how etheret switch work

[techie]

how does an ethernet switch decide which port to send which paket.

 

[DarkJedi]

Switches work in layer 2 of the OSI model, at this layer there is no IP address to identify the components of a network. What a switch actually uses is the phisycal address of a divice, this is called the MAC address.

When you plug a switch to the network, it asks for the MAC address of the components of the network, building a map of MAC address and ports, this way, when a packet is recieved, the switch reads out the layer 2 overhead. In this overhead is the information of the destiny MAC address and the origin MAC address.

The next step is for the switch to find out with port is connected to the MAC desitred, this is quiet simple since the swtch already got the MAC address of the devices conected to the network.

Hope this helps.

 

[techie]

Yes that helps. But two furhter questions.

1. How large is the size of the table. If it is variable, then how come no switch has this parameter on it s datasheet.

2. In the scheme you described, a switch requires to receive a full packet, decide which port to send it to and then retransmits it. Looks that this will have some problems. (a) There will be quite some delay in relaying a packet. (b) it will require some RAM (maybe 64kB or so) to store incoming packets from different ports and then retransmit them. Is this RAM onchip of the switch ASIC. (c) it will be extremely fast logic to process since many ports may be active at the same time.

 

[dainis]

Yes that helps. But two furhter questions.

1. How large is the size of the table. If it is variable, then how come no switch has this parameter on it s datasheet.

2. In the scheme you described, a switch requires to receive a full packet, decide which port to send it to and then retransmits it. Looks that this will have some problems. (a) There will be quite some delay in relaying a packet. (b) it will require some RAM (maybe 64kB or so) to store incoming packets from different ports and then retransmit them. Is this RAM onchip of the switch ASIC. (c) it will be extremely fast logic to process since many ports may be active at the same time.

1. It depends of switch chip (with internal, RAM) or of size of external RAM, and from division of this RAM for address table and packet buffers.
Count of MAC address table actually, is about 1K to 10K address.

2. Most of switches using store and forward algorithm, because incomming received packet is testing to good CRC, before transmitting. Some of switches also hawe forward architecture, which starting transmiiting after receiving packet header with MAC addreses.
Minimum of internal memory for switches is about 256 - 384 KBytes, maximum many MegaBytes.
For fast searching an address tables is using many metods, from HASH tables to CAM memorys.

 

[DarkJedi]

To round up the idea.

For a 3Com swith i was looking at today, they stated in their data sheet that the latency was about 2.7us in a store and forward architecture. the latency times you deal with is not very large, that is if you are only going through one switch, if you start bulding a larger network, latency is going to increase.

If you want more information on the amount of memory or the capacity of the MAC tables, you can check the switches data sheets, you con go to cisco or 3com or any other manufacturer for that, the state those features in their datasheets.

if you need more help, let me know, this actually seems like a topic i can give you a hand on, since i work for a networking ompany... at least for now :S...

 

[techie]

Thanks DarkJedi, I was actually looking to use ethernet for a non-coventional purpose. I am looking into the prospects of using ethernet to carry video frame data from a screen controller to it's LED modules. Ehternet seems a good choice because the technology is very mature and cheap and provides isolation and reliability.

 

[dolby.yang]

you can read IEEE 802.3 protocol for more detailed informations