connections can use 10BASE-T, 100BASE-TX, or
100BASE-FX media to create point-to-point connections. The network
interface cards (NICs) on both ends need to have full-duplex
capabilities. The full-duplex Ethernet switch takes advantage of the
two pairs of wires in the cable. This is done by creating a direct
connection between the transmit (TX) at one end of the circuit and the
receive (RX) at the other end. With these two stations connected
this way, a collision-free domain is created because the transmission and
receipt of data occurs on separate non-competitive circuits.
Ethernet usually can only use 50%-60% of the 10-Mbps available bandwidth because of collisions and latency. Full-duplex Ethernet offers 100% of the bandwidth in both directions. This produces a potential 20-Mbps throughput- 10-Mbps TX and 10-Mbps RX.
LAN switching eases bandwidth shortages and network bottlenecks, such as those between several PCs and a remote file server. A switch can segment a LAN into microsegments, which are single host segments. This creates collision-free domains from one larger collision domain. Although the LAN switch eliminates collision domains, all hosts connected to the switch are still in the same broadcast domain. Therefore, all nodes connected through the LAN switch can see a broadcast from just one node.
Switched Ethernet is based on Ethernet. Each node is directly connected to one of its ports or a segment that is connected to one of the switch's ports. This creates a 10-Mbps bandwidth connection between each node and each segment on the switch. A computer connected directly to an Ethernet switch is its own collision domain and accesses the full 10 Mbps.
A LAN that uses a Switched Ethernet topology creates a network that behaves as though it has only two nodes-the sending node and the receiving node. These two nodes share the 10-Mbps bandwidth between them, which means that nearly all the bandwidth is available for the transmission of data. Because a Switched Ethernet LAN uses bandwidth so efficiently, it can provide more throughput than Ethernet LANs connected by bridges or hubs. In a Switched Ethernet implementation, the available bandwidth can reach close to 100%.
Ethernet switching increases the bandwidth available on a network by creating dedicated network segments (that is, point-to-point connections) and connecting those segments in a virtual network within the switch. This virtual network circuit exists only when two nodes need to communicate. This is why it is called a virtual circuit, it exists only when needed and is established within the switch.
One drawback of switches is that they cost more than hubs. However, many businesses implement switch technology slowly by connecting hubs to switches to such a time that the hubs can be replaced.
Switching is a technology that decreases congestion in Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) LANs by reducing traffic and increasing bandwidth. LAN switches often replace shared hubs and are designed to work with existing cable infrastructures so that they can be installed without disrupting existing network traffic.