[Asterisk-Users] Applications of TDMoE

[Jayson Vantuyl]

On Tue, Jul 13, 2004 at 12:30:48PM +0100, luan wrote:
> Hi All,
> Please bear my ignorance but what is TDMoE used for? Illustrations
> with practical applications, scenarios or set ups will be most
> appreciated.
> Thanks
> Luan
TDMoE is for a situation where you need TDM reliability without
traditional TDM hardware.

Most modern computer people naturally think of taking digital voice and
dividing it into packets to be sent over a network.  Telephony people
didn't do it that way for a number of reasons (latency, virtual circuit
guarantees, etc).  Instead, they invented TDM (Time Division
Multiplexing).

Network packets can transfer data very flexibly because each stream is
broken into small chunks that each get sent with their own header
describing their contents.  Telephony people didn't like this because:

 * Packets can be out of order
 * More space for data is wasted by headers
 * There is no guarantee that every channel will be sent reliably.

TDM still takes the digital data in chunks, but they are spaced out with
a very tiny header (really only there to mark the beginning of the
packets) and then the channels data is concatenated in a specific order.
It's called TDM, because the channels are differentiated not by some
header bits, but by when they come over the wire (that is, the signal is
multiplexed by dividing up the signal in increments of time).  This is
roughly what happens on a T1.  This seemed much more natural to them
because it kept everything really simple and still preserved the idea of
a circuit in that digital lines were still broken into a certain number
of "channels" that can be "switched" much like a physical phone line.
This was also done during an age before packet networks had proven
themselves and its still useful today when a synchronous serial links
needs to be cleanly divided in terms of usage.

TDMoE is useful because it allows the above familiarity, flexibility,
and reliability of TDM but over inexpensive Ethernet instead of T1s or
E1s.

The easiest way to picture its use is to envision the scenario of
connecting two departmental PBXs.  Often, in larger businesses, multiple
PBXs are needed for different departments.  It is not desirable to have
them call each other via outside phone lines.  With traditional
equipment, someone might run a 25-pair trunk or T1 line between them.
TDMoE emulates a T1, but wraps the T1-encoded data in a small header
inside of an Ethernet frame.  This permits just hooking them up to the
same switch.

Note that this does NOT allow routing the data over the internet, it
only functions on a single Ethernet segment.

Once the TDMoE link is up, Asterisk just sees 24-lines that appear to be
a T1 instead of having to deal with all of the complexities of VoIP.
This is useful, since probably 75% of the utility of VoIP is really just
the fact that it can run over a network.  It's also handy because it
unifies the flexibility and cost-savings of a Ethernet with the
telephony-friendly aspects of a T1 (alarm codes, bundling trunks,
channelization).

Hope this helps.  Also, if anyone wants to cut/paste this into the Wiki,
I give my consent.  I'm just too lazy to do it myself right now.

-- 
Jayson Vantuyl