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Cisco 12000 Series Routers

Very Short Reach (VSR) OC-192/STM-64 Interface

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INTRODUCTION

Cisco Systems ® has developed VSR-1 technology, the first functional 10-Gbps SONET/SDH interface that provides a low-cost, standard solution optimized for very short reach (VSR) optical interconnections. As service providers move to 10-Gbps speeds to meet tremendous increases in traffic loads, VSR-1 helps enable cost-effective scaling of IP networks.
Service provider networks employ a variety of network elements, generally including IP routers (Layer 3), SONET, ATM, or wavelength switches (Layer 2), and transport systems (Layer 1). The interconnection (or peering) of each point of presence (POP) with the other POPs through the highest-bandwidth core WAN transport network is the basis of complete internetworking of all end users.
A typical POP architecture comprises numerous aggregation (or edge) switches and routers that connect to core (backbone or peering) routers and switches, which in turn connect to the WAN transport network over dense wavelength-division multiplexing (DWDM) terminals and wavelength routing optical cross-connect (OXC) systems (Figure 1). This architecture contains multiple high-bandwidth links that interconnect the core routers and the optical transport equipment.

Figure 1. Typical POP Architecture

As demand for bandwidth grows, the speed of the optical interfaces of these links has increased from 155 Mbps (OC-3/STM-1) to 10 Gbps (OC-192/STM-64).The distance of most of these links is less than 300 meters, as all this equipment is typically physically colocated within the same building. However, the 10-Gbps interface standards, SONET/SDH serial OC-192/STM-64, were designed for wide-area long-haul transmission. Therefore, these interfaces have been traditionally based on high-cost transceivers, with large high-speed, narrow-linewidth (often cooled) lasers.
To address the requirement for low-cost interconnects between colocated equipment, the Optical Internetworking Forum (OIF) has formally adopted two implementation agreements for VSR OC-192 interfaces. VSR-1 uses 850-nm lasers over 12 multimode fibers at 1.25 Gbps each for distances up to 300 meters. VSR-2 (or OIF-VSR-02.0) uses a 1310-nm laser in single-mode fiber at 10 Gbps for distances up to 600 meters.
VSR-1 employs parallel optics and is substantially less expensive for the shortest reaches. VSR-2 is a serial solution in which the primary advantage is that it can use installed single-mode fiber. However, the economic value is comparable to short-reach technology.
In intra-POP interconnections in which there is a hard requirement to use existing single-mode fiber, short reach is the most viable solution because the cost advantage of VSR-2 over short-reach equipment is slight. However, multimode ribbon fiber can be used in most intra-POP situations, hence VSR-1 is the interface of choice because of superior economic value. This white paper describes the application of VSR-1.

THE VSR-1 SOLUTION

Cisco ® VSR-1 technology is an economical solution for the interconnection of optical network elements residing in the same central office for distances up to 300 meters. The Cisco VSR-1 interface makes use of the technology advances in the lower-cost 850-nm laser and the 1.25 Gbps CMOS serializer/deserializer (SERDES), originally developed for Gigabit Ethernet, to offer a solution for interoffice interconnections with lower cost and lower power consumption than current OC-192 SONET serial solutions.

Figure 2. Cisco OC-192 VSR-1 Module

TECHNOLOGY OVERVIEW

The VSR-1 bi-directional interface comprises three main components (Figure 3):

• A low-cost, low-power integrated circuit (IC) converter that maps the standard 16x622 Mbps output SONET framer interface to and from the 12x1.25 Gbps parallel optics links

• A one-laser array transmitter

• A 12-detector array receiver

Both transmitter and receiver connect to the 12-fiber ribbon through standard Multicast Transaction Protocol/Multipath Optical (MTP/MPO) connectors. The VSR-1 link uses an array of 12 low-cost 850 nm lasers, each operating at the Gigabit Ethernet rate of 1.25 Gbps, whose signals are carried over a 12-fiber ribbon for distances up to 300 meters in 62.5-micrometer multimode (MM) fibers.

Figure 3. VSR-1 Bidirectional Interface

OPTICAL INTERFACE

A full-duplex physical layer electro-optic interface has been defined. The optical link specifications follow the Gigabit Ethernet standard and the corresponding Gigabit Ethernet optical link model, with the exception of fiber and connector specifications that are modified to meet design constraints imposed by engineering parallel components. The link power budget, and penalties, per data channel are summarized in Table 1. Table 1 indicates the 300-meter maximum guaranteed link distance for a system with four (0.5 dB loss) connectors.

Table 1. VSR-1 Optical Specifications

Parameter

Specification

Fiber

62.5-micrometer MMF

Fiber cable maximum attenuation

3.75 dB/km

Minimum modal bandwidth

400 MHz-km

Link power budget

6.0 dB

Maximum number of connectors

4

Maximum connector loss (per connector)

0.5 dB

Minimum operating range

2 to 300 m

Unallocated margin in link power budget

0.60 dB

To make use of the Gigabit Ethernet technology, an IC converter, as shown in Figure 4, maps the standard 300-pin MSA connector output of the OC-192 framer onto the parallel optical links and reassembles it after detection has been developed.

Figure 4. Converter IC Schematic

To this end, the 16-bit x 622-Mbps Low Voltage Differential Signal (LVDS) is 16-bit-word aligned at the input of the IC and byte-wise stripped to 10 data channels after being 8- and 10-bit encoded to ensure DC-balanced transmission. The data is source synchronous at twice the 622-MHz clock of the framer chip, which sets the chosen rate of 1.244 Gbps +/-20 ppm. Moreover, to enable the deskewing of the channels at the receiver (within the 80 nanosecond interchannel tolerance specification), the first three SONET/SDH A1 bytes of each channel are overwritten with a unique frame delimiter 8- and 10-bit code word.
At the receiver the individual channels are realigned, the A1 bytes are reinserted, and the SONET frame is reassembled so that the VSR-1 parallel optics link is completely transparent to the SONET interfaces with which it interconnects.
To make use of the additional two channels available using the standard parallel optics components, the IC converter also provides optional error detection and correction against single channel failure. More specifically, a protection channel, created by performing a bit-wise Exclusive-Or operation of the 10 data channels, is transmitted on channel 11, while channel 12 transmits a polynomial CRC16 calculated over virtual blocks of 24 bytes on each channel. Finally, the converter IC also provides automatic protection (detection and reversal) from polarity and fiber ribbon cable crossover.

OPTICAL FIBER AND CONNECTOR

The fiber optic ribbon cable contains 12 parallel 62 MM fibers, each meeting the requirements specified in IEC 793-2 (equivalent to Corning's Infinicor CL1000). The optical connector utilizes the MTP (MPO) with 12 MM fiber terminations, and maximum loss 0.5 dB per mating, conforming to IEC standard 1754.7. Component termination for equipment using OC-192 VSR-1 interfaces to the externally accessible transmit and receive optical connectors utilize male connectors.
Appendix A provides further detailed analysis of the specific ribbon cable and connector specifications and also refers to suppliers, lead times (which may be six to eight weeks), and ancillary equipment suppliers such as fiber scopes, field terminations, and cleaning solutions.

APPENDIX A: OPTICAL FIBER AND CONNECTOR SPECIFICATION

The fiber optic ribbon cable shall contain 12 parallel fibers. Each fiber shall meet the requirements specified in IEC 793-2: 1992 Type A1b (62.5 µm multimode) with exceptions noted in OIF2000.081, OIF2000.044, and OFC2001WS4 session. The ribbon cable shall have a maximum differential skew between fibers of 100 picosecond per meter.
The preferred optical connector shall be the MTP (MPO) with 12 MM fiber terminations. The orientation of the terminated cable is "keyed" and conforms to IEC standard 1754.7, as shown in Figure 5. The maximum connector loss shall be 0.5 dB per mating. Component termination for equipment using OC-192 VSR interfaces, the externally accessible transmit, and receive optical connectors shall be male connectors.

Note: MTP optical connectors have both male and female versions. While female-female connection is possible, care should be made to prevent this as it will not meet connector loss specifications.

Figure 5. MTP Connector Interface Orientation and Parallel Optical Link