Digital Trunk (T1)

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Digital Trunk (T1)
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DS1/T1 Basics (do E1s have robbed bit signaling?)
Digital Trunk (T1)
Frames transmitted back-to-back
A frame is 24 time slots plus one framing
bit 125 microseconds 1.544Mb/s
A time slot is 8 bits 64kb/s one talking
path one trunk
Bit1
Bit6
Bit7
Bit8
Bit3
Bit 2
Note In some trunks, the least significant bit
is used for signaling every 6th frame. This is
called robbed bit signaling. This bit the A bit
is used to indicate on hook/off hook status for
the trunk. Also, when dial pulse signaling is
used, the A-bit is used to represent the dial
pulseswhich are themselves a series of
on-hook/off-hook transitions.
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Transmitting a Time Slot across the packet network
Digital Trunk (T1)
Digital Trunk (T1)
Circuit-to-Packet Converter (gateway)
At the gateway, the 64kb/s 8-bit time slot is
converted from circuit to packet, compressed, and
sent via PVCs to the egress gateway for
conversion back to the circuit world.
What happens to the signaling bit? It gets
sent along also. It is interpreted as noise at
the egress gateway when reconstructing voice.
The egress gateway will also recover the
signaling bit and restore it to its place as
the least significant bit of every sixth
frame. However, this bit is subject to losses in
the network due to the re-coding and compression
done for the circuit-to-packet conversion as well
as packet losses due to transmission errors and
packet losses due to network congestion control
techniques such as dropping packets that contain
the least significant bits.
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Transmitting the signaling bit across a packet
network
PAD Packet Assembler/Disassembler
8-bits
PAD
Packet Voice to the Packet Network 12.8Kb/s
or less
1 bit every 6th frame
Signaling bit to the Packet Network 1.333 kb/s
One way to get the signaling bit across the
packet network reliably is to transmit the least
significant bit of the time slot as a separate
stream across the network in parallel with the
voice packetswithout running it through the
packet voice circuit-to-packet converter. Note
that the capacity needed to transmit the
signaling bit is 10 or more of the capacity
needed to transmit the packet voice...
Gateway
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Another technique for transmitting the signaling
bit across a packet network
PAD Packet Assembler/Disassembler
8-bits
PAD
Packet Voice to the Packet Network 12.8Kb/s
or less
1 bit every 6th frame
Signaling bit to the Packet Network 1.333 kb/s
Delta Mod
Gateway
Note that a signaling bit is transmitted every
1.5ms. (representing on-hook, off-hook, and dial
pulses which are themselves on-hook/off-hook
bursts lasting as short as 50 ms per pulse (25ms
on-hook 25ms off-hook) For a given trunk, the
signaling bit changes state very infrequently
w.r.t. the number of bits transmitted. A trunk
used at 80 occupancy (3 min/call) has about 16
calls/hr. (on the order of 40 signaling bit
transitions) Each call has about 10 digits of
dialing x 5pulses/digit ave. x 2 state changes
per pulse 100 transitions A dial pulse is
represented by about 35 signaling bits. I
propose inserting a function that only sends the
signaling bits when there is a change of state in
those bits...