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ethernet 100baseT switches and segment length

Posted on 2003-12-05
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Can any one tell me if there is:

A maximum number of switches in 100base2? (is a switch the same as a hub?)

A minumum seqment length?
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Question by:bearware
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by:MaB
ID: 9881876
I've never heard of a maximum number. There should not be a problem, just plug 'em in.

What you mean "minimum segment length". There is a maximum cable length of the cat5 twisted pair cable thats approx 100 m becase of the resistance in the cable and the needed detectable signal level, but no minimum length.
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by:bearware
ID: 9881927
I know for 10base there are minumum segment lengths.
I know there is a maxumum of 2 hubs.
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by:NicBrey
ID: 9881955
Switches does not split up broadcast domains, so a broadcast will be sent aout all switch ports - be received by each node on a layer 2 segment. Broadcast domains are split at layer 3. I reccommend that you do not have more than about 250 PC's on a segment without splitting them with a layer 3 device (multi layer switch or router). Having too many PC's on a segment will drag the network performance down severely.
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by:MaB
ID: 9881989
Interesting......didn't know that. Thanks for the enlightenment. Just out of quriosity, why is there a minimum length and a maximum of 2 hubs on 10 base?
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by:MaB
ID: 9882002
The difference between a switch and a hub is that a hub sends all incoming traffic out to all ports, a switch can register witch port you you are interested to send information to (in layer 2) and direct the traffic to the correct port giving less overhead on the network.
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by:NicBrey
ID: 9882079
I have not hear of a minimum lenght either. You are right about the max length.

Hubs are dumb network equipment. Let me explain why I say that.
Lets say you have a 24 port hub. All 24 ports are in one collision domain. Only one device can, at any time, send a packet or there will be a collision of packets, resulting in a resend. That is where CSMA/CD (carrier sence media access with collision detect) comes in play.
The PC's cards sense basicly when they can send. With a hub, if a packet is received on one port it is broadcasted out of all the other ports.

A switch is more intelligent in the sense that it builds a forwarding table. It learns wich MAC addresses are on a port and will only forward out the port where the destination MAC is located. It will only broadcast IP multicast addresses, broadcast addresses and unkown addresses.
Something can be done about the mutlicast addresses though - IGMP snooping or ICMP in the Cisco world

I might have been a liitle too concervative in the ammount of PC's in a layer 2 segment - you can most likely take that up to 500, providing that you are only using switches and no hubs - for reasons mentioned above.

You can definately have more than 2 hubs on a 10BaseT network. I have a client that just does not want to spend the money to upgrade his hubs to swithes and they are running up to 6 24 port hubs on some sites. Needless to say that performance will suffer.
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by:bearware
ID: 9882089
There is a limit of 2 HUBS between and node on 100baseT because the propergation delay would be to large to detect a colision.

For 10base2 I cant remember why there is a minumum segment length. may be because of impedence of propergation delay.....

Does any one know if switches are different to hubs in 100baseT with respect to Allowed topolagy.
A router/gateway will store and forward. so there is no need to have colision detection span the device.
For a hub the colision detection spans the hub.
But what about a switch.
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by:chicagoan
ID: 9882117
On 10BaseT the maximum number of the repeaters that can be used in one transmission path between two nodes is four, the maximum number of network segment between two nodes is five and no more than three of the five segments may have network stations attached to them (At least two segments should be inter-repeater links which connect only repeaters a hub is a repeater)). Maximum segment length is 100 meters. Minimum length between nodes is 2.5 meters, (though this is often violated in wiring closets). Maximum number of connected segments is 1024.
100Base-TX and Gigabit Ethernet cannot use multiple hubs to increase distance.

Maximum distance between pairs of switches remains 100m with UTP There is no limit on number of switches between any 2 nodes.



                                   
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by:bearware
ID: 9882121
Sorry for repeating stuff from nicbrey I write it before his comment arived.
But do the switches cope with colisions with out geting the whole network involved i.e. store and resend.


P.S. the 2 HUB limit is for 100base2.

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by:NicBrey
NicBrey earned 80 total points
ID: 9882128
Each switch ports is in it's own collision domain. No need for collision detection when talking about switches.
Only limitation is the number of PC's you want on the segment before splitting the broadcast domain.
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by:bearware
ID: 9882132
What is UTP
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by:NicBrey
ID: 9882144
There are no collisions on a switched network.
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by:NicBrey
ID: 9882154
Unshielded twisted pair.

It's normal cat 5 ethernet cable
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by:bearware
ID: 9882161
What if all nodes B to Z try to talk to node A at the same time?
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by:NicBrey
ID: 9882178
All packets will arrive on a different sdwitch port and will be destined out the same one. The switch buffer will handle it and send out all the packets without dropping any.
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by:MaB
ID: 9882225
On a hubbed network CSMA/CD in the ethernet standard is essential. Nodes wait with the send until the line to the destination is free (Carrier Sense). Then they try to send the package, several nodes could send a packet at the same time (Multiple Access), but if a collision occurs (Collision Detect) so the package don't arrive the nodes wait a predestined timelength before trying to send the package again. Since signals take different amount of time to travel across the network it is very inlikely that nodes resend the package in the exact same time.
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chicagoan earned 70 total points
ID: 9882373
>The switch buffer will handle it and send out all the packets without dropping any.
usually... hopefully... maybe

Switches have a finite amount of buffer, and when the stream to a port exceeds it's capacity there is no choice but to drop the packet. This is an important point in designing you network because as a switch is usually running the ports in full-duplex mode, there's no possibility to report collisions, and the only recourse is to drop packets, which must be  recovered by the much slower method of the higher-level client figuring out there was no ack and retransmitting.
In a collision sending environment this handled by the nic.
Therefore uplinks and servers which concentrate traffic should be considered for faster or multiple interfaces.
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