AN ATM CELL BASED TRANSMISSION SYSTEM ON A PON STRUCTURE

G. DU CHAFFAUT - D. CHAPELAIN - A. MADANI - S. CARPENTIER

Centre National d'Etudes des Tblbcommunications route de Tregastel - BP 40 - 22301 Lannion cedex - FRANCE

Tel:733) 96 05 36 04

Kev words :

Fiber sharing, passive optical network, customer access

connection, ATM, local loop, broadband

communications, transmission systems.

Abstract:

An ATM cell based transmission system has been studied

and optimised for fiber sharing in the customer access

connection; it is oriented towards B ISDN @revision of

both interactive and distributive services) and take into

account last CCITT works. A passive tree star topology

with standard SM fiber is proposed allowing at least eight

customers to be connected.

A n example of application to residential customers is

presented which takes into account possible evolutionary

steps. It leads to a 622 Mbit/s downstream on one

wavelength and a 2 to 20 Mbit//s per user upstream on

another wavelentgh.

1- Introduction :

Moving from the existing copper and coaxial

distribution network to an optical one takes a long

time and costs a lot of money. So intermediate steps

(e.g. FTTC) for a smooth introduction of the optical

fiber in the local loop are to be found. Introduced after

these steps, optical distribution systems would have to

FAX: (33) 96 05 35 90

be as flexible as possible to allow system evolutions,

without removing and replacing the passive optical

link. In the following, we focuses only on the FITH

phasis, describing an ATM transmission system and

giving examples of applications to residential

customers.

2- Video service Drovision :

In the case of residential applications, two kinds of

broadband services have been identified. Others are

not broadband enough to influence the line bit rate.

Video channels :

Improvements in videocoding could likely result in

distribution codecs at the following bit rates :

- normal quality (NQ)

- HDTV (Ha)

: 34 Mbit/s --> 17 Mbit/s

: 140 Mbit/s -- > 70 Mbit/s

A PON structure with a 622.080 Mbit/s downstream

allows x NQ plus y HQ to be carried depending on the

codec bit rate (e.g. 17 NQ channels at 34 Mbit/s or 35

NQ channels at 17 Mbit/s or 4 HQ channels at 140

Mbit/s or 8 HQ channels at 70 Mbit/s). One part of

these channels could be broadcasted to every customer

whereas the other part could be switched TV channels

selected by the customers connected on the same fiber.

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To improve the efficiency of the selection, a dynamic

allocation of the broadcasted and switched parts is

handled.

Interactive video se mice :

Fast video compression technology combined with CD

ROM disk possibilities on one hand and introduction

of low cost PC computers at home on the other hand

lead to new developments of interactive video

applications. A 1.5 Mbit/s bit rate is required for both

directions.

3- A Dassive tree sta r structure :

It seems that tree structure is more suitable for

distributive services. Indeed, it looks cheaper to

broadcast only once a video service to a group of

customers rather than duplicate this service for each

customer as needed with a point to point topology. The

most part of the line bit rate is due to distributive

services, so it is of great interest to favour such a

structure at least for residential customers.

Furthermore, this leads to a reduction of costs for

ducts, optical fibers and jointing, and decreases the

number of equipments at the central side.

The counterparts with fibre sharing are achromatic

couplers added at the division point and more

constraints reported on terminal equipments.

A convenient way to introduce digital distributive

services is the OLO concept as proposed in RACE

projects. It relies on the separation between interactive

and distributive services and allows independant

upgrades of the two kinds of services.

An early step for the introduction of optical fiber in the

local loop could be to broadcast analog video channels

using SCM with AM or FM modulations on a PON

structure (figure 1).

Then using the same structure with a digital

transmission system (an ATM one is proposed in the

following), it is possible to introduce digital video

distribution (figure 2).

A next step could be to provide a more integrated

system as explained figure 3.

The three cases described before have no active point

in the access connection to avoid degradation of the

optical line reliability, decrease the number of

maintenance entities to handle, and avoid civil works

and need of remote power supply.

: Sub. Carrier - c : +

. N T 2 I N T I -. * figure 7

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303.2.2 0121

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: N T Z T b l N T I I U b L T

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figure 3

5- ATM transmission svstem :

ATM has been choosen for the following reasons :

- We have to cope with a large number of service bit

rates and their possible evolution in the future

including variable bit rate services.

- ATM multiplexing is very convenient for non

redondancy transmission when sharing a medium (e.g.

transmission of distributive services).

- In case of switched TV issued from a remote emitting

point and routed through cross connect nodes, ATM

offers an interesting flexibility.

- Fiber sharing implies packet oriented transmission

for upstream.

- Confidency between users connected to the same

fiber requires to filter information flow. This is simply

done by reading cell headers.

- To move towards an ISDN network switching all

kinds of services, ATM is considered as a good

candidate.

A transmission system has been studied at the U

reference point which is described below :

Maintenance ceZZ : the PL OAM cell (figure 4) is

inserted every 134 cells. It carries F1 (regeneration

section OAM flow) and F2 (digital section OAM flow)

as defined in I610 draft recommendation and use a

specific pattern in the header. This cell is not passed to

the ATM layer. The phasing and access control fields

following the cell header are protected against line

errors.

Scrambling : a 15 stages self synchronized phased

scrambler is used with the following advantages :

- no error vultiplication or propagation

- reasonable hardware complexity

- high immunity against malicious user (no relation

between cell boundaries and scrambler length)

- good efficiency for transmission aspects

Synchronisation criterias : synchronisation process is

based on the recognition of a four bytes pattern and

confirmed using the CRC code from cell headers as

shown on the figure 5.

non scrumbled par/

01 22

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figura 4

303.2.3

Timing characteristics :

Propagation time : the maximum length retained to

connect subscribers to a local exchange (figure 6) is

6 km (hax). This covers about 93% of subscribers in

France (quite similar for other countries in Europe).

The maximum distance between the division point and

each subscriber does not exceed 500 meters (lma).

This gives the following maximum propagation times:

.

DisDersion time dtt for the DP-NT-DP delay is due to :

- dispersion time dtp from length differences between

DP and NTs :

dtp = 2.lmmtpd

tpd is the optical propagation time

dtp= 5 I . ~ S

- dispersion time dtd from optical and electronic

devices

dtd< = 1 ps

Upstream access control :

CMI code is prefered for the upstream specially

because of the easy and fast clock recovery associated

which is very suitable with transmission of non phased

blocks.

For low bit rate upstream (2 Mbit/s per user), there is

no need to take into account dtt. Access control to the

medium is obtained by decoding the MAC field

(Medium Access Control) carried by the PL OAh4 cell

(every 92.0 ps). Each NT is activated during a fixed

time slot (figure 7); if there is no information cell to

transmit, the block is completed with empty cells. A 4

cells fifo is needed in the NT to buffer the upstream

information flow (downstream at 622.080 Mbit/s).

I 1 usern 1-

Upgrading the system for higher bit rates (10 to 20

Mbit/s per user), leads to take into account dtt (figure

8). This is done when connecting a new NT. The LT

equipment sends a RFI message (Request For

Identification) to the concerned NT. The new NT

answers by sending a message including its UID (User

Identification) and PHS (Phasis Status). Then the LT

processes the correct delay to send to the NT. A PHL

(phasis to load) message is sent and decoded by the

new NT. As previously, a 4 cells fifo buffer is needed.

303.2.4 01 23

n r l pj!, mer f \ UDI, PHS I mer2 I

I user n I Rh

1

figure 8

7- User eauipment :

Considering the overcost on user equipment as soon as

the distance between NT1 and NT2 requires signal

regeneration and maintenance process, it is clearly

prefered on a technical and economic point of view to

set NT1 close to NT2, at least for residential

customers. Obviously, this implies to face and cope

with constraints due to national regulations.

A convenient implementation to reduce the user

equipment cost is to use a parallel interface between

NT1 and NT2. It is based on primitives exchanged

between ATM and TC layers (I321 CCIIT draft

recommendation ):

- the TC layer provides to the ATM layer: data

indication

- the ATM layer provides to the TC layer: data request

In case of no cell available (empty cells or cells issued

for other users in case of fiber sharing), no data is

passed through the parallel interface.

8- BiblioeraDhy :

CCITT SGXVIII draft recommendations Geneva May 1990

MOLENE system.

J. Abiven. ECHO DES RECHERCHES NO138 4th term

1989

Fiber optic point to multipoint interface. Configuration for

Broadband ISDN

Hiroshi Uno and Naoya Aragaki. J. LIGHTWAVE

TECHNOLOGY vol. 7 NO11 Nov. 1989

Experimental data link using SM fiber and ping pong

transmission

A. Jaillard, H. Prigent, Y. Guillauseau. EFOC LAN 1989

Broadband Upgrade Options for Passive Optical Networks

J.R. Stern, A.R. Beaumont, D.W. Faulkner, D.B. Payne,

J.W. Ballance IEEE Workshop on PON May 1990.

A Study of Optical Passive Bus Network K. IGUCHI

IEEE Workshop on PON May 1990.

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