Next-generation Parallel Optical Data Links

Parallel-optical data links are now available and designed into many next-generation telecom/datacom central-office switches and routers. In order to meet the continued growing Internet capacity over the next couple of years, parallel-optical data links are gaining more and more popularity. This article will introduce the next-generation parallel optical data links which will play continuing role in the future network of data communications.

What Is Parallel Optical Link

Parallel optical data link is a concept opposite to serial optical data link and also a replacement for many serial data communication links. In the more typical application of parallel optic link, one byte of information is split up into bits. And each bit is coded and sent across the a single fibers. Parallel optic links are often the most cost effective for 40 Gigabit transmission, and can transmit over distances exceeding 100 meters. Serial optical solutions can relieve bandwidth-distance, cable bulk, and EMI limitations of metallic interconnects. However, they still take up significant space and they are somewhat more expensive than copper interconnects. A much less space is possible with parallel optical module (e.g. 40G-QSFP-SR4-INT) rather than with multiple serial modules. Therefore, parallel optical module greatly increases interface density. Parallel optical link modules contain laser arrays, multichannel driver, receiver ICs and fiber-ribbon optical connectors, amortizing packaging costs over several channels.

Key Components of Parallel Optical Links

• Multifiber Connectors
  The connector is a critical enabling component, since its design ultimately determines the density, quality, and cost of fiber-optic interconnects. The connectors are most often used for multi-fiber links is the so-called MPO (multi-fiber push-on connector), also known by its most common vendor branded version, the MTP connector. The multifiber connectors trim costs for connector hardware, assembly, and cabling compared to single-fiber connector. This kind of connectors are not only save place for more efficient use of precious system board space, but their smaller port area also help reduce EMI. For various historical reasons, these connectors were standardized in rows of 12 fibers each, which isn’t a good match for data communication systems. Later, MPO/MTP connectors evolve to have 8-fiber row and 16-fiber row. The 40G interface is based on parallel striping of 10 Gbit/s serial channels. If we held up a standard 1 x 12 fiber MPO connector and looked back into the cable, the 4 leftmost fibers are used to transmit data, the middle 4 fibers are unused lanes, and the 4 rightmost fibers are used to receive data. Thus, we have a bidirectional interface with 4 x 10G in each direction. Following the same principle, we can use 10 Gbit/s links to build a duplex 100 Gbit/s channel, but we need an MPO connector with 2 rows of 12 fibers each. We leave the outermost fibers on either end of the rows unused, and use the remaining 10 fibers in the upper row to transmit data, and the remaining 10 fibers in the lower row to receive data.
• Fiber Cables
  As data rates increase, link loss budgets and insertion loss will significantly decrease, so if you have plans to re-connectorize your installed fiber cable plant, be sure that the infrastructure is adequate for this application. You’ll also need adapters to fan out the MPO connection into simplex connections which are compatible with existing test and measurement equipment, to verify link performance. This kind of cables include QSFP to SFP+ breakout cable and QSFP+ to QSFP+ direct attach cable.
• VCSEL-based Link Design
  The use of low cost VCSEL based laser transmitters is appealing for this application. While the advantages of discrete VCSELs have already placed them into serial link modules, the new parallel link modules will exploit the multichannel advantages of VCSELs arrays in parallel or WDM link modules.

Summary

Although some serial optical links can support high data rates (particularly for multi-data center connectivity or telecommunication systems), parallel optical links are a promising solution for more cost effective, higher data rate links within the data center. At shorter distances, such as within a data center rack or between adjacent racks (perhaps up to 100 meters), parallel optics are cost competitive with copper links. Thus, rather than using single-mode fiber, some laser optimized multi-mode fibers are often used. And an active optical cable allows for tradeoffs between the transmitter, receiver, and fiber parameters. We can also use existing 10 Gbit/s serial link technology and volumes by combining either 4 links to create a 40G channel, or 10 links to create a 100G channel.

1000Base-T SFP Module for Gigabit Ethernet

The Gigabit Ethernet technology is an extension of the 10/100-Mbps Ethernet standard. Gigabit Ethernet provides a raw data bandwidth of 1000 Mbps while maintaining full compatibility with the installed base of over 70 million Ethernet nodes. Gigabit Ethernet includes both full- and half-duplex operating modes. A Gigabit Ethernet is imperative for two reasons: faster systems and faster backbones. Gigabit Ethernet has the potential for low-cost products, freedom of choice in selecting the products, interoperability, and backward compatibility. Gigabit Ethernet supports existing applications, network operating systems, and network management; it requires a minimal learning curve for Ethernet network administrators and users. These investment preservation and risk minimization aspects are what make Gigabit Ethernet so attractive. With the development of Ethernet systems and the growing capacity of modern silicon technology, embedded communication networks are playing an increasingly important role in embedded and safety critical systems.

A known type of data communication device is a small form factor pluggable (SFP) module. Typically, the SFP module plugs into an interface slot in a circuit board populated with other communication devices used in an Ethernet-based system. The SFP module includes a second serial interface,interconnected with the circuit board slot, and a first serial interface, coupled to a serial link, such as a copper or fiber link, for communicating with remote link partners. The serial link, coupled with the first serial interface, may be a 10/100/1000 Base-T copper link, or a fiber link, for example. The SFP module also offers several significant advantages over its predecessor, the GBIC (Gigabit Interface Converter), including lower cost, lower power, and smaller size. Thus, with the SFP form factor, fiber Gigabit systems may be developed featuring similar port densities as copper-only systems using RJ-45 connectors.

The SFP transceiver MultiSource Agreements (MSA) document puts forward a specification for the development of optical SFP modules based on IEEE 802.3z, the Gigabit Ethernet Standard. Specifically, the MSA calls out 1000Base-X Physical Coding Sub-layer (PCS) which supports full-duplex binary transmission at 1.25 Gbps over two copper wire-pair SerDes (Serializer/Deserializer). Transmission coding is based on the ANSI Fiber Channel 8B/10B encoding scheme.

1000Base-X makes no provision for running at slower speeds. Thus, network device ports utilizing SFPs are dedicated to operating on fiber links at speeds of 1000 Mbps. However, more than 85% of office space inside buildings is category 5 copper. Thus, ports designed to use optical SFPs can not make use of this existing cabling.

For example, a customer may require a network device, such as a router, having both optical ports for long distance connections and RJ-45 copper ports for connecting to local devices. It is often the case that not all optical ports provided on a router are needed at a given time. However, with conventional SFPs these optical ports cannot be utilized to connect with local devices connected by standard copper cabling or operating at speeds lower than 1000 Mbps. But with a 1000BASE-T copper SFP transceiver, the customer could use their existing copper cabling infrastructure instead of replacing the infrastructure. Here are two good examples of 1000BASE-T copper SFP transceivers, the Finisar FCLF-8521-3 compatible 1000BASE-T SFP copper transceiver and Cisco Linksys MGBT1 compatible 1000BASE-T SFP copper transceiver from FS.COM. Both of them are designed for 100m reach over Cat 5 UTP cable with RJ-45 interface and support max data rate of 1000Mbps.

The 1000BASE-T copper SFP transceiver offers a flexible and simple method to be installed into SFP MSA compliant ports at any time with no interruption of the host equipment operation. It enables for seamless integration of fiber with copper LAN connections wherever SFP interface slots can be found. Such system is economical, it saves time, offers flexibility and eliminates the necessity for replacing entire devices once the customers have to change or upgrade fiber connections and you will benefit so much from it.

QSFP+ Direct Attach Copper Cables for EX Series Switches

Quad small form-factor pluggable plus (QSFP+) direct attach copper (DAC) cables are suitable for in-rack connections between QSFP+ ports of EX Series switches. They are suitable for short distances of up to 10 meters, making them ideal for highly cost-effective networking connectivity within a rack and between adjacent racks. This article will introduce EX Series switches and QSFP+ DAC for EX Series switches.

Introduction to EX Series Switches

EX Series switches deliver scalable port densities and carrier-proven high availability features that consolidate legacy switch layers, helping to reduce capital and operational expenses and advance the economics of networking. For example, the EX 4200 series Ethernet switches with Virtual-Chassis technology, deliver the same Gigabit Ethernet (GbE) and 10GbE port densities as traditional chassis-based switches, but at one-eighth the footprint and less than one third the cost. The EX Series switches are right-sized for campus, data center and remote office environments and feature many of the same carrier-class hardware and software architectures found in core routers that were purpose-built to support the convergence of data, voice, and video onto a single always-on network.

By alleviating the cost, complexity and risk associated with legacy switch infrastructures, the EX Series switches enable high-performance businesses to deploy a high-performance network infrastructure based on three key tenets – operational simplicity, carrier-class reliability, and integration and consolidation – to enable ubiquitous access to strategic assets, reduce network downtime and enhance overall security to shared assets across the extended enterprise.

Cable Specifications of QSFP+ DAC

QSFP+ direct attach copper (DAC) cable is hot-removable and hot-insertable. QSFP+ DAC mainly has two kinds. One is a cable that connects directly into two QSFP+ modules, one at each end of the cable. The cables use integrated duplex serial data links for bidirectional communication and are designed for data rates up to 40 Gbps. The other is a breakout cable consisting of a QSFP+ transceiver on one end and four SFP+ transceivers on the other end. The QSFP+ transceiver connects directly into the QSFP+ access port on the QFX Series device. The cables use high-performance integrated duplex serial data links for bidirectional communication on four links simultaneously. The SFP+ links are designed for data rates up to 10 Gbps each.

The following table describes the software support for QSFP+ passive DAC cable lengths on EX Series switches for Junos OS releases.

SWITCH	SOFTWARE SUPPORT ADDED	DAC MODEL NUMBER
EX44300 switches	Junos OS for EX Series switches, Release 13.2X51-D15 or later	EX-QSFP-40GE-DAC-50CM QFX-QSFP-DAC-1M QFX-QSFP-DAC-3M JNP-QSFP-DAC-5M
EX4300 switches	EX4300-24T, EX4300-24P, EX4300-48T, EX4300-48T-AFI, EX4300-48P, EX4300-48T-DC, and EX4300-48T-DC-AF switches—Junos OS for EX Series switches, Release 13.2X50-D10 or later EX4300-32F switches—Junos OS for EX Series switches, Release 13.2X51-D15 or later EX4300-24T-S, EX4300-24P-S, EX4300-32F-S, EX4300-48T-S, and EX4300-48P-S switches—Junos OS for EX Series switches, Release 13.2X51-D26 or later	EX-QSFP-40GE-DAC-50CM QFX-QSFP-DAC-1M QFX-QSFP-DAC-3M JNP-QSFP-DAC-5M
EX4550 switches	EX4550-32T-AFI, EX4550-32T-AFO, EX4550-32T-DC-AFI, EX4550-32T-DC-AFO, EX4550-32F-AFI, EX4550-32F-AFO, EX4550-32F-DC-AFI, and EX4550-32F-DC-AFO switches—Junos OS for EX Series switches, Release 13.2X50-D10 or later EX4550-32F-S switches—Junos OS for EX Series switches, Release 12.3R5 or later	EX-QSFP-40GE-DAC-50CM QFX-QSFP-DAC-1M QFX-QSFP-DAC-3M JNP-QSFP-DAC-5M

Conclusion

QSFP+ direct attach copper cables can provide cost-effective and reliable 40G speed connections for EX Series switches with distances reaching up to 10 meters. As the leading fiber optical manufacturer in China, FS.COM offers a wide selection of QSFP+ DAC with low cost but high performance. In addition, 10G SFP+ to SFP+ DAC (eg. HP JD096C), 25G SFP28 to SFP28 DAC, 40G QSFP+ to 4 XFP DAC, 100G QSFP28 to QSFP28 DAC, 100G QSFP28 to 4 SFP28 DAC are also available for your choice. All these DACs are with 100% compatibility and can be customized according to your special requirements.

QSFP+ Transceiver, AOC, DAC – Which Is You Choice?

The IEEE 802.3ba committee ratified the 40 Gigabit Ethernet standard and along with the general specification, defined a number of fiber optic interfaces. For 40GbE direct cabling between two devices using QSFP+ port, there are several options including QSFP+ transceiver option, QSFP+ DAC (Direct Attach Copper) cable, and QSFP+ AOC (Active Optical Cable). Among these options, each of them has its own merits. This article will compare them and point out which is more cost-effective for your networks.

QSFP+ Transceiver

The 40-Gigabit QSFP+ transceiver module is a hot-swappable, parallel fiber-optical module with four independent optical transmit and receive channels. These channels can terminate in another 40-Gigabit QSFP+ transceiver, or the channels can be broken out to four separate 10-Gigabit SFP+ transceivers. The QSFP+ transceiver module connects the electrical circuitry of the system with either a copper or an optical external network. The transceiver is used primarily in short reach applications in switches, routers, and data center equipment where it provides higher density than SFP+ modules.

Cables for 40 Gigabit Ethernet

Copper Cables: QSFP + to QSFP + copper cable and QSFP + to 4SFP + breakout copper cable are the two common types of QSFP + cables. QSFP + to QSFP + copper cables are hot-removable and hot-insertable. A cable consists of a cable assembly that connects directly into two QSFP + modules, one at each end of the cable. QSFP + to 4 SFP + breakout copper cable is with one QSFP + on one end and four SFP + on the other end. It allows a 40G Ethernet port to be used as four independent 10G ports thus providing increased density while permitting backward compatibility and a phased upgrade of equipment.

Fiber Cables: Active optical cable (AOC) assemblies were invented to replace copper technology in data centers and high performance computing (HPC) applications. The 40G QSFP + AOC is a parallel 40Gbps quad small form factor pluggable (QSFP +) active optical cable, which supplies higher port density and total system cost. The QSFP + optical modules provide four full-duplex independent transmit and receive channels, each are able of 10Gbps operation 40Gbps aggregate bandwidth of at least 100m multimode fiber.

Which to Choose?

Case 1: Distances Below 5 m
If the transmission distances is under 5 m, like the case that two switch ports are connected within the same rack or between racks located within the same room. The QSFP+ passive DAC cable is recommended. Take the Cisco compatible QSFP+ optics for example:

DAC	5m Cisco QSFP-H40G-CU5M Compatible 40G QSFP+ Passive Direct Attach Copper Cable	US$ 65.00
AOC	5m Cisco QSFP-H40G-AOC5M Compatible 40G QSFP+ Active Optical Cable	US$ 140.00
Transceiver & Cable	Cisco QSFP-40G-SR4 Compatible 40GBASE-SR4 QSFP+ 850nm 150m DOM Transceiver	US$ 85.00
	5M OM4 12-fiber MTP Fiber Optic Trunk Cable for 40GBASE-SR4	US$ 73.00

Case 2: Distances Below 100 m (< 5 m)
For the case that transmission distance is below 100 m but beyond 5 m, the QSFP+ AOC is an ideal choice for use. In this case, the QSFP+ passive DAC cable cannot achieve such long reach requirement because of its performance limitation. While the 40GBASE-SR4 QSFP+ transceiver can do this but the cost of the MTP trunk cable is higher. Thus, here, the QSFP+ AOC is preferred.

Case 3: Distances Above 100 m
If the transmission distance is longer than 100 m, DAC cables and AOCs are usually not recommended. In this case, the 40G QSFP+ modules which are used for short-reach applications are recommended.

40G QSFP+ cables can provide inexpensive and reliable 40G speed connections using either copper cables with distances reaching up to 30ft (10 meters length) or active optical cables reaching even 300ft (100 meters). Of course, all the above are just rough estimates of the optics. In reality, more details should be included according to your specific requirements.