Broadcast Tools DSC-20 Manual de Instalação descarregar pdf (Página 55)

Preliminary 8-2002 RSHD Smartcaster Installation Manual

Hubs and switches.

A hub is really nothing more than a wire directly connected to each and every port. What

goes in one port is sent out all ports at the time of transmission. If two or more nodes (a

node is anything capable of sending data on the network) transmit at the same time a

collision occurs -the data collides on the wire, becomes scrambled, and has to be

retransmitted later. The entire hub is called a collision domain* because any computer on

any port can collide with any other computer on any other port. Think of this a party line

or conference call. Everyone else can hear your conversation and must wait for you to

stop talking before they can speak. If two or more people talk at the same time (a

collision) the conversation has to be repeated.

*Note: A collision domain is any wire that can have 2 or more devices trying to

transmit at the same time, rendering all transmissions useless. After detecting

the collision, all units must reset and attempt to re-transmit the data. The

more traffic on a wire, the more likely collisions are to occur. The more

collisions, the more retransmits which means more traffic on the wire. (See the

problem?) Ultimately, the network gets REAL sluggish.

Switches are a vast improvement over hubs because they limit Collision Domains.

Switches move packets across the wire from port to port. They handle this task in

different ways depending on how the switch is configured. One way, called "store and

forward", will wait for an entire valid Ethernet frame before moving the packet to the

destination port. This was is very safe since you reduce the passing of bad frames nearly

completely. However, "store and forward" is also the slowest switching method. "Cut

through" switching will look at the packet coming in. It will look only at the beginning of

the packet where the source and destination Media Access Control (MAC) address is

kept. After that, it "cuts the packet through as it comes in", straight to it's destination port.

This form of switching is extremely fast since the switch never has to wait and examine

each Ethernet frame as it comes in. However, because the switch DOES NOT do this,

there is a chance it may pass a bad frame. So there are higher bad frame rates on switches

configured for "cut through" switching. Most common switches will allow both of these

formats to be assigned, and some even allow hybrids of these two methods. One such

hybrid is "Fragment Free" switching mode. The smallest legal Ethernet frame is 58 bytes.

Fragment free switching gets the first 58 bytes and then cuts the packet through to the

appropriate port. Performance is increased by not requiring a switch receive the entire

packet before forwarding it, and yet still cut down on the amount of network errors that

pass from one collision domain to another.

Switches store and forward or cut through packets on a per port basis. If the computers

on port one and two are transferring data, those packets are not replicated on every port -

freeing up the computers on all other ports to pass data without waiting for useless traffic

to get off the line. Now, if there is just one computer on each port, it cannot collide with

any traffic. This actually opens up the possibility of full duplex operation. If the computer

and the switch are the only things that will ever talk on the wire, then they never have to

listen for collisions because their transmit signals are hardwired to each others receive

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