PVCs: Circuit Emulation over a Packet Network
Digital Trunk (T1)
Switch
Digital Trunk (T1)
Switch
Digital Trunk (T1)
Switch
Switch
Circuit-to-Packet Converter
(gateway)
DS1/T1 Basics (do E1s have robbed bit signaling?)
PVCs: Circuit Emulation over a Packet Network
Digital Trunk (T1)
Frames transmitted back-to-back
Switch
Switch
F TS1 TS2 TS3
Bit1 Bit 2 Bit3
TS22 TS23 TS24
Bit6 Bit7 Bit8
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 pulses…which are themselves a series
of on-hook/off-hook transitions.
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
Transmitting a Time Slot across the packet network
PVCs: Circuit Emulation over a Packet Network
Digital Trunk (T1)
Digital Trunk (T1)
Switch
Switch
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.
Transmitting the signaling bit across a packet network
PVCs: Circuit Emulation over a Packet Network
PAD: Packet
Assembler/Disassembler
8-bits
PAD
1 bit every 6th frame
Gateway
Packet Voice to the Packet Network
12.8Kb/s or less
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 packets…without 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...
Another technique
for transmitting the signaling bit across a packet network
PVCs: Circuit Emulation over a Packet Network
PAD: Packet
Assembler/Disassembler
8-bits
PAD
1 bit every 6th frame
Packet Voice to the Packet Network
Delta Mod
12.8Kb/s or less
Signaling bit to the Packet Network 1.333 kb/s
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...