PVCs: Circuit Emulation over a Packet Network

Digital Trunk (T1)

Switch Switch

Digital Trunk (T1)

Switch

Digital Trunk (T1)

Switch

Circuit-to-Packet Converter(gateway)

PVCs: Circuit Emulation over a Packet Network

DS1/T1 Basics (do E1s have robbed bit signaling?)

Digital Trunk (T1)

Switch Switch

TS1 TS22 TS23 TS24TS3TS2F

Bit1 Bit6 Bit7 Bit8Bit3Bit 2

A frame is:+ 24 time slots plus one framing bit+ 125 microseconds+ 1.544Mb/s

Frames transmitted back-to-back

A time slot is:+ 8 bits+ 64kb/s+ one talking path+ one trunk

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 statusfor the trunk. Also, when dial pulse signaling is used, the A-bitis used to represent the dial pulses…which are themselves a seriesof on-hook/off-hook transitions.

PVCs: Circuit Emulation over a Packet Network

Transmitting a Time Slot across the packet network

Digital Trunk (T1)

Switch

Digital Trunk (T1)

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.

PVCs: Circuit Emulation over a Packet Network

Transmitting the signaling bit across a packet network

One way to get the signaling bit across the packet network reliably is totransmit 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...

8-bits PAD

PAD: Packet Assembler/Disassembler

Packet Voice to the Packet Network 12.8Kb/s or less

Signaling bit to the Packet Network 1.333 kb/s

Gateway

1 bit every 6th frame

PVCs: Circuit Emulation over a Packet Network

Another technique for transmitting the signaling bit across a packet network

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 transitionsA 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...

8-bits PAD

PAD: Packet Assembler/Disassembler

Packet Voice to the Packet Network 12.8Kb/s or less

Signaling bit to the Packet Network 1.333 kb/s

Gateway

1 bit every 6th frameDelta Mod