Ethernet in Metro and Long Haul Networks
(continued)

Gigabit Ethernet and Bit Rate Conversion

Nothing in the telephone world comes close to the utter simplicity of how Ethernet can convert bit rates. Right now, we run Ethernet at 10 Mbps, 100 Mbps, 1Gigabit, and 10 Gigabit.  The neat thing is that 10/100/1000 Mbps interfaces can exist on the same Ethernet switch and the switch will do the bit rate conversion for us. We take it for granted that we can plug our PCs into an Ethernet switch and it will figure out what kind of Ethernet card we have and automatically adjust to either 10 or 100 Mbps. 

Statistical Multiplexing

Gigabit Ethernet changes the way you look at statistical multiplexing.  Let me remind you that a standard Ethernet frame is 1500 bytes. This means a 1500 byte packet can be transmitted in roughly 12 microseconds across a 1 Gbps link, assuming that the link actually delivers the full 1 Gbps.  In reality, 8B/10B encoding reduces that a little, but not significantly.  If a voice packet has to wait around for even 100 microseconds before it can be forwarded, who cares?  You need to deliver voice packets in 150,000 microseconds, end to end to keep your voice users happy, otherwise, they will complain about delay. 

So how good does our network need to be? Well, let’s give the VoIP phones 100,000 microseconds to gather the voice, compress it and uncompress it at the other end. That leaves 150,000 – 100,000 = 50,000 microseconds for the network to deliver the voice frame. If you live in a Frame Relay world, you are used to measuring round trip delays in milliseconds. But in this example we are talking about gigabit speeds. Our measure is not milliseconds (thousandths of a second) but, rather, microseconds (millionths of a second).  If a Gigabit Ethernet switch delays voice packets by 100 microseconds by per hop, nobody will notice.  If you have 50,000 microseconds to deliver a voice frame and you use up 100 microseconds in one Ethernet switch, you have 50,000 – 100 = 49,900 microseconds left!  With Gigabit Ethernet, 1500 bytes, which seemed “big” a few years ago, now looks like a “cell” to me.

So how do we deliver optical Gigabit Ethernet service? The choices are:

• Dark fiber
• RPR (Resilient Packet Ring)
• A lambda in DWDM

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