| Author |
Message |
asm
Guest
|
 Posted:
Mon Oct 04, 2004 12:15 pm Post subject:
Is ISDN a synchronous protocol ? |
|
|
Hi All,
Since ISDN LAYER 2 is based on HDLC, can we consider ISDN to be a ASYNCHRONOUS
protocol. If not what makes it synchronous? dOES Q921/Q931 state its nature?
Please clarify.
Thanks,
ASM |
|
| Back to top |
|
 |
James Carlson
Guest
|
| Posted:
Mon Oct 04, 2004 6:23 pm Post subject:
Re: Is ISDN a synchronous protocol ? |
|
|
arut@post.com (asm) writes:
| Quote: | Since ISDN LAYER 2 is based on HDLC, can we consider ISDN to be a ASYNCHRONOUS
protocol. If not what makes it synchronous? dOES Q921/Q931 state its nature?
|
It's essentially synchronous.
ISDN is a suite of protocols and transports, not just one thing, so
it's hard to discuss in the abstract like that. The general idea is
one D (delta; signaling) channel and some number of B (bearer; data)
channels all (usually) atop one bitstream.
What makes it synchronous is that the bitstream doesn't have "start"
and "stop" marks in it. Instead, the receiver is expected to clock in
and decode every bit as it arrives. An asynchronous link (such as
commonly used to hook up a PC to a modem) uses start and stop bits to
frame each character.
In fact, the distinction between synchronous and asynchronous is a
_little_ bit fuzzy. A synchronous link usually doesn't have a
separate clock line, and instead derives the clocking by inspecting
the received data patterns -- somewhat like an asynchronous link. An
asynchronous link clocks bits within a "character" in much the same
way as a synchronous line. The distinction is that synchronous links
just run continuously (barring the effect of errors), while
asynchronous links start the clocking over on each character.
To make things more confusing, there are protocols (such as V.110 and
V.120) that allow you to run asynchronous data over ISDN channels.
--
James Carlson, IP Systems Group <james.d.carlson@sun.com>
Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677 |
|
| Back to top |
|
|
Phil McKerracher
Guest
|
| Posted:
Thu Oct 07, 2004 2:21 am Post subject:
Re: Is ISDN a synchronous protocol ? |
|
|
"asm" <arut@post.com> wrote in message
news:c8b782e3.0410040015.5f28b256@posting.google.com...
| Quote: | Since ISDN LAYER 2 is based on HDLC, can we consider ISDN to be a
ASYNCHRONOUS
protocol. If not what makes it synchronous? dOES Q921/Q931 state its
nature? |
ISDN isn't actually a protocol, it's a network. The physical part of that
network (layer 1) is most definitely synchronous. Terminals and other
network components cannot transmit data whenever they like, they must first
"lock on" (synchronise) to the network clock and then can only transmit at
very specific times.
Voice traffic is usually sent over the B-channels of an ISDN link
synchronously. It's just a continuous stream of data sent from the terminal
at 8,000 bytes (64,000 bits) per second with no start bits, stop bits,
framing, flags or other overhead.
However, D-channel signalling traffic and B-channel data such as internet
traffic are sent asynchronously over the synchronous underlying link by
dividing the data into packets.
My tutorial at www.mckerracher.org/isdn might help.
--
Phil McKerracher
www.mckerracher.org |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in