PMC Pushes Sonet Silicon

Unveils a chip that packs two gigabit Ethernet channels into a single Sonet OC48

December 10, 2001

4 Min Read
PMC  Pushes Sonet Silicon

PMC-Sierra Inc. (Nasdaq: PMCS) today announced a chip that sandwiches two gigabit-Ethernet channels into an OC48 Sonet wavelength (see PMC-Sierra Launches Arrow).

The chip, called the PM5397 ARROW-2xGE, implements recently finalized virtual concatenation (VC) and generic framing procedure (GFP) standards. Both standards have been devised to offer more bandwidth-efficient ways of packing Ethernet traffic into a Sonet/SDH transport network.

"Carriers are spending less, but what they are spending on is Sonet," says Steve Perna, VP and general manager of PMC-Sierra's optical networking division. This realization has encouraged PMC to place more emphasis on its Sonet developments while putting its packet processor work on the back burner (see PMC-Sierra Pulls Packet Silicon)

To understand what's so cool about virtual concatenation entails looking at how data is transported over existing networks. Simply put, the problem is that Sonet channels are the wrong size for carrying Ethernet traffic. In "concatenated" Sonet, the channel is simply treated as a fat pipe. Putting a single gigabit-Ethernet channel (1 Gbit/s), for example, into an OC48c (2.5 Gbit/s) wastes 58 percent of the bandwidth.

Table 1: Sonet virtual concatenation service applications


Bit rate

Without VC

With VC


10 Mbit/s

STS-1 (20%)

VT1.5-7v (89%)

Fast Ethernet

100 Mbit/s

STS-3c (67%)

STS-1-2v (100%)

Gigabit Ethernet

1 Gbit/s

STS-48c (42%)

STS-3c-7v (95%)

Low-speed ATM

25 Mbit/s

STS-1 (50%)

VT1.5-16v (98%)

Fibre Channel

200 Mbit/s

STS-12c (33%)

STS-1-4v (100%)

Fibre Channel

1 Gbit/s

STS-48c (42%)

STS-3c-7v (95%)

An improvement on this is so-called "channelized" Sonet, which allows carriers to carve up capacity in an OC48 link, say, into units of STS1 (51.4 Mbit/s), STS3 (155 Mbit/s), or STS12 (622 Mbit/s). But it's not possible to mix and match these units because there's no guarantee that the same combination of units will be available at both ends of the network, if at all.

Enter virtual concatenation. Its job, in a nutshell, is to "right size" Sonet channels, to allow more efficient provisioning of bandwidth.

PMC's new chip takes two gigabit-Ethernet channels and independently maps each of them into a STS48/STM16 channelized payload, using VC and GFP standards. Each gigabit-Ethernet channel most closely fits into an OC21v (also denoted STS3-7v , or seven units of STS3, in the table above). What's more, the capacity that's left over (two lots of STS3 worth) can be utilized to transport voice channels.

"This device allows you to physically have access to the gigabit Ethernet streams before they get mapped onto Sonet," Perna explains.

Transporting Ethernet over Sonet using VC is a better option than native Ethernet networks, Perna contends. For starters, it won't require carriers to get a forklift upgrade to their existing networks, which are predominantly built on channelized Sonet. To support VC, a carrier only has to upgrade the equipment on the ends of the connection. What's more, Sonet offers features that native Ethernet doesn't, such as bandwidth guarantees, and redundancy in the event of a cable break.

It's worth pointing out, however, that PMC is not the only chip maker that thinks support of Ethernet over Sonet is a smart move. In October, Agere Systems (NYSE: AGR) also unveiled what it dubbed "an add-drop multiplexer on a chip", which supports VC and GFP standards (see Agere Offers Single Chip ADM).

The chips from Agere and PMC are quite different, however. Agere's chip integrates a pointer processor with an STS1 granularity crossconnect. PMC's chip, on the other hand, doesn't include the STS1 crossconnect, but does have onboard SerDes allowing it to interface directly to an optical module on one side and a backplane on the other, if desired. "This is one of the highest integration devices we've ever produced," PMC's Perna claims.

It’s also worth noting that system developers like Sycamore Networks Inc. (Nasdaq: SCMR) have had equipment for more than a year that crams two gigabit-Ethernet channels into a 2.5-Gbit/s wavelength (see Gigabit Ethernet Goes Carrier Class). Sycamore, however, had to develop its own silicon to achieve this feat. The off-the-shelf chips now being offered by Agere and PMC make it easier for other system vendors to catch up.

— Pauline Rigby, Senior Editor, Light Reading

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