ATM (was Re: too many routes)

Michael Dillon michael at
Fri Sep 12 03:06:01 UTC 1997

>  Do you? When I mean voice, I mean true POTS. You present dialtone over
>IP ?
>(It could be done, btw), but POTS over ATM can exit right into the
>(DEXX = Big telco switch) Lets see you allocate an ESF B8ZS Clear
>Channel T1 over IP....

Are you sure you aren't referring to a DACS or DCS here? (Digital Access
and Cross-connect System). It's not quite the same as what is usually
referred to as a telco switch, i.e. 5ESS et al.

>   CBR in a "worse case scenario" ATM net, IS TDM, not SDM. NO variance
>against TAT allowed.

You missed CBR (Constant Bit Rate) in your definitions. CBR is an ATM
service used to do circuit emulation, i.e. deliver the same bits that a TDM
T1 would deliver.

>TDM = Time Division Multiplexing
>SDM = Statistical Division Multiplexing.

Stat muxing also known as X.25, frame relay, IP, ATM, or packet switching.
In other words rather than dividing the bitstream into fixed timeslots
(TDM) which guarantees that bits will arrive at the other end in a known
time interval, we try to avoid sending bits with no information content and
let another bitstream use the wire therefore the arrival time is not known
but is a statistical probability yadda yadda. TDM is DS3's divided into 28
DS1 timeslots which can each be divided into 24 DS0 timeslots.

>when running
>BEYOND the SCR, I lost the extra BW, and received FECN's ...... Slowing
>up the Ip.
>SCR = Sustained Cell Rate

Sustainable Cell Rate. This is an average rate, not a sustained flow of
cells. This is something like a frame CIR in that you can burst beyond the
SCR up to the Peak Cell Rate. It is used in ATM's VBR (Variable Bit Rate)

>FECN = Forward Explicit Congestion Notification
>BECN = Backwards Explicit Congestion Notification

These are used in Frame Relay. ATM has no BECN but has EFCI (Explicit
Forward Congestion Notification) instead of FECN.

>   Ahhh.. We await the completion, and proper interaction of RM, ILMI,
>and OAM.

Like IPv4, ATM is also evolving. Which one will win the evolutionary race?

>RM = Remote Management

Resource Management. Handled by special RM cells in the data stream.

>ILMI = a link management interface for ATM

Integrated Local Management Interface based on SNMP running through a
reserved VBR VC on the network.

Try to remember that there are many people on the list who are not that
familiar with ATM. And given that ATM switches can't route IP packets that
is probably OK. Only a few companies need to give serious thought to the IP
and ATM together.

Michael Dillon                    voice: +1-650-482-2840
Senior Systems Architect            fax: +1-650-482-2844

"The People You Know.  The People You Trust."

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