Column

Wireless Networks, Wired Bottlenecks

Network Computing Mobile Observer, May 2, 2007

By Peter Rysavy

Are you feeling backhaul constrained? You may not know it, but that may be the reason you're not getting the throughput you're expecting with your wireless applications.

While there is plenty of press about advances in wireless technologies, especially about increasing speeds and lower latencies, it's actually likely that wireline technologies will provide the next boost for wireless application performance. That's because today, many networks are constrained by the backhaul from the wireless access point or cell site. If you've ever been at a wireless hotspot and done a throughput test, you've probably noticed a maximum rate just under 1.5 Mbps. That's because many wireless hotspots use a T1 circuit for backhaul, which has a maximum supported rate of 1.544 Mbps. So it doesn't really matter if you're using IEEE 802.11b or IEEE 802.11g, your throughput is going to only be as fast as the weakest link in the chain. Whether it's the 6 Mbps maximum over-the-air throughput of 802.11b after protocol overhead, or the 25 Mbps of maximum over-the-air throughput of 802.11g--both are constrained by the T1 backhaul. IEEE 802.11n will eventually increase range but it won't do anything for throughput.

It's no different in municipal Wi-Fi networks, many of which today provide 1 Mbps service, again due to backhaul constraints. The same situation exists at home, though cable modem and DSL speeds for many users are faster than T1. As an aside, I find this a little ironic given that a T1 used to be the gold standard for Internet connectivity, but is now much slower than consumer-oriented alternatives.

When we turn to cellular networks, it's the same story. Many cell sites have a couple of T1s going to them, which have to support voice and data traffic. If a cellular operator wants to provide, say 7.2 Mbps High Speed Data Packet Access (HSDPA) service, those bits may travel over the air at that blistering rate, but then grind down to backhaul-circuit throughput rates. My understanding is that this is not really a problem today because the number of 3G data users is still relatively small, and throughput is not advertised at these higher rates. Expectations today are more in the 1 Mbps range for peak 3G throughputs. But as the number of users ramps up, and as 3G has to start competing with other emerging technologies--such as WiMAX--operators will have to beef up the backhaul, possibly to four or five times the capacity they have now. That's a big jump, but I believe this is inevitable.

There is no shortage of market activity to address this trend. If the wireless site is already serviced by fiber, provisioning additional virtual T1s is easy. However, there are a lot of other optical transport approaches available, including metro Ethernet, Pseudowires (emulation
of native services over packet switched networks), Multiprotocol Label Switching, as well as traditional SONET/SDH. There are also point-to-point radio options, including 60 GHz unlicensed radios that offer 100 Mbps throughput over a mile or so, and plenty of options below 60 GHz. In fact, this radio market is extremely competitive, with prices coming down quickly, and is likely to be the most attractive option for sites where fiber is not feasible.

Carriers are generally tight lipped about what they are doing, but I think they are all very active in their efforts to beef up both their backhaul and core networks to deal with much higher data flows. Assuming wide-area wireless data finally takes off, solving this infrastructure problem is essential for both current 3G networks as the number of users increases, and especially for enhanced 3G offerings such as HSPA+ and EV-DO Rev B. Further out, systems such as WiMAX, 3GPP Long Term Evolution and Ultra Mobile Broadband will require even more backhaul capacity. Interestingly, it may ultimately be the backhaul and core network architecture that plays a bigger role in overall performance and operators with strong wireline networks may be at an advantage here.

There's lots happening on the home front as well. VDSL2 can provide up to 100 Mbps of theoretical throughput over copper wire, though only over fairly short distances. But 25 Mbps may be practical. Similarly, we are seeing cable modem rates approaching 10 Mbps, and fiber to the home offering 30 Mbps. Hopefully, before long, you'll be seeing the full capability of that Wi-Fi or 3G connection. But until then, make sure you understand where the bottlenecks are.

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