Friday, August 6, 2010

When Does Passive Optical LAN Make Sense?

by Lisa Huff

Are you purchasing a Transparent LAN service? Then you probably want to consider POL. Inter-building Transparent LANs often have distance limitations, which are currently overcome by installing significantly more expensive 1310 and 1550nm transceivers. As an active network, these higher cost modules are needed on both ends of every connection, and where seven-to-eight buildings are involved, the dollars spent can add up quickly.

With one long reach transceiver needed at the central office (or fed out of an enterprise data center), POLs can offer significant savings in multi-building campus environments. It is important to note how much more expensive 1550nm modules are as compared to their 850nm counterparts. At 10-Gigabit, a 10GBASE-SR (850nm) optical module costs approximately $1300/port (switch + transceiver cost). A comparable 10GBASE-ER (1550nm) longer reach device that is needed for an inter-building connection costs around $11,000/port (switch + transceiver) or nearly ten times as much. When connecting multiple buildings in a campus setting, these costs add up quickly, and a POL network can be a much more economical solution. The POL system uses 1310/1550nm diplexer optics and while more expensive than 850nm can still cover entire campuses at a fraction of the cost of the 1550nm Ethernet-based transceivers. And, since the signal from these devices can be split to as many as 64 users instead of 1, the cost-per-end-user is drastically reduced.

Passive optical LANs are being touted by their equipment suppliers as THE most cost effective solution for medium-to-large enterprises. According to Motorola, you can save more than 30-percent on your network infrastructure and as your number of users increases, so does your savings.

In our recent research for our POL report, we found that there is a subset of vertical markets – specifically, not-for-profits – that may be ripe to implement this technology. But how does this affect the data center network?

We’ve done our own cost analysis and the reason why POL is so cost effective as compared to a traditional switched-Ethernet network is because you can eliminate lots of copper and MMF cabling as well as workgroup switches. But, in the data center, you still need to connect to your WAN routers. With a POL, you could cover as many as 96 end users with one 4-port blade in an enterprise aggregation switch and ONE 10G uplink port to the WAN router. The equivalent switched-Ethernet network would need four workgroup switches connected to a core switch through 12 uplink Gigabit Ethernet ports and TWO 10G uplink ports from the core switch to the WAN router. So by installing POL, you may be able to cut your router uplink ports in half. I wouldn’t mind saving 10’s of thousands of dollars on router ports – would you?

Of course, this is all assuming a totally non-blocking architecture, which, in reality, isn’t necessarily needed. A switched-Ethernet oversubscribed network covering a 132-user, 4-floor building is still less expensive than a POL. For the details, see our POL report.

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