packet over sonet interface

Packet over SONETInterfaceA Design Strategy

Ravishankar KJinu Koshy

WHITE PAPER - December 2001

Wipro TechnologiesInnovat ive Solut ions, Qual i ty Leadership.

© Wipro Technologies Page :

White Paper

Table of Content

Introduction ....................................................................................................01

Packet over SONET Technology Evolution .........................................................01

Packet over SONET architecture ......................................................................02

Wipro’s Packet Over SONET Porfolio ................................................................03

Designing Packet over SONET Interfaces ..........................................................06

Design Approaches for Packet over SONET Interface ..........................................07

Wipro is best suited to provide a POS solution ...................................................08

Conclusion ......................................................................................................09

About the Authors ............................................................................................09

About Wipro ....................................................................................................10

Wipro in Telecommunication Technologies .........................................................10

Packet over SONET Interface

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Introduction

Synchronous Optical Network / Synchronous Digital Hierarchy (SONET/SDH) and opticalfiber have dominated the technology-scenario for building large-scale, high-speed,Internet Protocol (IP)-based networks. The high-bandwidth capacity available fortransporting data is the major reason for its explosive use in the Internet and largeenterprise data networks.

The problem with conventional networks, when conversion happens to ATM (IP-over-ATM)is the overhead imposed by ATM cell headers (5-bytes out of every 53-bytes), sometimesreferred to as the cell tax. Additional overhead is added by AAL5 (padding, 8-byte trailer). IPachieves only about 80 percent of the available line rate when operating over ATM.

This paper describes the Packet Over SONET/SDH (POS) technology and Wipro's FPGAcore implementing a SONET/SDH to POS mapper. It is addressed to engineeringmanagers who wish to add the Packet Over SONET/SDH Interface to their product range.Wipro's FPGA core and engineering skills could be leveraged for arriving at a cost-effective, early-to-market solution for POS.

1. Packet over SONET Technology Evolution

RFC 1662 describes a point-to-point protocol (PPP), HDLC-like framing method toencapsulate data for transmission in SONET/SDH payload.

IP-over-SONET as opposed to IP-over-ATM eliminates the overhead imposed by the celltax. IP achieves 95 percent of the line rate when running over SONET. The added capacitywhen running IP-over-SONET is very compelling when expensive wide-area or otherwisebandwidth-constrained links are used for interconnecting backbone routers.Packet over SONET (PoS) technology, thus allows efficient transport of data over SONET/SDH

The advantage of PoS is its flexibility that allows it to be used in a variety of transportapplications. For example, it finds use in network backbone infrastructures and dataaggregation or distribution on the network edge and in the metropolitan area. Router PoSinterfaces are frequently connected to Add Drop Multiplexers (ADMs), terminating point-to-point SONET/SDH links.


White Paper Packet over SONET Interface

2. Packet over SONET architecture

Packet Over SONET details

The Packet over SONET sequence is depicted below in a diagram. The blocks areexplained below in the following sections:

Point to Point Protocol

The Point-to-Point Protocol is designed for simple links, which transport packetsbetween two peers. These links provide full-duplex simultaneous bi-directionaloperation, and are assumed to deliver packets in order. It is intended that PPP providea common solution for easy connection of a wide variety of hosts, bridges and routers.

PPP is comprised of three main components:

n A method for encapsulating multi-protocol datagrams.n A Link Control Protocol (LCP) for establishing, configuring, and testing the datalink

connection.n A family of Network Control Protocols (NCPs) for establishing and configuring

different network layer protocols.

EncapsulationThe PPP encapsulation provides for multiplexing of different network-layerprotocols simultaneously over the same link. RFC 2615 specifies the use of PPPencapsulation over SONET/SDH links. PoS frames are mapped into SONET/SDHframes and they sit in the payload envelop as octet streams aligned on octetboundaries.

Link Control ProtocolIn order to be sufficiently versatile to be portable to a wide variety of environments,PPP provides a Link Control Protocol (LCP). The LCP is used to automaticallyagree upon the encapsulation format options, handle varying limits on sizes of

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................................................................................................................................Figure 1: Packet over SONET Blocks

MappingScrambling

PPP OverSONET/

SDHRFC 2615

RFC 1662 PPP

in HDLC-likeframing

PPPRFC 1661

IP Pack


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packets, detect a looped-back link and other common mis-configuration errors,and terminate the link.

Network Control ProtocolsPoint-to-Point links tend to exacerbate many problems with the current family ofnetwork protocols. For instance, assignment and management of IP addresses,which is a problem even in LAN environments, is especially difficult over circuit-switched point-to-point links (such as dial-up modem servers). A family of NetworkControl Protocols handles these problems (NCPs). There are many NetworkControl Protocols available. One of these is IPCP, a network control protocol for IPwhich assigns the IP addresses through negotiations and brings up the interface.

HDLC Framing

PoS uses PPP in High-Level Data Link Control (HDLC)-like framing (as specified inRFC 1662) for data encapsulation at Layer 2(data link) of the Open SystemInterconnection (OSI) stack.

PoS Security through scrambling

RFC 2615 recommends payload scrambling and a safeguard against bit sequences,which may disrupt timing. In addition to high-bandwidth efficiency, PoS offers secureand reliable transmission for data. Reliable data transfer depends on timing integrity.SONET/SDH timing information is obtained by filtering state transitions through aphased-locked loop (PLL). A severe drift of the receiving clock will cause signal or dataloss. Although this is highly unlikely in normal conditions, it is prudent to guard againstincorrect payload bit sequences that could cause such problems.

3. Wipro’s Packet Over SONET portfolio

Wipro has developed FPGA and software that caters to the needs of a Packet over SONETdesign. These are the FPGA implementation for the Packet over SONET and the softwareimplementation of the PPP module.

Wipro Packet Over SONET Controller

Wipro has designed and implemented a Packet over SONET Controller. The details ofthis design are described in the following sections.

Packet over SONET Controller Features

The following are the features of the Wipro Packet over SONET Controller Core:

n Implements RFC 2615 POS and RFC 1662 HDLC— like framing specificationsn Supports STM-4 (OC-12) framersn Supports 16- or 32-bit FCS verification


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n (Optional) Provides statistics for the number of received packets and errorpackets received

n (Optional) Provides signals for packets received with abort, CRC error, short, long, address and control error, protocol error, and dropped packets.

Block Diagram

The following section provides the details of the Wipro Packet over SONET SONET/SDH Controller. The shaded blocks are the ones for which development is alreadycomplete in FPGA. Based on the needs of the customer, the SONET/SDH Interface aswell as the POS PHY Interface can be implemented.

Figure 2: SONET/SDH Maper to POS Details................................................................................................................................

PO

S-P

HY

PL3/4 interface

PPP De-encapsulation

MPLS Logic

Protocol Field Filtering

IP Layer Statisticstics

TCP/UDP Data Generation

(eg. PRBS)

TCP/UDP Header

IP Header insertion

Datagram Generation (Optional)

Protocol Field insertion

MPLS Logic

PPP Encapsulation

PPP/IP Bypass

HDLC DE-FRAMER

FrameDelineator

Destuffer Logic

FCS Validation

HDLC Statistcs

HDLC FRAMER

IDLE PatternInsertion logic

Packet AbortSequence

Insertion logic

Byte Suffing

FCSComputation

Scrambler

DeScrambler

LEGENDBlock Ready with Wipro

Generic SDH Interface


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Description

The Wipro Packet over SONET controller implements RFC 2615 POS (Packet OverSONET) and RFC 1662 HDLC— like framing which allows an 8-bit data stream to beencapsulated into packets. The transmitter takes data and performs flag, address,control, and protocol field insertion, escape character stuffing, and FCS insertion. Thereceiver extracts data, checks address, control, and protocol fields, performs escapecharacter de-stuffing and verifies the FCS.

FCS computationThe transmit data will be passed through a FCS computation block. The FCS willbe computed over the entire incoming packet including padding. (Without the flagsequences and the stuffed bytes). This module does computation of FCS usingthe CRC-CCITT or CRC-32 method. The default method used is CRC-32.

Packet abort sequenceDuring Transmission, this module ensures the insertion of the Packet AbortSequence in case of under run of packet data.

MPLS label insertionFollowing the protocol, the MPLS field will be inserted if the MPLS is enabled. Thelength of the MPLS field is specified as 1,2 or 3 labels.

Frame delineationThe receiver will receive a parallel data with respect to the receiver clock. The startof frame will be indicated by the start flag sequence ‘7E’.

The data will be received only if the DATA_ENABLE is asserted. The assertion ofthis DATA_ENABLE indicates that the data is being received. No data will beavailable when this signal is de-asserted. A continuous reception of 7E willindicate an idle frame and will be ignored by the receiver. The flag sequencefollowed by a non-7E data will indicate the beginning of a frame and will bereceived. The start flag will be dropped/ ignored and the following data will bepassed for further processing.

MPLS validationThe user defined MPLS values will be stored in the buffer against which thereceived MPLS labels will be validated.

FCS ValidationThe receive data will be passed through a FCS validation block. The FCS will becomputed over the entire incoming packet including padding. (Without the flagsequences and the stuffed bytes). This module does validation of FCS using theCRC-CCITT or CRC-32 method. The default method used is CRC-32. The FCSreceived from incoming stream will be validated against computed FCS.


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DestufferThe descrambled data will be fed to the destuffer. Here the bytes stuffed for addingtransparency during transmission, i.e. to prevent false interpretation of ‘7E’ in thedata as flag sequence, will be removed.

4. PPP Module

As part of WIPTCP, a TCP/IP stack for Embedded systems, Wipro has developed amodule that conforms to the RFC-1661. This module defines the PPP encapsulation,option negotiation mechanism that is able to negotiate a rich assortment ofconfiguration parameters. Wipro has implemented LCP and IPCP, a network controlprotocol for IP (NCP for IP) that assigns the IP addresses through negotiations andbrings up the interface.

This PPP module is available on x86 and MPC8xx platform on VxWorks OperatingSystem. Designers could use this module to implement the PPP Finite State Machine.

5. Designing Packet over SONET Interfaces

This section explains the design considerations for Packet over SONET interface to aSONET/SDH line card.

SONET/SDH LINE CARD with POS Interface

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SystemInterface

SONET/SDH/

Mapperto POS

SONET/SDH

FramerSERDES

Clock Extraction

PLLsOptical

Transceiver

PROCESSOR SD RAM FLASH

Figure 3: SONET/SDH Line Card with POS Interface................................................................................................................................

The above diagram depicts a SONET/SDH Line card with POS Interface.


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The Optical Transceiver performs Optical to Electrical conversion as well as Electricalto Optical conversion. The Clock Extraction module extracts the clock in the receiveside from the receive traffic. The SERDES module is a serializer / deserializer forparallel to serial conversion and vice-versa. The SONET/SDH Framer performs framingas per standards. The POS Mapper performs SONET/ SDH mapping to POS. TheSystem Interface module is the interface to the switch. The Processor Interfaceallows software control of the devices on board.

Design Approaches for Packet over SONET interface

There are two approaches that could be adopted to design a Packet over SONETinterface. These approaches are discussed below:

FPGA based approachThe Packet over SONET standard could be implemented in an FPGA such as theone described in Section 5 of this document.

The advantage of the FPGA based approach is the flexibility it provides in terms offuture feature enhancements.

If the existing SONET/SDH line card has an FPGA in the system interface side,Wipro could study the existing board design and recommend changes. The FPGAcould be upgraded with an image that includes the POS implementation as well.The issues that need to be considered here is the co-existence of the POS logicwith the existing logic and the overall logic code fitting in the FPGA- if need be,larger-sized FPGA with the same footprint needs to be included.

If the existing SONET/SDH line card does not have an FPGA in the system interfaceside, the board could be respun to include an FPGA. This FPGA then couldimplement the POS standard.

Framer based approachIn case a new line card with POS interface needs to be designed, the POSinterface could be achieved by using an Off-the-shelf framer that supports POSframing as well. Wipro could design and implement this card.

Testing Packet over SONET/SDHThere are a couple of key issues to be addressed when testing Packet overSONET/SDH implementation:

a. Byte stuffing/destuffing logic

A IP packet containing all 7D/7E results in doubling of packet size after insertioninto HDLC frame. This causes maximum stress on byte stuffing and destuffinglogic. This tests the flow control mechanism between the line card and the routersegress/ingress buffer.

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b. Send minimum size packets at full rate

This tests the maximum rate of FCS calculation and also is a good test of overall linecard’s frame handling capacity.

The above two aspects test the handling of data at Layer 2 at wire speed rates.

6. Wipro is best suited to provide a POS solution

Engineering shift

There is a definite shift in product engineering today, away from “developing everythingin house”. Network equipment vendors are increasingly looking out for components,so that they could concentrate on fine-tuning system architecture.

Engineering relationships across company-boundaries are taking birth so as toleverage the competencies available. With product development cycles crashingsignificantly, faster time-to-market assumes high emphasis. By contracting relevantportions of development to companies like Wipro, engineering departments couldfocus on development of newer modules.

In this context, Wipro’s building blocks in the form of the Packet Over SONET/SDHController Core could be used to add POS interfaces to existing communicationequipment.

The Wipro Advantage

Optical Networking is a complex technology that requires several years of experiencefor developing high quality solutions. WIPRO has a great deal of expertise andpractical system “know-how” with Optical Networking technology.

Wipro has undertaken key projects in the Optical-Networking arena that includeturnkey product development from market analysis through production. Wipropossesses two decades of experience in VLSI and system software design.

This technological strength is supported by world-class procedures, which ensure ourcustomers that the product they receive is fully operating, and fully tested, thus savingseveral weeks in time-to-market.

Wipro’s building blocks and expertise on POS can help bring customers significanttime-to-market advantage to custom configure communication equipment with a POSinterface.

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7. Conclusion

Clearly, Packet over SONET/SDH is finding favor with network managers in their quest tooptimize their networks for the dominant applications. POS takes advantage of theexisting SONET/SDH infrastructure to better support IP

Wipro’s Packet Over SONET building blocks and expertise should be useful for cost-conscious engineering managers who are planning to add POS Interface to the existingrange of Internet infrastructure equipment.

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About the Authors

Ravishankar K is a specialist in VLSI and System Design at Wipro technologies and hasbeen working for 6 years in the area of FPGA, ASIC and board designs in Networking andMultimedia systems.

Jinu Koshy is a Software specialist at Wipro Technologies and has been working for 9years in the area of Telecom Software. His current area of work includes OpticalNetworking.

Contact Us

if you need to contact us regarding any clarification of feedback, mail us [email protected]

7.1 Acronyms

ADM Add Drop Multiplexer

ATM Asynchronous Transfer Mode

FPGA Field Programmable Gate Array

HDLC High Level Data Link Control

IPCP Internet Protocol Control Protocol

LCP Link Control Protocol

NCP Network Control Protocol

OSI Open System Interconnection

POS Packet Over SONET / SDH

PPP Point-to-Point Protocol

RTOS Real Time Operating System

SDH Synchronous Digital Hierarchy

SERDES Serializer - Deserializer

SONET Synchronous Optical Network

About Wipro

Wipro Technologies is a part of Wipro Limited (NYSE:WIT) and is a leading global providerof high-end IT solutions. The IT solutions provided include application developmentservices to corporate enterprises and product engineering services to technologycompanies. The company's top clients include Nortel, Cisco, Canon, Fujitsu, Microsoftand Alcatel, to name a few.

With over 10000 consultants Wipro integrates people processes and technologies todeliver innovative solutions that enable implementation of product development strategiesefficiently and cost-effectively. Wipro is also the first IT services company in the world toachieve the highest level of quality certification, the SEI-CMM Level 5. Wipro currently runslarge development facilities for industry leaders and start-ups alike, and has more than4000 consultants focused on communication hardware and software alone.

Wipro in Telecommunication TechnologiesWipro solutions for the technology sector are in areas of Optical networking, Datanetworking, Wireless networking, Voice switching, and Network Management systems.These solutions span across a vast spectrum of technologies in the hardware (VLSI,FPGA, Board design, DSP, RF), embedded software (VxWorks, EPOC, pSOS) and NetworkManagement software (CORBA, UML, SNMP, TL1) and many more. With over 10 years ofexperience in the telecom space, Wipro offers engineering services like architecting,design, development, verification, validation and maintenance.

© Copyright 2001. Wipro Technologies. All rights reserved. No part of this document may bereproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic,mechanical, photocopying, recording, or otherwise, without express written permission from WiproTechnologies. Specifications subject to change without notice. All other trademarks mentioned hereinare the property of their respective owners. Specifications subject to change without notice.

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