PLC and SONET/SDH Networks Bridging with

Ethernet

Carolina Pérez

, Enrique Areizaga

, Daniel Múgica

, Elena Terradillos

, Amaya

Pardo

ROBOTIKER-TELECOM, Parque Tecnológico de Zamudio, Edificio 202, 48170 Zamudio

(SPAIN)

ROBOTIKER

Abstract. Power Line communications (PLC) provides “Broadband Ethernet”

connectivity directly to the customer’s socket in home without additional

cabling. However, PLC networks do not provide global end-to-end connectivity

and need to rely on incumbent’s telecom networks. In order to lower the cost of

CAPEX and OPEX the network interconnection is done at “Ethernet” level.

Most of Incumbent’s networks are based on SONET/SDH Rings, and efficient

transport of Ethernet over those technologies is a prime requirement. Ethernet

over SDH/SONET (EoS), enables internet services over existing SONET/SDH

systems using a simple structure. However, SONET/SDH is a TDM technology

optimized for voice, and the standard rates are bandwidth inefficient when data

is transported. With virtual concatenation it is possible to provide fine

granularity in the transport of data traffic over SONET/SDH. The combination

of these two technologies (EoS and virtual concatenation) in the same system

will allow remote LANs to be connected together at lower costs in a very

simple and bandwidth efficient way.

1 Introduction

Power Line Communications technologies provide ubiquitous connectivity within

buildings (Multi tenant and Multi Dweller), allowing every computer or device with a

CPE card or box to be plugged into any socket of any room.

Broadband networks aim to provide High Speed connections at low cost. Protocol

translations usually bring complexity and add cost; therefore the telecom industry is

pursuing solutions that keep protocol connectivity end to end. The good old Ethernet

is breaking its barriers within the LAN world and preparing its invasion into the

WAN. The low cost of ports, as well as being well known by the communications

industry, make Ethernet the best candidate to bring low cost broadband.

A typical architecture to provide End-to-End Ethernet connectivity over power

lines will consist of:

• In-home network, based on the low voltage network, and its network elements:

− CPE devices that connect end-users to a PLC in-home network;

Pérez Sáez C., Areizaga E., Múgica D., Terradillos E. and Pardo A. (2004).

PLC and SONET/SDH Networks Bridging with Ethernet.

In Proceedings of the 1st International Workshop on Shaping the Broadband Society, pages 12-20

DOI: 10.5220/0001405500120020

 SciTePress

− Home Gateways that connect the PLC in-home network with the PLC access

network.

• Access network, based on the low voltage distribution network, and its network

elements:

− Home Gateways;

− MV/LV Gateways that connect the access network and the aggregation network.

• Aggregation network, based on a medium voltage distribution network, where the

network elements are MV/LV gateways that connect the MV network with the LV

voltage network, bypassing the transformer. Those gateways can handle the

Ethernet traffic generated by 10 to 100 end-users.

• Regional/Core networks based on incumbents SONET/SDH rings. In order to

provide Ethernet connectivity an Edge Node is required that handle the Ethernet

traffic generated by 3-7 medium voltage transformers and mapped it efficiently

into the SONET/SDH rings.

Fig. 1. Ethernet over PLC network example

2 Mapping Ethernet in SONET/SDH

SONET/SDH technology provides a highly resilient, fully managed fiber

infrastructure with back-office systems already in place to deploy global services, but

the existing SONET/SDH transport structures have been optimized to support

traditional TDM voice applications. However, data traffic is now dominating the

transport networks, and the standard rates are bandwidth inefficient when data is

transported. Tab. 1 shows the SONET/SDH digital hierarchies, with these standard

rates.

Table 1. SONET/SDH Digital Hierarchies

SONET SDH Bit Rate

STS-1 STM-0 51.84 Mbit/s

STS-3 STM-1 155.52 Mbit/s

STS-12 STM-4 622.08 Mbit/s

STS-48 STM-16 2488.32 Mbit/s

STS-192 STM-64 9953.28 Mbit/s

STS-768 STM-256 39812.12 Mbit/s

This inefficiency has forced the development of a new mechanism that provides

fine granularity to transport data traffic (Ethernet, ATM, IP, etc.) through

SONET/SDH networks. This mechanism is called virtual concatenation.

Another breakthrough technology in the SONET/SDH world nowadays is Ethernet

over SONET/SDH (EoS). EoS technology is expected to greatly simplify and reduce

the cost of maintaining corporate networks that span several geographic sites. Fig. 2

shows an EoS network example. Two new different methods for mapping Ethernet

frames over SONET/SDH payload envelopes have being specified: Link Access

Procedure-SDH (LAPS) standard by ITU-T [2] and Generic Framing Procedure

(GFP) draft standard by the T1X1.5 working group [7].

Fig. 2. Ethernet over SONET/SDH network example

Using these new technologies we are trying to develop an EoS mapper with virtual

concatenation for a SONET/SDH terminal multiplexer system.

The aim of the project is to map Ethernet frames into OC-3/STM-1 structures using

the ITU-T standards X.86 (Ethernet over LAPS) and G.707/Y.1322 and the ANSI

standard T1X1.5 (along with GFP contributions).

In this paper we briefly describe these new technologies, virtual concatenation and

Ethernet over SONET/SDH using both LAPS and GFP.

ADM

EXISTING

SDH RING

ADM

MV to SDH

GATEWAY

EoS DEVICE

MV to SDH

GATEWAY

3 Concatenation in SONET/SDH

The SONET/SDH structures define a synchronous optical hierarchy with flexibility to

carry many different capacity signals. The payload of the basic signal is structured in

virtual tributaries (VT), in SONET, or tributary units (TU), in SDH, providing in this

way for the transport of lower rate services.

Payloads that exceed the standard payload capacities can be transported using

concatenation. There are two methods of concatenation: contiguous and virtual

concatenation. Contiguous concatenation groups the payload of several signals

forming a payload that is transported as a whole through the network. In virtual

concatenation, the individual payloads associated to a concatenated group are

transported individually through the network. Virtual concatenation requires

concatenation functionality only at the path termination equipment, while contiguous

concatenation requires concatenation functionality at each network element. Virtual

concatenation only requires up-grading the ends of the path, so the up-grade costs are

lower.

Virtual concatenation is defined at two levels: high order and low order. High order

virtual concatenation group the payload of different signals of 51.840 Mbit/s or

155.52 Mbit/s, while low order virtual concatenation group the payload of different

VTs/TUs which have lower rates such as 1.544 Mbit/s, 2.048 Mbit/s, etc. Tab. 2 and

Tab. 3 show the capacity of virtually concatenated tributaries in SDH and SONET

respectively.

Bit rates for LAN services are typically 10 Mbit/s and 100 Mbit/s. Other services,

e.g. ATM cells stream, may vary from a few Mbit/s to several tens of Mbit/s. These

bit rates can be fitted in the virtual concatenated payloads, improving the use of the

bandwidth. Besides, voice and data can be sent using the same transport structure.

Associated with virtual concatenation there is also a new methodology for hitlessly

changing the payload allocated to virtually concatenated SONET/SDH SPEs. This

methodology, called Link Capacity Adjustment Scheme (LCAS), allows to

accommodate the SPE (adding or removing tributaries) to situations like requested

increases or decreases in capacity requirements or link failure conditions [6].

4 Ethernet Over LAPS

LAPS protocol and specification was introduced in ITU-T Recommendation

X.85/Y.1321 (IP over SDH using LAPS) and was defined as a type of HDLC,

including data link service and protocol specification which are used to the network of

IP over SDH [1]. LAPS allows the encapsulation of IPv6, IPv4, PPP and other upper

layer protocols, and is fully compatible with RFC 2615 (PPP over SONET/SDH)

when the address field is set to “11111111”. This protocol provides a point-to-point

unacknowledged connection-less-mode service over SDH.

ITU-T Recommendation X.86 describes how to adapt Ethernet frames to LAPS, so

they can be later transported through SDH networks. The relationship between the

different entities is shown in Fig. 3.

IEEE802.3 M AC frame

R econciliation subla

LAPS

Rate Adaption

SDH

MII

Fig. 3. Relationship between Ethernet and LAPS and SDH

Table 2. SDH Capacity of Virtually Concatenated VCs

Carried in X Capacity (kbit/s) In steps of (kbit/s)

VC-11-Xv VC-3 1 to 28 1600 to 44800 1600

VC-11-Xv VC-4 1 to 64 1600 to 102400 1600

VC-11-Xv Unspecified 1 to 64 1600 to 102400 1600

VC-12-Xv VC-3 1 to 21 2176 to 45696 2176

VC-12-Xv VC-4 1 to 63 2176 to 137088 2176

VC-12-Xv Unspecified 1 to 64 2176 to 139264 2176

VC-2-Xv VC-3 1 to 7 6784 to 47448 6784

VC-2-Xv VC-4 1 to 21 6784 to 142464 6784

VC-2-Xv Unspecified 1 to 64 6784 to 434176 6784

Table 3. SONET Capacity of Virtually Concatenated VTs

Carried in X Capacity (kbit/s) In steps of (kbit/s)

VT1.5-Xv STS-1 1 to 28 1600 to 44800 1600

VT2-Xv STS-1 1 to 21 2176 to 45696 2176

VT3-Xv STS-1 1 to 14 3328 to 46592 3328

VT6-Xv STS-1 1 to 7 6784 to 47448 6784

VT1.5-Xv STS-3c 1 to 64 1600 to 102400 1600

VT2-Xv STS-3c 1 to 63 2176 to 137088 2176

VT3-Xv STS-3c 1 to 42 3328 to 139776 3328

VT6-Xv STS-3c 1 to 21 6784 to 142464 6784

VT1.5-Xv Unspecified 1 to 64 1600 to 102400 1600

VT2-Xv Unspecified 1 to 64 2176 to 139264 2176

VT3-Xv Unspecified 1 to 64 3328 to 212992 3328

VT6-Xv Unspecified 1 to 64 6784 to 434176 6784

5 Ethernet Over GFP

GFP defines a standard framing procedure for octet-aligned, variable-length payloads

(Ethernet, PPP/HDLC, etc.) for subsequent mapping into SONET synchronous

payload envelopes. Fig. 4 shows the relationship between payloads, GFP and SONET.

Ethernet IP/PPP

Other client signal

GFP – Client Specific Aspects

Transport Path: SDH, SONET, OTN

GFP - Common Aspects

Ethernet IP/PPP

Other client signal

GFP – Client Specific Aspects

Transport Path: SDH, SONET, OTN

GFP - Common Aspects

Fig. 4. Relationship between Payloads and GFP and SONET

6 SONET/SDH Mapping of LAPS and GFP frames using Virtual

Concatenation

Once the LAPS or GFP frame has been created it must be mapped into a

SONET/SDH structure for its transport across the network. There are many different

ways to do this, as defined in [4] and [5], but the following are some of the most

interesting examples:

• Using a VT1.5/TU-12 structured STS-3/STM-1 and virtual concatenation.

• Using a STS-1-SPE/VC-3 structured STS-3/STM-1 and virtual concatenation.

• Using a STS-3c/STM-1.

• Using a STS-1-SPE/VC-3.

The first method is an example of low order virtual concatenation, and the second

is an example of high order virtual concatenation. The third method is an example of

contiguous concatenation, while the fourth one does not make use of any kind of

concatenation.

We are going to describe briefly the first two mapping methods proposed, as they

make use of virtual concatenation, allowing for fine granularity and better bandwidth

efficiency.

7 Low order virtual concatenation

The first method proposed is an example of low order virtual concatenation, and

allows us to create virtual pipes of multiples of 1.544 Mbit/s in SONET or 2.048

Mbit/s in SDH.

Virtual concatenation allows us to create a synchronous payload envelope (SPE),

different from those in the standard set, to efficiently transport the LAPS or GFP

payload. As an example, if we want to transport a payload of around 10 Mbit/s

(Ethernet) we would need to virtually concatenate 7 VT1.5s. In Fig. 5 we show the

SPE created by virtually concatenating X different VT1.5s.

VT1.5 SPE # X

1X Xx25

VT1.5-Xv SPE

VT 1.5 S P E # 1

Fig. 5. X virtually concatenated VT1.5s SPE structure.

The payload (LAPS or GFP frames and inter-frame fill octets) is mapped in X

individual tributary SPEs which form the virtually concatenated SPE. Each of the

tributaries SPE has its own POH, and is transported individually through the network.

8 High order virtual concatenation

The second method is an example of high order virtual concatenation. LAPS or GFP

frames are inserted in the payload of 1 to 3 virtually concatenated STS-1s/VC-3s. As

an example, the SPE created by virtually concatenating X different STS-1s is shown

in Fig. 6.

As in the case of low order virtual concatenation, each STS-1 has its own POH and

is transported individually through the network.

9 EoS Mapper System Overview

Using the technologies described we are developing a system to transfer voice (T1s

and/or E1s) and Ethernet data (10Mbps and 100Mbps) over an OC-3 (SONET) or

STM-1 (SDH) link. The main innovation introduced consists in the use of virtual

concatenation to map Ethernet data using LAPS or GFP encapsulation. The system’s

interfaces include an OC-3/STM-1 optical interface, eight 10Mbps/100Mbps Ethernet

ports, and up to 28 T1s and 16 E1s.

13059

ST S-1 S PE#1

13059

X x 8 4

STS-1-Xv SPE

payload capacity

STS-1 SPE #X

STS-1 SPE #1

= Stuff Columns

Fig. 6. X virtually concatenated STSs SPE structure

The SONET/SDH overhead terminator, T1 and E1 mappers, framers and LIUs and

Ethernet MAC and PHY are based on commercial application specific integrated

circuits (ASICs), while the Ethernet over LAPS or GFP framing and the subsequent

SONET/SDH mapping is performed by a FPGA. This FPGA supports four mapping

modes: low order virtual concatenation, high order virtual concatenation, contiguous

concatenation and without concatenation.

10 Conclusions

PLC networks provide a good alternative within the home, access and aggregation

networks. However, in order to offer end to end Ethernet connectivity, there is a need

to be interconnected to metro and regional networks through SONET/SDH networks.

In order to provide an efficient way to transport the Ethernet traffic coming from

the PLC networks into the metro/regional networks, new ways of encapsulation and

transmission are requested in the SONET/SDH world.

The Next Generation SONET/SDH networks will incorporate Ethernet data

encapsulation and virtual concatenation.

This paper has sketched the current alternatives for “Ethernet encapsulation”, one

within the HDLC world (LAPS) and a GFP. From both, GFP is nowadays gaining

momentum due to its flexibility, bandwidth efficiency, single mechanism for

multiprotocol encapsulation and interoperatibility.

Besides, the Next Generation SONET/SDH will incorporate the Virtual

concatenation mechanism, eliminating the bandwidth waste that results from a

mismatch between the granularity required for Ethernet data transport and the

bandwidth offered by traditional TDM structures. One step forward will be the

inclusion of link capacity adjustment schemes (LCAS).

Therefore, in order to provide end-to-end efficient Ethernet transport between PLC

access/aggregation networks and SONET/SDH metro/regional networks, there is only

the need to up-grade the path termination edges at the SONET/SDH rings with GFP

encapsulation, Virtual concatenation and LCAS.

The solution presented in this paper will reduce the CAPEX and OPEX and thus

provide a solution for efficient low cost high speed Ethernet transmissions.

OPERA (Open PLC European Research Alliance for new generation PLC

integrated network) is a project funded by the European Commission. The Sixth

Framework Programme (FP6) sets out the priorities - including those of the

Information Society Technologies (IST) - for the period 2002-2006. The OPERA

project will make a significant contribution within the IST area “Broadband for All”.

The activities carried out within the work package 2 of the OPERA project are related

to the work described in this paper. One of the objectives of this work package aims

to optimize the interconnection of the PLC access network to the backbone network to

improve competitiveness of PLC to other broadband access technologies.

References

1. ITU-T: Recommendation X.85/Y.1321. “IP over SDH using LAPS”. 03/2001.

2. ITU-T: Recommendation X.86. “Ethernet over LAPS”. 02/2001.

3. IEEE 802.3. “CSMA/CD Access Method and Physical Layer Specifications”,

2002.

4. ITU-T: Recommendation G.707/Y.1322. “Network node interface for the

synchronous digital hierarchy (SDH)”. 12/2003.

5. ANSI: Synchronous Optical Network (SONET) - Basic Description including

Multiplex Structure, Rates, and Formats (Includes T1.105a-2002): May, 2001.

6. ITU-T: Recommendation G.7042/Y1305: “Link Capacity Adjustment Scheme

(LCAS) for SONET Virtually Concatenated SPEs”. 02/2004.

7. ITU-T: Recommendation G.7041/Y130.: “GFP: Generic Framing Procedure”

12/2001. Amendment and Corrigendum 12/2003.