TDM Circuit Emulation over Packet (CEP-TDM)Tom Johnson

tom_johnson@litchfieldcomm.com

(presented by Tim Frost, tim.frost@zarlink.com)

IETF 56, 19 March 2003http://www.ietf.org/internet-drafts/

draft-johnson-pwe3-tdm-00.txt

Why do we need another TDM draft?

• Deadlock between the two major contributions in the PDH space for almost two years(draft-anavi-tdmoip and draft-vainshtein-cesopsn)

• Several attempts at a compromise failed

• Previous compromise presented at IETF 55 (Atlanta) didn’t survive for various reasons

• New design team formed following Atlanta to attempt to break the deadlock

• Design team consisted of:Tom Johnson, Sasha Vainshtein, Yaakov Stein, Prayson Pate, Max Riegel, Shahram Davari and Tim Frost

Advantages of previous solutions

• draft-anavi-tdmoip

– based on tried and tested AAL1 and AAL2 techniques

– ease of data-plane inter-working with ATM CES

– CAS signaling carried within the data packet for guaranteed synchronization

– large number of deployments in the field

• draft-vainshtein-cesopsn

– simple payload structure based on native TDM framing

– same payload structure regardless of attachment type

– constant delay, regardless of service bit-rate

– no pointers buried within the payload

What is draft-johnson?

RTP Header(may be suppressed)

PSN and Multiplexinglayer headers

CEP-TDM Header

TDM Fragment

0 Sequence numberFlags Structure pointer

0 1 5 18 31

CEP-TDM Header control word format:

Unstructured Emulation Mode

• treated as a pure bitstream, with no knowledge of contents

• regular segmentation

• default payload length 376 bytes, to give ease of inter-working with ATM CES (one packet = 8 ATM cells)

• option to use other payload lengths to improve efficiency or reduce latency

• broad agreement on this mode

– although some debate on size of default payload,e.g. should it be reduced to say 188 bytes (4 cells)?

Structured Emulation: NxDS0 with CAS• based on TDM multiframe format, with an AAL1-like CAS sub-structure,

with a whole multiframe as the default payload:

• option to use an arbitrary payload size, with structure pointer indicating first timeslot of multiframe structure

• easy to inter-work with ATM (based on same structure)• guarantees relationship of CAS to data

CAS1CAS1

First 125us frame

Second 125us frame

Last 125us frame

CAS substructure

Timeslot 1Timeslot 2Timeslot 3

CAS2pad

Timeslot 1Timeslot 2Timeslot 3

Timeslot 1Timeslot 2Timeslot 3

Structured Emulation: NxDS0 without CAS• Still based on TDM multiframe format, with a whole multiframe

as the default payload size:

• option to use an arbitrary payload size, with structure pointer indicating first timeslot of multiframe structure

First 125us frame

Second 125us frame

Last 125us frame

Timeslot 1Timeslot 2Timeslot 3Timeslot 1Timeslot 2Timeslot 3

Timeslot 1Timeslot 2Timeslot 3

w w w . Z A R L I N K . c o m