1、Point-to-point techniquesThis paper gives a brief review of typical point-to-point techniques that can beapplied in offering optical access to residential and business users. Some of thepresented techniques can also be applied as local area solutions, i.e. to carry databeyond the ONUs.1 Plesiochrono
2、us Digital HierarchyPlesiochronous Digital Hierarchy (PDH) is an older technique developed forconventional voice transport. PDH is still widely used, e.g. in mobile networksto transport data between the base stations and base station controllers. The PDHconcept has a hierarchical structure with line
3、 rates of 64 kbit/s, 2.048 Mbit/s,8.448 Mbit/s, 34.368 Mbit/s and 139.264 Mbit/s, which are known as E0, E1,E2, E3 and E4, respectively. The standard Pulse Code Modulation (PCM)technique is used to decode the analogue voice signal to digital form. One digitalPCM coded voice channel is carried in one
4、 E0.2 Synchronous Digital HierarchySynchronous Digital Hierarchy (SDH) and its American counterpartSynchronous Optical Network (SONET) are widely used in transport networksworldwide. Although these techniques were developed already in 1980.s, itseems that they are not going to be replaced for a long
5、 time. One reason for thisis the recent enhancements made to the standards and another reason is the largeinvestment made in these techniques. For these reasons operators are not willingto replace their SDH/SONET networks with other solutions in a short time scale.SDH is a hierarchical transport con
6、cept with line rates of 155.52 Mbit/s, 622.08,2488.32 Mbit/s and 9953.28 Mbit/s, named as STM-1, STM-4, STM-16 andSTM-64, respectively. Each line rate has its own frame format and the OAMfunctionality is an integral part of the frame structure. Irrespective of the linerate, transmission of each fram
7、e lasts exactly 125 s.2.1 Next-generation SDHThe conventional SDH is optimised for transporting voice channels, which setsquite different requirements for the network than data transport. It has beenpredicted that enterprises will move to packet based voice services in a fewyears, and the number of
8、conventional voice-service-users will eventuallydecrease close to zero. The technique enabling the packet based voice services iscalled Voice over Internet Protocol (VoIP), which has gained a lot of interest.This implies that in the future the legacy SDH cannot support the growing dataservice demand
9、. It is also good to keep in mind that the legacy service users arenot going to vanish over night. The next-generation SDH has a transportmechanism, which enables the concurrent existence of the legacy and newservices over the same network, without interfering each other.The technique behind the nex
10、t-generation SDH is commonly known as the Dataover SDH (DoS) concept. DoS is a transport mechanism that provides means toaccommodate various data interfaces efficiently into SDH. Most importantly, theallocation of bandwidth between these interfaces can be done without disturbingthe existing SDH traf
11、fic, enabling the use of legacy services together with newservices provided by DoS. The DoS scheme consists of three techniques:Generic Framing Procedure (GFP), Virtual Concatenation (VC) and LinkCapacity Adjustment Scheme (LCAS) all standardised by ITU-T and AmericanNational Standard Institute (ANS
12、I). Several interfacing techniques have alreadybeen standardised for DoS and will be defined when the need emerges 22. Generic Framing Procedure (GFP) provides a simple encapsulationmechanism for frame-based data traffic (Ethernet, IP/PPP, RPR, FibreChannel, ESCON etc.) over a TDM transport path, e.
13、g. SDH/SONET orOptical Transport Network (OTN). Virtual Concatenation (VC) provides a flexible bandwidth allocationscheme, moving granularity and utilisation of SDH link capacity that is closeto that of Ethernet. Link Capacity Adjustment Scheme (LCAS) provides end-to-endsignalling for dynamic adjust
14、ment of capacity when using VC on SDH links.3 EthernetWhile the electrical 10 and 100 Mbit/s Ethernet connections are used in theaccess network, the optical 1 and 10 Gbit/s Ethernet (GbE) connections arebecoming more popular in the core network. Presently, 1 GbE signals can becarried electrically an
15、d optically, but the 10 GbE signals only optically.However, the electronic 10 GbE version is under development (standardapproval expected in July 2006). IEEE has recently defined also an optical100 Mbit/s interface for the subscriber access 9. 1 GbE links exploit 8B10Bblock-coding resulting in 1.25
16、Gbit/s line rate and 10 GbE uses 64B66B blockcodingresulting in 10.3 Gbit/s line rate. OAM functionality requires anadditional control protocol.The reasons for Ethernet.s popularity are the relatively low-cost equipment, easeof use and installation and the use of Ethernet in the residential data tra
17、nsport.Ethernet is deployed in the core network mainly by new service providers nothaving existing network infrastructure and by other providers not supportinglegacy voice transport, e.g. cable operators.3.1 Ethernet access servicesEthernet access services are a logical evolution towards packet base
18、d useraccess. These services offer configurable bandwidth (more flexibility thanavailable in E1 or STM-1 transport), can be used over fibre or copper lines andhave a simple interface to subscriber equipment. Most importantly, the Ethernetaccess services enable scalable revenue model for carriers as
19、they can sell almostany size data pipes to customers and many types of services can be provided tocustomers over the same physical connection. In addition, services can be addedor improved without any changes to the customer interface. However, one thingis certain. In order to support the Ethernet a
20、ccess services, the underlyingnetwork has to support the Ethernet transport services 23.3.2 Ethernet transport servicesThe Ethernet transport services are also known as the Ethernet Wide AreaNetwork (WAN) services. Basically, these services can be classified into pointto-point and multipoint-to-mult
21、ipoint services. Despite the categorisation, somescalability and reliability challenges are present in the all-Ethernet metronetworks 24, 25 as given in the list below. Restrictions on the number of customers . Virtual Local Are Networkidentifier (VLAN ID) used for traffic identification is 12 bits
22、and thusrestricts the carrier to 4096 customers. Service monitoring . Ethernet does not have embedded mechanism forservice monitoring. Scaling the L2 backbone . Spanning tree protocol does not scale forbandwidth, but blocks some number of ports to prevent loops in ringtopologies. Service provisionin
23、g . VLAN tag needs to be carried over the wholenetwork, which is not an easy task and results in impossible networkscaling. Interworking with legacy deployments . Frame Relay is widelydeployed and there is a need for functionality enabling Ethernet andFrame Relay services to work together.Point-to-p
24、oint service . This is also known as Ethernet Line Service (ELS) andit can be divided into two types of service: Ethernet Wire Service (EWS) andEthernet Relay Service (ERS). EWS is the Ethernet analogue to the private lineor private wire service 24, where a single physical connection is establishedb
25、etween two sites. Depending on the used transport method (frame encapsulationor native entity transport), different functionality can be provided between theend-points. ERS is the Ethernet analogue to FR service 24, where multiplepoint-to-point connections are established between two sites. This req
26、uires theco-ordination of Ethernet VLAN IDs and enables service multiplexing betweensites. As a result, it offers Hub-and-Spoke enterprise connectivity as well asInternet Service Provider (ISP) to customer connectivity.Multipoint-to-multipoint service . This is also known as Ethernet LANService (E-L
27、AN) and it can also be divided into two categories: EthernetMultipoint Service (EMS) and Ethernet Relay Multipoint Service (ERMS). EMSis the WAN analogue to the multipoint Ethernet LAN capability 24. Unicastframes are delivered using standard self-learning and forwarding capabilities ofthe Ethernet
28、bridges and broadcast frames are replicated to all sites. Forexample, EMS is a good way to connect multiple campuses together. ERMS is acombination of EMS and ERS and it enables simultaneous use of multipoint layer-2 services and Internet access over the same UNI. It is exploited by ISPs that areint
29、erested in offering service multiplexed multipoint-to-multipoint services.4 Optical Transport NetworkSDH cannot fully exploit the possibilities of the WDM technique. To ease themanagement tasks in the WDM networks, ITU-T study group 15 has defined aset of recommendations (G.709.x) for optical transp
30、ort 26. These includearchitectural, interfacing and management issues, which all are described by asingle term Optical Transport Network (OTN). Since OTN can be considered asan improved version of the SDH concept, it has many similarities with SDH,such as hierarchical network structure and frame for
31、mat. OTN has methods tomanage and monitor optical paths and channels in a similar way as SDH does inelectrical transport systems. OTN defines frame mappings for multiple opticallayer client signal types, e.g., SDH and GFP. Moreover, Automatic SwitchedOptical Network (ASON) specification G.8080 follo
32、wing the AutomaticSwitched Transport Network (ASTN) recommendation G.807/Y.1302 has beencreated to enable automatic provisioning of OTN. Currently, the OTNhierarchical structure defines line rates of 2666 Mbit/s, 10709 Mbit/s and43014 Mbit/s for OTU-1, OTU-2 and OTU-3, respectively.5 SummaryAlthough
33、 PDH and SDH are the most widely used point-to-point techniques,they do not fit well for data transport. The DoS concept has been developed toenhance data transport capabilities of SDH and most lately it has been extendedalso to PDH 27. The enhancements improve performance of the legacytransport con
34、cepts and therefore PDH and SDH can be considered equally goodcompetitors to Ethernet. In the long run, issues like wavelength channelmanagement and automatic network configuration will gain importance in theoptical access networks. These are not supported by the above mentionedtechniques and OTN can be seen as the solution. Table summarises somecharacteristics of the point-to-point techniques.Table . Summary of point-to-point techniques.
