question on internetworking basic

[sixth_sense]

Question:-1

I am not getting the meaning of this line from Todd lammle's ccna book:

"If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions."

whats the relation of detecting collision and half duplex operation of a hub connected with a switch ?

Question:-2

Why this said that individual switch ports are individual collusion domain?

anyone can explain?

Thanks,

[n2-goes]

Hello,

Half duplex means that it is sending and listening at the same time. With a hub you have more than one device that shares the medium. It is statistically possible to have to station that will listen and start sending a packet at the same time. In this case the 2 packests will collide. Both stations will sense that a collision has occur with the listening function.

In full duplex the collision detection is disabled. It means that the switch port can only be associated with one device.

HTH,

Nadine.

[sixth_sense]

hi Nadine,

you told "Half duplex means that it is sending and listening at the same time." --- are you sure about it?

In my sense,

simplex is: only one way communication.

half-duplex: both way communication, but any one way at a time.

full-duplex: both way simultaneous communication.

Check this out:

http://www.webopedia.com/TERM/h/half_duplex.html

--------------------

[Richard Burts]

Nadine is correct in saying Half duplex means that it is sending and listening at the same time. And that is not disagreement with your reference half-duplex: both way communication, but any one way at a time.

In half duplex the NIC can only successfully transmit or receive at a time but not both. Full duplex means that it can transmit and receive at the same time successfully.

One of the key concepts in understanding this distinction is collisions. A collision is an event on the wire when there is simultaneous transmit and receive. In full duplex the NIC accepts the collision and continues to transmit while it receives. In half duplex the NIC is listening while it transmits, as Nadine said. While it is transmitting it listens and if it detects incoming data, it means that some other station is also transmitting and in half duplex when the NIC detects the collision, it stops transmitting its frame, waits for a timer, and attempts to transmit.

So the main difference between half duplex and full duplex is that half duplex is checking for collisions and if it detects a collision it will initiate retransmission. In full duplex the NIC ignores any collision that may occur.

[sixth_sense]

thanks all of you here. I am now clear and happy with the answer of my question:1.

:D

[scottmac]

Regarding a hub: A hub is essentially emulating a piece of cable; it directly replaced a segment of coax. Under the old system (with coax), each station was half duplex, it can send or receive at any given time. While transmitting, it was also listening to the cable so that if a collision occurred, it could be detected and acted upon (send a JAM, fall back for a (not really) random period of time and try again when the wire appeared clear of traffic.

A hub is a single collision domain: only one device can be talking at a time or a collision occurs.

Regarding a switch: A layer 2 switch is essentially an improved multi-port bridge. A bridge (or switch) separates / divides / defines a collision domain: there can be a transmitting device on each side of a two-port bridge without causing a collision. Since a switch is basically a large multi-port bridge (functionally), each port represents a collision domain.

When a device is talking to another device between two ports on the same switch, a "circuit" is created through the switch fabric between the two ports, which becomes the collision domain for those two devices (but only those two devices). Multiple pairs

of devices can be communicating at the same time, each pair on their own "circuit"; each pair on their own collision domain.

Because of the improved architecture of the switch fabric (versus the hub emulating a chuink of cable), full-duplex operation is enabled, allowing each device to be actively transmitting and receiving at the same time (but NOT listening for collisions, since collisions are "impossible" on a full-duplex connection). Each device can be talking and listening to separate and different othe devices (if the device's drivers are written to do so).

So depending on the setup, the book could be slightly wrong saying that "each port is a separate collision domain"; actually it should be "each pair of ports on the same fabric represents a collision domain" (if you're talking only about devices / hosts connected only to a switch) OR, if you had a hub connected to each port, then each port would truely be a separate collision domain and the switch would be working as a multi-port bridge.

There were some "hubs" that would operate in full-duplex (they were also usually multi-speed "hubs" that allowed 10/100 connections), these devices were actually a hybrid and not really "just" a hub.

Switches, as well as bridges, still pass broadcasts and multicasts. There is no chance of collision since all traffic passing through a switch is buffered (first in, first out - FIFO), usually by port.

FWIW

Scott

[mberrocal]

well actually each port in a switch IS a separate collision domain. There is no way that two ports directly connected to two hosts in full-duplex mode (say hostA to fastethernet0/1 and hostB to fastethernet0/2) will ever have a collision (because collisions can't happen in full-duplex). So "each pair of ports on the same fabric represents a collision domain" doesn't sound right to me, not in a switch. Of course in this scenario there wouldn't be collisions (due that the link is full-duplex). if the port is for any reason half-duplex then there might be collisions, yet it would still be a separate collision domain per switch port.