Carrier Ethernet: The native approach

Ericsson Review No. 3, 200784

Introduction

IP and Ethernet are becoming ubiquitous – IP packets make up the majority of traffi c carried over the world’s communication net-works (by volume, if not yet by value) and more often than not this traffi c is presented to the network in Ethernet frames. In short, Ethernet is increasingly becoming the bearer technology of converging networks.

The advantage of Ethernet has always been its ability to leverage high volumes, thanks especially to its dominance in the enterprise market. Throughout its thirty-year history, Ethernet has shown an extraordinary capac-ity to adapt and grow. It is popular largely

because of its ability to self-confi gure, based on the key concepts of

“learning bridge” (fl ooding and associating learned destination addresses with bridge ports); and “spanning tree” (the protocol used to avoid loops). Despite this, these features have weaknesses

in the context of large-scale public networks. The learning bridge procedure, for example, broadcasts unknown addresses, resulting in fl ooding, which clearly does not scale well. Similarly, the spanning tree protocol often makes poor use of underlying transport re-sources. Moreover, standard Ethernet lacks key public network features, in particular for

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operation, administration and maintenance (OAM) and for isolating customers.

Public networks are evolving into what Ericsson terms Full Service Broadband, which carries a steadily widening range of rich multimedia services to fi xed and mo-bile devices over a common network with carrier-class characteristics, such as scalabil-ity, robustness, and resilience.1 Current de-velopments in Ethernet as a public network transport technology include

the defi nition of standardized services to be provided by an Ethernet-based public network;fl exibility, to enable scaling a network to a global size while supporting many concur-rent service networks;comprehensive OAM mechanisms for monitoring service quality and service lev-el agreements (SLA), and for detecting and locating faults and misconfi gurations; andthe creation of a highly scalable transport control plane solution that facilitates rapid restoration of service and supports auto-mated provisioning.

Several technologies including generic framing procedure/synchronous digital hi-erarchy (GFP/SDH) or multiprotocol label switching (MPLS) can be used to transport carrier-grade Ethernet services. The focus of this article, however, is on the evolution of native Ethernet technology to carry these services.

Convergent network architecturesThe increasing dominance of IP and Ether-net is enabling a convergence of networks, allowing for a wide range of services to be carried over the same infrastructure. Mobile backhaul, business services and residential services are applications with important re-quirements for a converged network archi-tecture.

Mobile backhaulOperators typically try to situate mobile base stations where they can provide maxi-mum coverage and be backhauled to the core network at minimum cost. At present, more than 60% of all mobile base stations are con-nected by microwave links into main back-haul networks of leased lines.

While microwave will still be the domi-nant technology for last-mile backhaul, new, higher-bandwidth mobile services require

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Carrier Ethernet: The native approachHoward Green, Sylvain Monette, Jonathan Olsson, Panagiotis Saltsidis and Attila Takács

This article reviews the developments and emerging standards of native Ethernet technology that give Ethernet the features of a packet transport technology for public networks. The increasing dominance of IP and Ether-net is enabling a convergence of networks and allowing for a wide range of services (to business and residential customers and for mobile backhaul) to be carried over the same infrastructure.

TERMS AND ABBREVIATIONS

3GPP Third Generation Partnership

Project

ATM Asynchronous transfer mode

BCB Backbone core bridge

BEB Backbone edge bridge

B-VID Backbone VLAN ID

CCM Connectivity check message

CFM Connectivity fault management

C-VLAN Customer VLAN

Diffserv Differentiated services

DSL Digital subscriber line

DSLAM DSL access multiplexer

ESP Ethernet label-switched path

EVC Ethernet virtual connections

GELS GMPLS-controlled Ethernet label

switching

GFP Generic framing procedure

GMPLS Generalized MPLS

HSI High-speed internet

HSPA High-speed packet access

IEEE Institute of Electrical & Electronics

Engineers

IP Internet protocol

IPTV IP television

I-SID Service instance identifi er

IT Information technology

ITU International Telecommunication

Union

LAN Local area network

LMP Link-management protocol

MA Maintenance association

MAC Media access control

MEF Metro Ethernet Forum

MEN Metro Ethernet network

MEP MA endpoint

MIP MA intermediate point

MPLS Multiprotocol label switching

MSTP Multiple spanning tree protocol

NNI Network-to-network interface

OAM Operations, administration and

maintenance

OPEX Operating expenses

OSPF-TE Open shortest path fi rst – traffi c

engineering

PBBN Provider backbone bridged network

PBB-TE Provider backbone bridging – traffi c

engineering

PON Passive optical network

QoS Quality of service

RAN Radio access network

RSVP-TE Resource reservation protocol

– traffi c engineering

SDH Synchronous digital hierarchy

SLA Service level agreement

SPB Shortest-path bridging

S-VID S-VLAN ID

S-VLAN Service provider VLAN

TDM Time-division multiplexing

UNI User network interface

VDSL Very high-speed DSL

VLAN Virtual LAN

VPN Virtual private network

Review3.07.indd 84Review3.07.indd 84 07-10-08 10.26.4707-10-08 10.26.47

Ericsson Review No. 3, 2007 85

greater use of fi ber closer to base stations. At the same time, 3GPP mobile technolo-gies are migrating toward the use of IP and Ethernet (although SDH- and ATM-based backhaul still have much life left in them). Key requirements for this approach include timing accuracy and low delay for voice transport, and the emulation of E1 and asyn-chronous transfer mode (ATM) bearers to aid migration.

Business services: transformation of leased-line and VPN serviceBusinesses want to interconnect multiple sites reliably, transparently and at local area network (LAN) speed (currently 1Gbps or 10Gbps). Doing so will allow them to con-solidate information technology (IT) infra-structures, provide full-speed application access for nomadic users, and signifi cantly reduce internal expenses. Operators are thus looking to replace separate, low-speed time-division multiplexing (TDM), Frame Relay or ATM leased-line networks with new con-nectivity services, such as point-to-point Eth-ernet and virtual private networks (VPN).

Businesses also want to improve their high-speed internet (HSI) access to improve effi ciency and to extend transparent appli-cation access to remote offi ces and mobile workers.

Residential servicesThe bandwidth available to residential users is rapidly approaching standard LAN speed, thereby enabling a wide range of new appli-cations.

IPTV

Good IPTV service emphasizes interactiv-ity and personalization and meets high user expectations for service availability, quality, and responsiveness. This requires redundan-cy, fl exibility and scalability in the network, and the combination of unicast with multi-cast capabilities.2

High-speed internet access

The network must scale to provide the re-quired “internet” capacity. Many users today both consume and produce video-rich media, driving the need for more symmetric access and increased aggregation bandwidth. A range of background applications, such as rich media podcasting, peer-to-peer media distribution methods, and automatic soft-ware updates, are fi lling up the troughs with varying levels of network load.

Multimedia and voice service

Conversational services are expanding from sim-ple voice to rich interactive multimedia. These services span a range of bandwidths, many of which will require low transport delay.

Common architecture The demand for a convergent network is be-ing driven by a common set of requirements and trends:

Fixed and mobile users demand greater bandwidth, using technologies including VDSL2, passive optical network (PON), active Ethernet, and high-speed packet ac-cess (HSPA). Greater bandwidth calls for shorter dis-tances between end users and copper or radio access network (RAN) equipment. This, in turn, drives fi ber deployment deeper into the access network. Growing demand for bandwidth is not being matched with increases in revenue, which means operators must somehow cut the cost per transported bit. Operators are increasingly sharing network infrastructure as a way of reducing costs. Operators want to retire old technologies in order to manage and limit their own ar-eas of expertise. Operators want to reduce the number of network sites and centralize complex func-tions in order to cut operating expenses (OPEX). Many incumbent operators want to cash in on their property portfolios by vacating sites where possible.

Operating multiple services (and businesses) on a common network requires effective sep-aration of traffi c and support for monitoring service levels. However, pure class-of-service techniques, such as differentiated services (Diffserv), cannot do this alone. In addi-tion, different kinds of applications and traf-fi c must be allocated to resource partitions defi ned by a virtualizing layer on top of the physical network bearers.

This virtualization layer augments the tra-ditional role of transport layers by supporting packet networking, giving rapid topology protection while accommodating hierarchical network layering to aid scalability. The packet-transport layer also provides a stable basis for monitoring trends and planning investments and medium-term capacity.

Carrier Ethernet services

The Metro Ethernet Forum (MEF) describes

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Figure 1Top: E-Line service type.Middle: E-LAN service type.Bottom: E-Tree service type.



Carrier Ethernet as a ubiquitous, standard-ized, carrier-class service with fi ve distin-guishing attributes: standardized services, scalability, reliability, quality of service (QoS) and service management.

MEF has defi ned the requirements put on network reference points including the user-network interface (UNI) and network-to-network interfaces (NNI). The MEF archi-tecture is based on Ethernet virtual connec-tions (EVC), where an EVC is an associa-tion of two or more UNIs over one or more Metro Ethernet networks (MEN) that trans-port Ethernet frames. Each EVC has a set of service attributes (service type, multiplex-ing support, bandwidth profi les, and per-formance assurance) that are used to defi ne services in a fl exible way and to standardize SLAs.

The service attributes are refl ected in the Ethernet service types E-Line, E-LAN, and E-Tree, which can be defi ned on a per-port basis, or multiplexed on a shared port.

E-LineThe E-Line service type (Figure 2, top) is a point-to-point service that connects two UNIs over the MEN (providing an Ethernet leased line). Many fl avors of service can be defi ned using E-Line, from simple symmet-rical best-effort service without performance guarantees to a multiplexed service that con-nects UNIs of different speeds with band-width profi les and stringent performance requirements.

E-LANThe E-LAN service type (Figure 2, mid-dle) supports multisite enterprise LAN services by connecting multiple UNIs in a multipoint-to-multipoint fashion. To cus-tomers, this gives the appearance of being a bridged Ethernet network. A UNI in an E-LAN service may send service frames to any other UNI that is a member of the EVC.

E-TreeThe E-Tree service type (Figure 2, bottom) is a rooted-multipoint service that is suitable for IPTV distribution and mobile backhauling. One or more UNIs are defi ned as leaves and one or more as roots. The UNIs that serve as leaves can only exchange service frames with UNIs that are roots, whereas root UNIs can send service frames to other root UNIs and to all leaves in the EVC.

The MEF is working to defi ne Ethernet

Figure 3Example of provider bridging (PB) and provider backbone bridging (PBB) format.

Figure 2Provider backbone bridged network (PBBN).



services for enterprises and mobile backhaul networks. Mobile operators can thereby use the MEF service types to replace traditional leased lines, to offl oad high-speed packet access (HSPA) and to support migration to Ethernet networks.

Ethernet scalability

The IEEE 802.1Q standard is being extend-ed to evolve Ethernet technology for use in large public networks. Scalability extensions include

802.1ad, Provider Bridge (PB); and 802.1ah, Provider Backbone Bridge (PBB).

Thanks to functions that permit mapping between VLAN labels, provider bridges can separate VLAN service instances in customer domains (C-VLAN) from service provider VLANs (S-VLAN) in the service provider domain. Service providers can thus separate traffi c and constrain broadcasts in the net-work while preserving customer VLAN in-formation. The size of provider bridge net-works is limited by number of S-VLANs, however.

Provider backbone bridging introduces the backbone edge bridge (BEB), which en-capsulates provider bridge frames in a pro-vider backbone frame (Figure 4) labeled with backbone source and destination MAC ad-dresses. Using existing bridged and virtual bridged LAN (VLAN) technologies, pro-vider backbone bridges allow practically un-limited scaling of provider bridge networks – to at least 16 million (224) service instances. Provider backbone bridge techniques are compatible and interoperable with provider bridge techniques.

A provider backbone bridged network (PBBN) comprises a set of backbone edge bridges, possibly interconnected by pro-vider bridges deployed as backbone core bridges (BCB). Figure 5 shows how customer S-VLAN service instances (for example, an E-LAN instance for a large distributed en-terprise) are interconnected in a PBBN:

A 24-bit I-SID (service instance identifi er) identifi es a customer S-VLAN service in-stance in the backbone. The instances are identifi ed by unique S-VID tags in dif-ferent client domains (thus removing the scaling restriction on total number of ser-vice instances).The provider backbone edge bridge trans-lates between S-VID and I-SID (mapping is provided at service setup).

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Carrier Ethernet OAM

Operation, administration and maintenance functions are required to monitor SLAs, detect and locate failures and misconfi gu-rations, and to measure quality trends and impairments. The IEEE and ITU are cur-rently working to standardize OAM. A key component of this work is connectivity fault management (CFM, IEEE 802.1ag), which specifi es protocols, procedures, and managed objects. These elements facilitate the dis-covery and verifi cation of the path through bridges and LANs and the detection and isolation of a connectivity fault to a specifi c bridge or LAN. CFM establishes managed objects, called maintenance associations (MA), to bring structure to the exchange of CFM messages. The scope of a maintenance association is determined by the manage-ment domain (MD), which describes a net-work region where connectivity and perfor-mance is managed. Each MA associates two

or more maintenance association endpoints (MEP) and allows maintenance association intermediate points (MIP) to support fault detection and isolation. CFM messages are sent in each MA to verify connectivity and to isolate faults (Figure 6).

Fault detectionFault detection uses the continuity check protocol to detect both connectivity failures and unintended connectivity between main-tenance associations. Each MEP can periodi-cally transmit a multicast connectivity check message (CCM) and track CCMs received from other MEPs in the maintenance asso-ciation. A connectivity check can detect ser-vice cross-connect, duplicate MEP confi gu-rations, missing or unexpected MEPs, data loss, and jitter.

Fault verifi cation and isolationFault verifi cation and fault isolation are ad-ministrative actions typically performed after

Figure 4Backbone service instances in a PBBN.



fault detection. The functions also confi rm successful initiation or restoration of connec-tivity. The administrator uses the loopback protocol to perform fault verifi cation. Send-ing a high volume of loopback messages can test bandwidth, reliability, and jitter.

Path discoveryPath discovery uses the multicast linktrace protocol to determine, link by link (from one MEP to another), the path taken to a target MAC address.

Carrier Ethernet control

Provider backbone bridging – traffi c engineering (PBB-TE)Carrier-grade networks rely on explicit control of path routing so that traffi c can be engineered to allocate bandwidth, assure diverse backup path routing, and select path performance as required by the SLAs. Most major network providers currently deploy IEEE 802-based networks and will need traffi c engineering to balance load and protect switching. PBB-TE (IEEE 802.1Qay) will enable network providers to engineer connections in a PBB network.

Service providers can employ PBB-TE in the service domain of a PBBN to confi g-ure resilient, SLA-driven, point-to-point Ethernet trunks that fulfi ll stringent QoS and traffi c-management requirements. The trunks allow carriers to engineer traffi c-managed circuits that can be monitored, along with the rest of the 802.1ah network, using 802.1ag protocols. Paths generated by PBB-TE may be used to guarantee route di-versity (for protecting paths), to balance net-work load, and to guarantee performance.

In standard provider backbone bridged networks, traffi c engineering is limited by the multiple spanning tree protocol (MSTP) control plane protocols, which populate the bridge fi ltering tables. PBB-TE replaces the MSTP control plane with either a manage-ment plane or an external control plane, and populates the bridge fi ltering tables of the component bridge relays by creating static fi ltering table entries (Figure 7).

PBB-TE is a connection-oriented Ethernet technology that uses a statically confi gured tuple {DA-MAC, VID, port} of fi ltering en-tries to create PBB-TE paths. Because for-warding is based on the destination MAC address and VLAN ID, the tuple can be viewed as a 60-bit Ethernet label, and the

Figure 5Overview of the OAM architecture.

Figure 6Example PBB-TE network.



constructed PBB-TE path, as an Ethernet label-switched path (ESP).

The external PBB-TE management/con-trol plane maintains and controls the topol-ogy information to support point-to-point and multipoint Ethernet switched paths over the PBBN. The PBB-TE topology can coexist with MSTP and the new shortest-path bridg-ing (SPB) technology, by allocating B-VID spaces to each path. PBB-TE takes control of a range of B-VIDs from the backbone core bridges (BCB) and backbone edge bridges (BEB) of the PBBN. The external manage-ment or control plane can enforce the con-nection admission control function without modifying existing Ethernet bridges.

GMPLS for carrier EthernetGeneralized MPLS (GMPLS, RFC 3471) is emerging as a key unifying technology for out-of-band control in many packet transport technologies. GMPLS, which began with the MPλS initiative for controlling wavelengths, now supports a wide variety of transport bear-ers and is deployed in many large networks to support rapid automatic restoration. That is, it extends the MPLS control protocols (in particular RSVP-TE and OSPF-TE) and adds a separate link-management protocol (LMP) to verify connectivity and correlate the data and control planes. In addition, it enables the automation of topology discovery, path provisioning (including backup paths), and rapid restoration.

Given that GMPLS is out of band, it can be used, with appropriate parameter extensions, for any data plane technology. The extensions for Ethernet under discussion in IETF are known as GMPLS-controlled Ethernet label switching (GELS). The core part of the ex-tension is the defi nition of a 60-bit Ethernet forwarding label or tuple {DA-MAC, VID}.

A single set of control plane protocols for different transport layers in a network will be a major step forward, reducing operating costs and allowing for new multilayer traffi c-engineering methods.

Conclusion

Ericsson is helping to drive the evolution of Ethernet as a public network technology and as a part of Full Service Broadband, by work-ing to bring together the virtues of Ethernet (low cost, ease of confi guration) and current transport technologies (stability, fault toler-ance, monitoring and diagnosis) to ensure high levels of reliability and low-cost packet

transport (reducing both capital and operat-ing expenditures).

This is increasingly a matter of network simplifi cation, by means of self-discovery and diagnosis. The main focus is thus on highly capable OAM functions and automation of provisioning via a common transport control plane based on GMPLS.

REFERENCES

Ericsson White Paper: “Full Service Broadband Metro Architecture” June 2007

www.ericsson.com/technology/whitepapers/fsb_metro_rev_b.pdf

Arberg, P., Cagenius, T., Tidblad, O., Ullerstig, M., and Winterbottom. P.: Network

infra structure for IPTV. Ericsson Review, Vol. 84(2007)3, pp. 79-83

IEEE 802.1Q-2006 D0.1 Draft: Virtual Bridged Local Area Networks

IEEE 802.1ag D8.0 Draft: Virtual Bridged Local Area Networks – Amendment 05: Connectivity

Fault Management, Feb 2007

IEEE 802.1ah D3.5 Draft: Virtual Bridged Local Area Networks – Amendment 06:Provider

Backbone Bridges, April 2007

IEEE 802.1Qay Draft: Provider Backbone Bridge – Traffi c Engineering, May 2007

MEF 10.1 Ethernet Service Attributes, Phase 2 www.metroethernetforum.org/pdfs/

standards/mef10.1.doc

RFC3473 L. Berger, Editor, “Generalized Multi-Protocol Label Switching (GMPLS) Signaling

Resource ReserVation Protocol-Traffi c Engineering (RSVP-TE) Extensions,” January 2003

Draft-PBBTE Don Fedyk et al, “GMPLS control of Ethernet,” work in progress, March 2000

Draft-EXP Loa Andersson et al, “Extension to RSVP-TE for GMPLS Controlled Ethernet

– An experimental approach,”, work in progress, January 2007

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Figure 7GMPLS – unifi ed control of multiple data planes.


Carrier Ethernet: The native approach

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