Multitech RF500S Uživatelský manuál stáhnout pdf (Strana 21)

6.1.8 Wiring Topology

UTP Ethernet uses a point-to-point topology. Each Ethernet outlet must be directly connected to a hub port.

The hub regenerates the signals and allows devices to talk to each other, remember the party line analogy.

Cable must run directly between the outlet and the hub it cannot be spliced. CDMA/CA scheme used by

Ethernet places a limit on how may wire segments and hubs can be between devices. For 10Mbps Ethernet

use the 5-4-3 rule, maximum of 5 wire segments and 4 hubs between devices, however only 3 of those hubs

can have devices attached. Because 100Mbps Ethernet is faster the rules are more stringent. A maximum of

two Class II hubs, and the distance between hubs is limited to less then 5 meters. Class I hubs cannot

connect directly to another hub. For all intents and purposes 100Mbps systems are limited to a single hub.

Where hubs need to be cascaded the solution is to use an Ethernet switch. Switches do not simply repeat

incoming packets on all ports. A switch examines each incoming packet, reads the destination address and

passes it directly to the proper port. Multiple conversations can occur simultaneously as opposed to only

one in a hub. Total switch bandwidth is greater then in a hub. A 100Mbp/s hub shares 100Mbp/s among all

devices. A switch segments traffic betweens pairs of ports. A non-blocking 16-port 100Mbp/s Ethernet

switch has a maximum throughput of 800Mbp/s. This assumes 8 pairs of connections evenly divided

between the 16 ports, even though each port is limited to 100Mbps. Switches have another advantage

because collisions no longer occur switches support full duplex communication. This means individual

computers can be transmitting at the same time they are receiving. This doubles throughput of our

hypothetical 16-port switch to 1.6Gbp/s. In actual use the advantage will not be as great but switches offer

tremendous advantage over hubs. A typical installation may use a switch in the main wiring closet. Since a

switch feeds each drop if more Ethernet ports are needed a hub can be added.

The switch determines connections based on MAC address. Every Ethernet controller has an address. The

switch monitors packets as they arrive and associates a port with a specific MAC address. When the switch

does not know which port to use it broadcasts the incoming packet to all ports, much like a hub. When the

device responds the switch knows which port it is connected to.

Hub Hub

10Mbps 5-4-3 Rule

Hub Hub

100Mbps - Class II hub

5 Meters Max

Hub

100Mbps - Class I hub

Figure 3 Connectivity rules for Ethernet and Fast Ethernet

6.2 Alternatives to Wired Ethernet

Wired Ethernet is the predominant commercial LAN. It is also popular in new home construction. The cost

of installing additional network wiring is low if done when the house is being built.

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Multitech RF500S Uživatelský manuál Strana 21