Abstract:

This article introduces a new networking technique for metro network? EOS and CWDM under built-out MSTP. The article describes the superiority of EOS under built-out MSTP against EOS under integrated MSTP and against POS. The emphasis is on network solution of built-out EOS. In addition, this paper also introduces technical advantage of applying CWDM to metro network, as well as networking scheme using CWDM. 

In recent years, two aspects in metro network market are worth noting: MSTP and CWDM. The advantage of MSTP lies in that it can fully utilize existing transmission resource and facilitate integration of multiple services. On the other hand, since protection mechanism of SDH is better than IP route convergence and spanning tree convergence, therefore, compared with traditional pure IP network, reliability of IP network that takes advantage of SDH transmission is much higher. There are two technical approaches for implementing MSTP in IP layer: EOS (Ethernet over SDH) under integrated MSTP and EOS under built-out MSTP. Discussion in this paper is focused on new built-out EOS. 

Due to significant cost competitiveness in medium and short distance metro network market compared with DWDM, CWDM is also an ideal platform for transporting multiple services, therefore, is widely applied in various network layers of metro network today. 

1. Built-out EOS technique

Typical application method of integrated MSTP is to develop interface card that can support multiple services including Ethernet, ATM and new broadband services like video and image, based on existing SDH equipment. Main advantage of integrated MSTP is management integration and equipment simplicity. The disadvantages of integrated MSTP include no capability of supporting old SDH equipment, incompatibility of multi-service cards among various vendors. In addition, multiplicity of protocols of multiple vendors makes difference in SDH mapping and encapsulation (for instance, PPP encapsulation, LAPS encapsulation, GFP encapsulation etc.). It also leads to difference in performance (such as data speed, flow control, time delay and packet loss etc.).

In order to overcome drawbacks of traditional built-in MSTP solution, a new built-out MSTP solution emerges. EOS of FiberHome Networks is such a representative solution. In this solution, EOS is an independent intermediate equipment in between the SDH equipment and Ethernet equipment, just like middleware in software industry. This EOS equipment and technical solution is an adhesive bonding SDH and Ethernet together, and is designed based on a China-proposed international IP standard ? ITU-T Rec. X.86. The EOS semiconductor chip is developed independently by FiberHome Networks. The new EOS solution features remote monitoring function, remote alarm indication and flow control capability, high efficiency, as well as low time delay and jitter. The new EOS solution is compatible to old and new SDH equipment, also compatible to SDH equipment of different vendors. This solution has great cost competitiveness as compared with POS. Since its first release a year ago, EOS solution has been favored by various domestic communication carriers, and applied in several hundred key transmission links.

Comparison with “POS + SDH/WDM” solution: “POS + SDH/WDM” solution means adoption of POS technique in high-speed router or core switch with SDH or WDM acting as transmission platform. POS interface port of high-speed router or core switch is connected to SDH equipment or WDM equipment. Usually, 2.5G POS is used in core layer while 155M POS is mostly used in metro network area. This situation is more popular and matured. But in edge nodes of metro network, only a few equipment can provide slots for POS interface card, and such equipment is relatively expensive. EOS technique imposes on particular requirement on edge equipment of metro network and has advantage of flexibility. In addition, successful development of EOS chip based on our own expertise makes price of EOS equipment greatly reduced.

Network application: EOS supports point-to-point, star and ring topologies, and supports mapping and translating 10/100M and 1000M Ethernet onto E1, and SDH 155M, 622M and 2.5G.

Examples of network application are given below.

Point-to-point application: This topology is generally applied in backbone. In this example, EOS equipment provides interfaces on both SDH side and Ethernet side. EOS equipment with Ethernet Over 155M SDH is a typical example of such equipment: it has a 100M optical/electrical Ethernet interface card and a 155M optical/electrical SDH interface card. For EOS equipment with 4?GE Over 2.5G SDH, it has 4 GE interface cards and a 2.5G interface card. EOS equipment with 6?GE Over 2.5G SDH can also be provided. When in support of virtual concatenation, EOS equipment can provide function of mapping Ethernet onto multiple E1s. In middle-west areas in Chinese territory, peak traffic flow of broadband service in district-level cities and underlying counties generally exceeds 100Mbps, quite a number of EOS equipment with 100Mbps Ethernet Over 155M SDH of FiberHome Networks are applied today for reliable data service connection between cities and counties.

Star topology: Rack-mounting type EOS equipment in central office equipment room adopts modular structure. For instance, B2100 series product of FiberHome Networks can support EOS conversion for maximum 12 channels. EOS equipment in branch office adopts standalone type that can support EOS conversion for one channel. This is suitable for carriers or large customer that have multiple branches or organizations.

Ring topology: In ring structure, dual-channel (bi-directional) EOS equipment is adopted. Every Ethernet switch or router connects to Ethernet interface card of EOS equipment in two directions. Switching function is implemented in Ethernet switch or router equipment. This solution makes full use of SDH protection mechanism, and its reliability is far much better than traditional Ethernet ring. This is suitable for building high-reliability IP metro network.

2. CWDM technique

Metro network is characterized by short distance and complex service interface. Applying DWDM in metro network will lead to high cost. On the other hand, CWDM (coarse wavelength division multiplexing) that came into being for two years is widely applied in such environment. Currently, many communication carriers in the globe have applied CWDM in many places, as they considered appropriate. Some experts even proposed idea of leasing wavelength, thus opening a new dimension for broadband market beyond traditional fixed bandwidth leasing and fiber leasing. 

Main features of CWDM are: simple implementation, small equipment size, low power consumption, and low cost (about one third to one fifth of DWDM). Therefore, using CWDM equipment in metro environment brings early and cost-effective network deployment. As a comparison, laying new fiber cable will deal with a series of issues including administrative procedures, city environment, geographical condition for deployment, materials, manpower, project schedule etc.

Capabilities of CWDM when applied in metro network include: 1) 8 service channels can be multiplexed in a fiber pair; 2) It supports multiple services covering IP, SDH, ATM, POS, image, Fiber Channel etc.; 3) Up to 20G service bandwidth can be provided in only one fiber at present; 4) physical layer path protection and multiplex section protection greatly enhanced reliability of data network, the switching time is less than 50ms, and protection efficiency is mush high than Layer-3 redundant route protection. 

Networking options for metro network adopting CWDM: 

1. Point-to-point: At present point-to-point system with 4 or 8 wavelength channels is more popular. It has greater cost advantage compared with approach of laying new fiber cable.

2. Linear chain with a central node: This is suitable for linking headquarter to branch offices along a route, the intermediate nodes generally adopt OADM type CWDM equipment.

3. Ring network: Adopting CWDM equipment in core network or access network layer of metro network facilitates fiber saving and multiple services integration. It is the most typical application of CWDM. 

Schematics of applying CWDM ADM equipment in core network

As shown in the figure above, CWDM OADM equipment helps overlap multiple logical rings carrying multiple services including SDH service, IP service, and video and image service, on a physical optical ring. Each logical ring carries independent service without mutual interference.

In this type of “physical ring + logical ring” networking approach, in fact, a whole logical ring consists of segment-by-segment addition of “point-to-point” WDM transmission lines, hence there is no restriction on total length of a ring: the more the nodes in a ring, the longer the total distance. Generally, spacing between two adjacent nodes will not exceed 80km. If this spacing does exceed 80km, then O/E repeater or optical amplifier must be used.

Network management

Network management information is transmitted through in-band channel or out-of-band channel. In-band mode management requires optical supervision channel (OSC) unit card that provides in-band management supervision channel operating at 1310nm. In this case, all functional units in CWDM equipment including OTU, OADM, OMU, ODU, ORU etc should have corresponding optical monitoring device and module at 1310nm. Out-of-band mode of management uses the upper Ethernet data interface port on NMU unit; the management information is transmitted on public data network. 

Protection function

Multiplex section (MS) protection: For this protection mechanism, 1+1 multiplex section protection of CWDM equipment, and switchover from working line to standby line in event of signal loss (LOS) are implemented on data plane. The switching time less than 50ms ensures no loss of transmitted service. On the other hand, on control plane, MS protection mechanism accepts configuration command issued by NMU, performs configuration of working line and standby line, and reports information on optical power, line operation status of working line and standby line etc. during the polling time. 

Path protection: It supports 2-fiber unidirectional wavelength path (channel) switched ring (UWPSR). The switching time is less than 50ms that ensures no service loss in every optical channel. Such protection mechanism provides carriers with 1+1 optical channel protection in optical domain when they are building their ring network. Such 1+1 channel protection ring has self-healing feature, similar to SDH self-healing ring. 

All above functions help greatly widen the application area of CWDM, and make data transmission more secure and reliable.