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The OIF's OTN implementation agreement (IA) allows a packet fabric to also switch OTN traffic. The result is that operators can now use one switch for both traffic types, aiding IP/ Ethernet and OTN convergence. Source: OIF

Improving the switching capabilities of telecom platforms without redesigning the switch as well as tinier 100 Gigabit transmitters are just two recent initiatives of the Optical Internetworking Forum (OIF).

The OIF, the industry body tackling design issues not addressed by the IEEE and International Telecommunication Union (ITU) standards bodies, has just completed its OTN-over-Packet-Fabric protocol that enables optical transport network (OTN) traffic to be carried over a packet switch. The protocol works by modifying the line cards at the switch's input and output, leaving the switch itself untouched (see diagram above). 

In contrast, the OIF is starting a 100 Gigabit-per-second (Gbps) transmitter design project dubbed the integrated dual-polarisation quadrature modulated transmitter assembly (ITXA). The Working Group aims to expand the 100Gbps applications with a transmitter design half the size of the OIF's existing 100Gbps transmitter module.

The Working Group also wants greater involvement from the system vendors to ensure the resulting 100 Gig design is not conservative. "We joke about three types of people that attend these [working group] meetings," says Karl Gass, the OIF’s Physical and Link Layer Working Group vice-chair. "The first group has something they want to get done, the second group has something already and they don't want something to get done, and the third group want to watch." Quite often it is the system vendors that fall into this third group, he says.

OTN-over-Packet-Fabric protocol  

The OTN protocol enable a single switch fabric to be used for both traffic types - packets and time-division multiplexed (TDM) OTN - to save cost for the operators. 

"OTN is out there while Ethernet is prevalent," says Winston Mok, technical author of the OTN implementation agreement. "What we would like to do is enable boxes to be built that can do both economically."

The existing arrangement where separate packet and OTN time-division multiplexing (TDM) switches are required. Source: OIF

Platforms using the protocol are coming to market. ECI Telecom says its recently announced Apollo family is one of the first OTN platforms to use the technique.

The protocol works by segmenting OTN traffic into a packet format that is then switched before being reconstructed at the output line card. To do this, the constant bit-rate OTN traffic is chopped up so that it can easily go through the switch as a packet. "We want to keep the [switch] fabric agnostic to this operation," says Mok. "Only the line cards need to do the adaptations." 

The OTN traffic also has timing information which the protocol must convey as it passes through the switch. The OIF's solution is to vary the size of the chopped-up OTN packets.  The packet is nominally 128-bytes long. But the size will occasionally be varied to 127 and 126 bytes as required. These sequences are interpreted at the output of the switch as rate information and used to control a phase-locked loop.

Mok says the implementation agreement document that describes the protocol is now available. The protocol does not define the physical layer interface connecting the line card to the switch, however. "Most people have their own physical layer," says Mok.

100 Gig ITXA 

The ITXA project will add to the OIF's existing integrated transmitter document. The original document addresses the 100 Gigabit transmitter for dual-polarisation, quadrature phase-shift keying (DP-QPSK) for long-haul optical transmission. The OIF has also defined 100Gbps receiver assembly and tunable laser documents.

The latest ITXA Working Group has two goals: to shrink the size of the assembly to lower cost and increase the number of 100Gbps interfaces on a line card, and to expand the applications to include metro. The ITXA will still address 100Gbps coherent designs but will not be confined to DP-QPSK, says Gass.

"We started out with a 7x5-inch module and now there is interest from system vendors and module makers to go to smaller [optical module] form factors," says Gass. "There is also interest from other modulator vendors that want in on the game."

The reduce size, the ITXA will support other modulator technologies besides lithium niobate that is used for long-haul. These include indium phosphide, gallium arsenide and polymer-based modulators.

Gass stresses that the ITXA is not a replacement for the current transmitter implementation. "We are not going to get the 'quality' that we need for long-haul applications out of other modulator technologies," he says. "This is not a Gen II [design].

The Working Group's aim is to determine the 'greatest common denominator' for this component. "We are trying to get the smallest form factor possible that several vendors can agree on," says Gass. "To come out with a common pin out, common control, common RF (radio frequency) interface, things like that."

Gass says the work directions are still open for consideration. For example, adding the laser with the modulator. "We can come up with a higher level of integration if we consider adding the laser, to have a more integrated transmitter module," says Gass.

As for wanting great system-vendor input, the Working Group wants more of their system-requirement insights to avoid the design becoming too restrictive. 

"You end up with component vendors that do all the work and they want to be conservative," says Gass. "The component vendors don't want to push the boundaries as they want to hit the widest possible customer base."

Gass expects the ITXA work to take a year, with system demonstrations starting around mid-2013.