Whitepaper c ein_backhaul_10jul

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© Tech Mahindra Limited 2009

Migrating to Carrier Ethernet in Mobile Backhaul

A whitepaper By Pramila A, Sasindran M Prabhu 10th July, 2009

Abstract:

Backhaul, the part of the network from the wireless tower tothe Mobile switching center, has always been an expensivepart of the total service model. Growth in service demand hasdrastically increased the bandwidth requirement in thebackhaul, while revenue per bit is going down.

To meet the dual challenge of rapid bandwidth growth and newservices, wireless service providers and backhaul providersneed to implement backhaul solutions optimized for cost andperformance.

Migrating to Carrier Ethernet is one of the solutions for many ofthe backhaul issues. Migration from legacy backhaul to CarrierEthernet technology in the RAN (Radio Access Network) offersmany benefits for both service providers and also to thecustomers.

This white paper provides the importance and benefits ofCarrier Ethernet migration, key challenges in migration andCESoETH (Circuit Emulation Service over Ethernet - one ofthe migration methodologies).

Based on the Resource Capabilities and Market trends inMobile Backhaul, Tech Mahindra can plan to involve inRequirement, Design, Development and Testing of CESoETH-IWF area.

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Table of Contents Introduction .................................................................................. 3 Acronyms and Abbreviations ....................................................... 4 Importance and Benefits of Carrier Ethernet Migration ................ 5 Key Challenges in Carrier Ethernet Migration .............................. 5

Migration Strategy ................................................................... 5 Packet Offload over Carrier Ethernet 6 Emulation over Carrier Ethernet 6 Dual Stack RAN 7 Full Ethernet 7

Carrier Ethernet Services for Mobile Backhaul ........................ 7 E-Line 7 E-LAN 8 E-Tree 8

Synchronization and Clock Recovery ...................................... 8 CES over Carrier Ethernet ........................................................... 8

CES Interworking Function ...................................................... 9 CES Architecture - IWF Direction 9 TDM Line Service over MEN 10 Functional Components and Interfaces 10 CES Interfaces 10 Functional Elements 11

User-Network Interface .......................................................... 12 Summary ................................................................................... 13 Tech Mahindra’s Plan: ............................................................... 14 References ................................................................................ 15

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Introduction Mobile backhaul includes networks and network technologies in RAN (Radio Access Network) and Core network (MSC, GPRS, VLR, HLR etc.). As per Metro Ethernet Forum, it refers to the network part in the RAN (Radio Access Network) between BTS (Base Transceiver System) and BSC (Base Station Controller). Traditionally, it uses TDM (Time Division Multiplexing) and ATM (Asynchronous Transfer Mode) network technologies. But the next generation equipment and networks are based on Ethernet.

Many mobile service providers are in the process of adapting their RAN’s to incorporate innovative high-speed data services such as 3G and 4G HSPA (Third and Fourth Generation High-speed packet access), WiMAX (Worldwide Interoperability for Microwave Access) and 1xEV-DO (CDMA Single carrier EVolved Data Optimized). As the volume of such bandwidth-intensive traffic grows, the costs for RAN backhaul grow correspondingly, lowering ARPU (Average Revenue per User).

The average revenue per megabit for data service is far lower than for traditional voice and text messaging. But consumers are demanding mobile broadband services at affordable prices. Adapting traditional circuit-switched transport architectures to support these new services are quite expensive. Mobile service providers are therefore looking for alternative ways to scale bandwidth in the RAN (Radio Access Network) while reducing their growing operating expenses. Carrier Ethernet technology is one of the best ways in the RAN for backhaul to significantly increase the performance while lowering the operating expense.

A number of Ethernet innovations have emerged to ensure that Carrier Ethernet provides Carrier-Grade packet transport. It also meets the performance criteria for mobile backhaul applications. They are

Provider Backbone Transport

Pseudo wire technology (PWE3)

Circuit Emulation Services (CES) technologies [16].

This white paper deals with CESoETH (Circuit Emulation Services over Ethernet) methodology for RAN backhaul migration. Circuit Emulation Service allows the transport of synchronous circuits such as T1/E1 over asynchronous networks. It is originally developed to allow T1/E1 to run over ATM, CES (Circuit Emulation Service) now can be extended to work over Ethernet Networks.

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Acronyms and Abbreviations ARPU Average Revenue per User ATM Asynchronous Transfer Mode BTS Base Transceiver System BSC Base Station Controller BS Base Station CE Customer End CEN Carrier Ethernet Network CES Circuit Emulation Service CESoETH Circuit Emulation Service over Ethernet ECDX Emulated Circuit Demultiplexer/Multiplexer Function ECID Emulated Circuit Identifier EFT Ethernet Flow Termination E-LMI Ethernet-Local Management Interface EVC Ethernet Virtual Connection GIWF Generic Interworking Function GSM Global System for Mobile Communications LAN Local Area Network LTE Long Term Evolution HSDPA High Speed Downlink Packet Access HSPA High Speed Packet Access MEN Metro Ethernet Network NC Network Controller (Same as BSC) OAM Operation Administration and Maintenance PBT Provider Backbone Transport PDA Personnel Digital Assistant PDH Plesiochronous Digital hierarchy PSN Packet Switched Network PWE3 Pseudo Wire Edge-to-Edge Encapsulation RNC Radio Network Controller RAN Radio Access Network SONET/SDH Synchronous Optical Network/ Synchronous Digital Hierarchy TDM Time Division Multiplexing TSP TDM Service Processor UNI-C User-Network Interface - Customer side UNI-N User Network Interface - Network Edge UMB Ultra Mobile Broadband UMTS Universal Mobile Telecommunications System WiMAX Worldwide Interoperability for Microwave Access

5 © Tech Mahindra Limited 2009

Importance and Benefits of Carrier Ethernet Migration The number of subscribers using mobile are increasing extensively, which in turn increases the bandwidth per subscriber. Also increase in availability of mobile data devices such as 3G handsets, laptop cards and PDA has driven demand for new mobile data, video and multimedia services. These services are made possibly by 3G technologies like HSDPA.

This demand for mobile data services is further accelerated by 4G technologies like WiMAX, UMB and LTE which are now beginning to be deployed. The challenge for wireless operators is to support more subscribers, satisfy higher bandwidth requirements per subscriber and also to increase the ARPU.

The wireless operators must reassess their current backhaul infrastructure. The current 2G networks uses PDH, SONET/SDH leased lines or microwave to carry TDM voice traffic from BTS to BSC. Similarly, 3G networks use leased lines to provide ATM backhaul between the UMTS node and RNC. Huge increase in traffic created by mobile data services makes this model unsustainable.

This allows wireless operators to tune to Carrier Ethernet to provide a more flexible shared bandwidth access infrastructure. Ethernet has dominated the LAN for years. Now, with the addition of number of key technology enablers, Ethernet is capable of carrier grade performance and is referred to as Metro or Carrier Ethernet. The widespread adoption of Ethernet has made it more cost-effective than any other networking technology. Ethernet offers wireless operators both the bandwidth and cost points needed to support mobile multimedia services.

Key Challenges in Carrier Ethernet Migration

Migration Strategy RAN backhaul migration to carrier Ethernet [1], [8], [9], [13] from legacy networks will not happen in overnight. It is required to make use of the existing infrastructure and network elements as much as possible before migrating to full Ethernet in RAN backhaul. Reference model for the Mobile Backhaul is shown below.

Fig 1. Mobile Backhaul Reference Model

RAN BTS can either be a single base station or collection of several base stations. RAN BSC is nothing but a single network controller or several network elements. Few migration strategies


mentioned below, which are used to migrate from legacy network to full Ethernet in the RAN backhaul.

Packet offload over Carrier Ethernet

Emulation over Carrier Ethernet

RAN Dual Stack

Full Ethernet

Packet offload and emulation over carrier Ethernet can be used where RAN BTS and BSC cannot be directly connected to UNI as they have non-Ethernet based interfaces such as TDM or ATM. These two migrations require GIWF (Generic Inter Working Function), which in turn is connected to UNI. RAN Dual Stack and Full Ethernet can be connected directly to UNI via Ethernet interface without the use of GIWF as they are Ethernet based interface.

Packet Offload over Carrier Ethernet

In this scenario, access network is split into two parallel networks, legacy network and MEN. The legacy network is used to carry the traditional voice traffic. Where as the MEN is used to carry the data traffic. It is highly appropriate where an operator wants to offload low priority high bandwidth traffic from the legacy network to MEN in order to scale after network demand.

Fig 2. Packet offload over Carrier Ethernet

Emulation over Carrier Ethernet

In this, the legacy network is replaced by the MEN. BSC and BTS are connected to MEN via GIWF. All traffic from BTS/BSC is transported over MEN using Ethernet services. Emulation of TDM/ATM circuits can be achieved by CES or Pseudo wire technology. In this white paper, CESoETH (Circuit Emulation Services over Ethernet) technology is used for RAN backhaul migration from legacy network to Carrier Ethernet Network.

Fig 3. Emulation Over Carrier Ethernet


Dual Stack RAN

It is similar to Packet offload, but BTS/BSC is directly connected to UNI via an Ethernet interface and to MEN eliminating the need for a GIWF. Low priority high bandwidth traffic is offloaded via MEN and Legacy is used for high priority voice traffic.

Fig 4. Dual Stack RAN

Full Ethernet

All traffic from RAN base station is directly transported via UNI over the Carrier Ethernet Network.

Fig 5. Full Ethernet

Carrier Ethernet Services for Mobile Backhaul Carrier Ethernet specifies the service attribute and related parameter values for mobile backhaul Ethernet services [6], [7], [16] for a given service types. They are

E-Line (Point-to-Point)

E-LAN (Ethernet LAN Service)

E-Tree (Rooted Multipoint)

E-Line

Most of the mobile backhaul networks today are point-to-point services. The Ethernet Line services can be used to emulate existing service offerings with a point-to-point relationship between BSC and BTS. Multiple EVCs (Ethernet Virtual Connections) can also be used between BSC and BTS. EVCs are purely based on services. If multiple EVCs used between BTS and BSC, then maximum of 4095 Base Stations can be connected per Network Controller UNI.


E-LAN

E-LAN service is required when some mobile operators access other services from one or more UNIs at the same time. For example, UNI is in mobile operator site and its BTS can support different technologies like legacy GSM and WiMAX. Each technology may have a specific EVC assigned to transport mobile backhaul traffic and different UNI Peers. This E-LAN service is used to address this need.

E-Tree

E-Tree service is used when mobile operators with multiple sites may want to interconnect them to provide services other than LAN. These services may be distributed from a single or several centralized sites where the distribution sites are named as roots and all the remaining sites are named as leaves. Traditionally in mobile backhaul, BTS sites needs to exchange the service frames with only BSC and not within the BTS’s. This kind of behavior is possible by the use of E-Tree service.

Synchronization and Clock Recovery Synchronization [1], [2], [3] is important as RAN backhaul migrating to Ethernet network which loses TDM clock source. When the TDM circuit traffic is transported via CES, the continuous signal is broken into packets at the MEN-bound IWF. And it is reassembled into a continuous signal at the CE-bound IWF.

The continuous frequency of the TDM service clock is disrupted when the signal is mapped into packets. In order to recover the service clock frequency at the egress of the CES connection, the IWF must employ a process that is specific to the CES interface type. It is the responsibility of CES-IWF to preserve the service clock of TDM service through the MEN. Many synchronization options are available for packet based network. CESoETH IWF can make use of following options for TDM clock to the TDM-bound IWF:

Clock from the incoming TDM link

External reference clock source

Free running oscillator

Clock from Ethernet interface (synchronous Ethernet)

CES over Carrier Ethernet Circuit Emulation Services [2], [3], [11], [16] allows TDM traffic exchanged by the end users by the means of Carrier Ethernet Network (Packet-switched Network).


Fig 6. Circuit Emulation over Carrier Ethernet

CES requires an interworking function to connect the BTS/BSC to the Carrier Ethernet via UNI. Interworking function used in CES is called as CES-IWF. It has two interfaces, Ethernet and Non-Ethernet.

From Non-Ethernet interface, it receives the TDM traffic from BTS/BSC (Non-Ethernet interface), coverts it into Ethernet frames and finally send it to Carrier Ethernet Network. It also receives Ethernet frames from CEN via Ethernet interface, recreates the TDM traffic and forwards it to BTS/BSC.

Conversion of TDM traffic to Ethernet packets requires splitting the TDM traffic into parts of a predefined size. And then it adds an Ethernet header into the predefined payload to form an Ethernet packet.

RAN backhaul migration to carrier Ethernet by the use of Circuit Emulation requires two major functional blocks. They are

CES Interworking function

UNI (User-Network Interface)

CES Interworking Function

CES Architecture - IWF Direction

CES is a bidirectional service consisting of two symmetrical data flows in the opposite directions. For each emulated circuit, there is a pair of CES interworking functions, MEN-bound IWF and TDM-bound IWF.

Fig 7. IWF Flow Direction


MEN-bound IWF is responsible for packetization of the TDM traffic, encapsulation into Ethernet frames and forwarding them into the Ethernet network. TDM-bound IWF is responsible for extracting the TDM data from the Ethernet frames and recreating the TDM traffic.

TDM Line Service over MEN

This service provides TDM interfaces to customer and transfers data across the MEN instead of legacy circuit switched TDM network. Two CES interworking functions are provided to interface TDM services to Ethernet. These IWF are connected via MEN using point-to-point EVC. Three major functional blocks involved in the conversion of TDM data to Ethernet packets are TSP (TDM Service Processor), CES-IWF and Ethernet Flow Termination (EFT).

Fig 8. TDM Line Service over MEN

The main responsibility of TSP is to convert the TDM service offered to the customer into a form that the CES-IWF can accept. TSP can be a Framer device, converting a fractional DS1 service offered to the customer into N*64 Kbits/s service for transport over MEN. TSP and CES-IWF can co-exist.

CES-IWF is responsible for data packetization/de-packetization, sequencing, synchronization, TDM signaling, alarms and performance monitoring.

EFT is transport processing function. It accepts the information from CES-IWF or from Ethernet Interface as its input, adds/removes Ethernet header information. Finally it sends to Ethernet interface/CES-IWF.

Functional Components and Interfaces

CES Interfaces

Two main interfaces in the CES-TDM domain are TDM service interface and CES TDM interface. In TDM service interface, the TDM service is either handed over to Customer or TDM network operator. CES TDM interface is between Inter working functions. Multiple IWF can make use of same Ethernet interface. This can be achieved by the use of Emulated Circuit De-multiplexer /Multiplexer Function. This function (ECDX) is in the Ethernet domain. It interfaces the CES


payload to the Ethernet Flow Termination which is responsible for Layer 2 information. Layer 2 information is compliant with IEEE 802.3 standard.

Fig 9. CES components and Interfaces

Functional Elements

There are four major functional elements in the CES IWF. They are

TSP ( TDM Service Processor)

CES-IWF (Circuit Emulation Service – Interworking function )

ECDX (Emulated Circuit De-multiplexer/Multiplexer Function)

EFT (Ethernet Flow Termination)

TSP is an optional functional element and is in the TDM domain. It is required only when TDM service from the customer is fractional T1/E1. Its main responsibility is to multiplex several fractional TDM services into single service which is to be emulated. It takes either TDM service/CES TDM service and converts it into CES TDM service/TDM service respectively.

CES-IWF is the adaptation layer which interfaces the CES application to Ethernet layer. It is responsible for encapsulation and de-capsulation, payload formation and extraction, carriage of TDM signaling and alarms, synchronization, TDM performance monitoring.

ECDX function is in the Ethernet domain. It has two main functionalities - MEN bound, TDM bound. In the MEN bound, it generates Emulated circuit identifier (ECID) which is unique to the TDM bound CES IWF and adds it to every received Ethernet Frames. And also it assigns Ethernet type to identify the Ethernet frames performing CESoETH adaptation function. In TDM bound, it extracts the ECID value, Ethernet type and finally sends the CES payload to CES-IWF.

Ethernet Flow Termination function takes the CES payload from the adaption layer and adds the Destination MAC, source MAC and FCS (Frame check sequence). Towards MEN, it generates the Ethernet Frame format of IEEE 802.3 standard and sends it to the MEN. In the TDM direction, it accepts the Ethernet frame from the MEN and verifies the FCS. If it is correct, then it extracts the ECID, Ethernet type and sends the CES payload to the ECDX function based on the ECID value for passing it to the appropriate CES-IWF.


User-Network Interface User-Network Interface [5] is an interface between Customer and network provider. It is required to connect CES Interworking function to MEN. It has two major functional elements namely, UNI-C (Customer Edge) and UNI-N (Provider Edge).

Fig 10. User-Network Interface

These two functional elements exchange configuration information and EVC status, link status and end to end connectivity by the use of E-LMI (Ethernet Local Management Interface) protocol [4] and Ethernet OAM [10], [19] respectively.

E-LMI protocol and procedures are used to configure the customer side UNI automatically by the use of information from the provider UNI. It also used to provide the status of UNI and EVC information to the UNI-C. The scope of E-LMI protocol is between UNI-C and UNI-N (i.e. between Customer Edge and MEN).

E-LMI messages are based on IEEE 802.3 untagged MAC-frame format. It uses STATUS and STATUS ENQUIRY messages to exchange the information between UNI-C and UNI-N. E-LMI protocol includes following procedures:

Notifies status of EVC.

Notifies the addition/deletion of EVC to the CE.

Notifies the CE, the availability state of a configured EVC (active, not active, partially active).

Communication of UNI and EVC attributes to the CE.

Ethernet Link Management enables service providers to monitor and troubleshoot single Ethernet link. It is used only in point-to-point Ethernet link. These Ethernet frames (Link OAM) will not propagate beyond a single hop within an Ethernet network. This is achieved by the use of Link OAM (IEEE 802.1ah) and its main features are Discovery (identify the peer link), Link Monitoring, Remote Fault Detection and Remote Loopback.

Ethernet Service management (Connectivity Fault Management) [17] allows the service providers to manage each customer service (EVC) individually. It operates on per service basis. For example, if an EVC fault is detected, the service provider edge device notifies the CE device about the failure. So the traffic can be rerouted to a different path more quickly than the failure detected by the routing protocol being run by the CE device. Ethernet CFM uses three messages


together to help administrators debug Ethernet networks. They are Continuity Checks, Link trace and Loopback.

CES-IWF and UNI are the major functional elements in the CESoETH migration methodology for RAN backhaul. CES-IWF is used to connect the non-ethernet (TDM) interface from customer edge to MEN via UNI and vice versa. UNI on the customer edge collect the configuration information from UNI on Network edge and auto configure itself by the use of E-LMI protocol. Link management and service management can be achieved in layer 2 by the use of Ethernet OAM.

Summary RAN Backhaul migration gains the benefits of Carrier Grade Ethernet and its services by using CESoETH technology. It converges 2G, 3G and 4G backhauls together. It helps the Service Provider to offer high bandwidth services for the customer and also maximize their ARPU.


Tech Mahindra’s Plan: Based on the Capabilities in Mobile Backhaul, Tech Mahindra will be able to contribute in the following areas. Based on the market potential and the opportunities from the vendors, we will be able to select a few among these areas.

CES IWF Module

TDM Service Processor

CES- interworking function

Emulated Circuit De-multiplexer/Multiplexer function

Ethernet Flow Termination

Ethernet services and OAM

E-Line, E-LAN and E-Tree services

E-LMI

Link OAM

Service OAM

Activities

Requirement Analysis

Design & Development

Testing & Validation


References [1] MEF Technical Specification MEF 22, “Mobile Backhaul Implementation Agreement” [2] MEF Technical Specification MEF 3, “Circuit Emulation Service Definitions, Framework and Requirements in Metro Ethernet Networks” [3] MEF Technical Specification MEF 8, “Implementation Agreement for the Emulation of PDH circuits over Metro Ethernet Networks” [4] MEF Technical Specification MEF 16, “Ethernet Local Management Interface (E-LMI)” [5] MEF Technical Specification MEF 20, “User Network interface (UNI) Type 1 Implementation Agreement” [6] MEF Technical Specification MEF 6.1, “Ethernet Services Definitions – Phase2” [7] MEF Technical Specification MEF 10.1, “Ethernet Services Attributes – Phase2” [8] White paper from MEF, “Carrier Ethernet for Mobile Backhaul Implementation Agreement”, Feb 2009. [9] White Paper from Nortel, “Mobile Backhaul Evolves with Carrier Ethernet”. [10] White Paper from Cisco, “Ethernet Operations, Administration, and Maintenance”. [11] White Paper from Huawei, “Technical White Paper for Circuit Emulation Service over PSN”. [12] Presentation from MEF, “Carrier Ethernet for Mobile Operators”, May 2008. [13] White Paper from Ciena, “Practical TDM Service Migration to a Converged Ethernet Infrastructure”. [14] White Paper from Cisco, “Cisco RAN Optimization solution for GSM and UMTS Backhaul Optimization: Applications”. [15] Technical Paper from MEF, “Metro Ethernet Services – Technical Overview”. [16] Paper from MEF, “Introduction to Circuit Emulation Services over Ethernet”. [17] Technology White Paper from Alcatel, “End-to-End Ethernet Connectivity Fault Management in Metro and Access Networks”. [18] Paper from Corrigent Systems, “3G Wireless Backhaul”. [19] White Paper from RAD Communications, “Ethernet OAM”.

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