What the "Channel" mean in facebook chat application?

Facebook uses publish/subscribe pattern for their chat app. I used Firebug to catch request in facebook site and I saw that every request to receive message from facebook server almost link to an address with pattern : http://[number].[numbe开发者_Python百科r].chanel.facebook.com. There is about 0->20 channels which mean number in address pattern will range in [0-20]. I don't know why they need to use channel as subdomain ? Does it relate to divide processing chat message to multiple service (server) to increase server performance ? or just a subdomain name ? What is your opinion ? I'd like to hear your interesting idea on this.

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I believe they use sub-somains because there's a limit on how many connections per domain a browser can keep open.

See How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers?

So, those sub-domains are not "channels".

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These are special servers which handle only chat messages and notifications. Source: Rodrigo Schmidt's lecture (https://www.facebook.com/rodrigo)