Backhaul, the part of the network from the wireless tower to the Mobile switching center, has always been an expensive part of the total service model. Growth in service demand has drastically increased the bandwidth requirement in the backhaul, while revenue per bit is going down.To meet the dual challenge of rapid bandwidth growth and new services, wireless service providers and backhaul providers need to implement backhaul solutions optimized for cost and performance.
Migrating to Carrier Ethernet is one of the solutions for many of the backhaul issues. Migration from legacy backhaul to Carrier Ethernet technology in the RAN (Radio Access Network) offers many benefits for both service providers and also to the customers.
This white paper provides the importance and benefits of Carrier Ethernet migration, key challenges in migration and
CESoETH (Circuit Emulation Service over Ethernet - one of the migration methodologies).
Based on the Resource Capabilities and Market trends in Mobile Backhaul, Tech Mahindra can plan to involve in Requirement, Design, Development and Testing of CESoETH- IWF area.
IntroductionMobile backhaul includes networks and network technologies in RAN (Radio AccessNetwork) 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 (BaseTransceiver System) and BSC (Base Station Controller). Traditionally, it uses TDM(Time Division Multiplexing) and ATM (Asynchronous Transfer Mode) networktechnologies. But the next generation equipment and networks are based onEthernet.
Many mobile service providers are in the process of adapting their RAN’s toincorporate innovative high-speed data services such as 3G and 4G HSPA (Third andFourth Generation High-speed packet access), WiMAX (Worldwide Interoperability forMicrowave Access) and 1xEV-DO (CDMA Single carrier EVolved Data Optimized). Asthe volume of such bandwidth-intensive traffic grows, the costs for RAN backhaulgrow correspondingly, lowering ARPU (Average Revenue per User).
The average revenue per megabit for data service is far lower than for traditionalvoice and text messaging. But consumers are demanding mobile broadband servicesat affordable prices. Adapting traditional circuit-switched transport architectures tosupport these new services are quite expensive. Mobile service providers aretherefore looking for alternative ways to scale bandwidth in the RAN (Radio AccessNetwork) while reducing their growing operating expenses. Carrier Ethernettechnology is one of the best ways in the RAN for backhaul to significantly increasethe performance while lowering the operating expense.
A number of Ethernet innovations have emerged to ensure that Carrier Ethernetprovides Carrier-Grade packet transport. It also meets the performance criteria formobile backhaul applications. They are
This white paper deals with CESoETH (Circuit Emulation Services over Ethernet)methodology for RAN backhaul migration. Circuit Emulation Service allows thetransport of synchronous circuits such as T1/E1 over asynchronous networks. It isoriginally developed to allow T1/E1 to run over ATM, CES (Circuit Emulation Service)now can be extended to work over Ethernet Networks.
Acronyms and AbbreviationsARPU Average Revenue per UserATM Asynchronous Transfer ModeBTS Base Transceiver SystemBSC Base Station ControllerBS Base StationCE Customer EndCEN Carrier Ethernet NetworkCES Circuit Emulation ServiceCESoETH Circuit Emulation Service over EthernetECDX Emulated Circuit Demultiplexer/Multiplexer FunctionECID Emulated Circuit IdentifierEFT Ethernet Flow TerminationE-LMI Ethernet-Local Management Interface
EVC Ethernet Virtual ConnectionGIWF Generic Interworking FunctionGSM Global System for Mobile CommunicationsLAN Local Area NetworkLTE Long Term EvolutionHSDPA High Speed Downlink Packet AccessHSPA High Speed Packet AccessMEN Metro Ethernet NetworkNC Network Controller (Same as BSC)OAM Operation Administration and MaintenancePBT Provider Backbone Transport
PDA Personnel Digital AssistantPDH Plesiochronous Digital hierarchyPSN Packet Switched NetworkPWE3 Pseudo Wire Edge-to-Edge EncapsulationRNC Radio Network ControllerRAN Radio Access NetworkSONET/SDH Synchronous Optical Network/ Synchronous Digital HierarchyTDM Time Division MultiplexingTSP TDM Service ProcessorUNI-C User-Network Interface - Customer sideUNI-N User Network Interface - Network EdgeUMB Ultra Mobile BroadbandUMTS Universal Mobile Telecommunications SystemWiMAX Worldwide Interoperability for Microwave Access
Importance and Benefits of Carrier Ethernet MigrationThe number of subscribers using mobile are increasing extensively, which in turn increases thebandwidth per subscriber. Also increase in availability of mobile data devices such as 3Ghandsets, 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, UMBand LTE which are now beginning to be deployed. The challenge for wireless operators is tosupport more subscribers, satisfy higher bandwidth requirements per subscriber and also toincrease the ARPU.
The wireless operators must reassess their current backhaul infrastructure. The current 2Gnetworks uses PDH, SONET/SDH leased lines or microwave to carry TDM voice traffic from BTSto BSC. Similarly, 3G networks use leased lines to provide ATM backhaul between the UMTSnode and RNC. Huge increase in traffic created by mobile data services makes this modelunsustainable.
This allows wireless operators to tune to Carrier Ethernet to provide a more flexible sharedbandwidth access infrastructure. Ethernet has dominated the LAN for years. Now, with theaddition of number of key technology enablers, Ethernet is capable of carrier grade performanceand is referred to as Metro or Carrier Ethernet. The widespread adoption of Ethernet has made itmore cost-effective than any other networking technology. Ethernet offers wireless operatorsboth the bandwidth and cost points needed to support mobile multimedia services.
Key Challenges in Carrier Ethernet MigrationMigration StrategyRAN backhaul migration to carrier Ethernet [ 1 ], [ 8 ], [ 9 ], [ 13 ] from legacy networks will nothappen in overnight. It is required to make use of the existing infrastructure and networkelements as much as possible before migrating to full Ethernet in RAN backhaul. Referencemodel 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 isnothing 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 RANbackhaul.
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 cannotbe 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 connectedto UNI. RAN Dual Stack and Full Ethernet can be connected directly to UNI via Ethernet interfacewithout 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. Thelegacy network is used to carry the traditional voice traffic. Where as the MEN is used to carrythe data traffic. It is highly appropriate where an operator wants to offload low priority highbandwidth 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.
It is similar to Packet offload, but BTS/BSC is directly connected to UNI via an Ethernet interfaceand to MEN eliminating the need for a GIWF. Low priority high bandwidth traffic is offloaded viaMEN and Legacy is used for high priority voice traffic.
Fig 4. Dual Stack RAN
Full EthernetAll traffic from RAN base station is directly transported via UNI over the Carrier EthernetNetwork.
Fig 5. Full Ethernet
Carrier Ethernet Services for Mobile BackhaulCarrier Ethernet specifies the service attribute and related parameter values for mobile backhaulEthernet 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 Lineservices can be used to emulate existing service offerings with a point-to-point relationshipbetween BSC and BTS. Multiple EVCs (Ethernet Virtual Connections) can also be used betweenBSC 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 service is required when some mobile operators access other services from one or moreUNIs at the same time. For example, UNI is in mobile operator site and its BTS can supportdifferent technologies like legacy GSM and WiMAX. Each technology may have a specific EVCassigned 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 themto provide services other than LAN. These services may be distributed from a single or severalcentralized sites where the distribution sites are named as roots and all the remaining sites arenamed as leaves. Traditionally in mobile backhaul, BTS sites needs to exchange the serviceframes 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 RecoverySynchronization [ 1 ], [ 2 ], [ 3 ] is important as RAN backhaul migrating to Ethernet network whichloses TDM clock source. When the TDM circuit traffic is transported via CES, the continuoussignal is broken into packets at the MEN-bound IWF. And it is reassembled into a continuoussignal at the CE-bound IWF.
The continuous frequency of the TDM service clock is disrupted when the signal is mapped intopackets. In order to recover the service clock frequency at the egress of the CES connection, theIWF 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 optionsfor 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 bythe means of Carrier Ethernet Network (Packet-switched Network).
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 toBTS/BSC.
Conversion of TDM traffic to Ethernet packets requires splitting the TDM traffic into parts of apredefined size. And then it adds an Ethernet header into the predefined payload to form anEthernet packet.
RAN backhaul migration to carrier Ethernet by the use of Circuit Emulation requires two majorfunctional 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 oppositedirections. For each emulated circuit, there is a pair of CES interworking functions, MEN-boundIWF and TDM-bound IWF.
MEN-bound IWF is responsible for packetization of the TDM traffic, encapsulation into Ethernetframes and forwarding them into the Ethernet network. TDM-bound IWF is responsible forextracting 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 interfaceTDM services to Ethernet. These IWF are connected via MEN using point-to-point EVC. Threemajor 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 serviceoffered to the customer into N*64 Kbits/s service for transport over MEN. TSP and CES-IWF canco-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 EthernetInterface as its input, adds/removes Ethernet header information. Finally it sends to Ethernetinterface/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 networkoperator. 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
TSP is an optional functional element and is in the TDM domain. It is required only when TDMservice from the customer is fractional T1/E1. Its main responsibility is to multiplex severalfractional TDM services into single service which is to be emulated. It takes either TDMservice/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 isresponsible 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, TDMbound. In the MEN bound, it generates Emulated circuit identifier (ECID) which is unique to theTDM bound CES IWF and adds it to every received Ethernet Frames. And also it assigns Ethernettype 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 theDestination MAC, source MAC and FCS (Frame check sequence). Towards MEN, it generates theEthernet Frame format of IEEE 802.3 standard and sends it to the MEN. In the TDM direction, itaccepts the Ethernet frame from the MEN and verifies the FCS. If it is correct, then it extracts theECID, Ethernet type and sends the CES payload to the ECDX function based on the ECID value forpassing it to the appropriate CES-IWF.
User-Network InterfaceUser-Network Interface [ 5 ] is an interface between Customer and network provider. It isrequired to connect CES Interworking function to MEN. It has two major functional elementsnamely, 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 statusand end to end connectivity by the use of E-LMI (Ethernet Local Management Interface) protocol
E-LMI protocol and procedures are used to configure the customer side UNI automatically by theuse of information from the provider UNI. It also used to provide the status of UNI and EVCinformation 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 andSTATUS ENQUIRY messages to exchange the information between UNI-C and UNI-N. E-LMIprotocol 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 singleEthernet 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 useof Link OAM (IEEE 802.1ah) and its main features are Discovery (identify the peer link), LinkMonitoring, Remote Fault Detection and Remote Loopback.
Ethernet Service management (Connectivity Fault Management) [ 17 ] allows the service providersto manage each customer service (EVC) individually. It operates on per service basis. Forexample, if an EVC fault is detected, the service provider edge device notifies the CE deviceabout the failure. So the traffic can be rerouted to a different path more quickly than the failuredetected 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, Linktrace and Loopback.
CES-IWF and UNI are the major functional elements in the CESoETH migration methodology forRAN backhaul. CES-IWF is used to connect the non-ethernet (TDM) interface from customeredge to MEN via UNI and vice versa. UNI on the customer edge collect the configurationinformation 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 EthernetOAM.
SummaryRAN Backhaul migration gains the benefits of Carrier Grade Ethernet and its services by usingCESoETH technology. It converges 2G, 3G and 4G backhauls together. It helps the ServiceProvider 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 thefollowing 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
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 PDHcircuits 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 1Implementation 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 overPSN”.[12] Presentation from MEF, “Carrier Ethernet for Mobile Operators”, May 2008.[13] White Paper from Ciena, “Practical TDM Service Migration to a Converged EthernetInfrastructure”.[14] White Paper from Cisco, “Cisco RAN Optimization solution for GSM and UMTS BackhaulOptimization: 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 FaultManagement in Metro and Access Networks”.[18] Paper from Corrigent Systems, “3G Wireless Backhaul”.
[19] White Paper from RAD Communications, “Ethernet OAM”._________________________
