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Stardate
20030824.1307 (On Screen): During the blackout, a lot of people were annoyed when their cell phones didn't work. In some cases the cells didn't have backup power, and in many cases they were overloaded. Glenn Reynolds quotes an article which says that the system didn't have enough excess capacity, and says:
I don't think that's much of an excuse, and I think that cell-phone technology is mature enough that it's fair to start expecting the kind of robust reliability that we've seen from landline services. This is too important to ignore.
It's more complicated than that. Cell phones are one of the many kinds of businesses where most of the expense is capital and up-front, and must be paid just to get into the business. That expense isn't related to the number of actual customers. Airlines are like this; the jets have to be purchased, and they cost nearly the same to fly whether they're full or empty. Passenger railroad and long distance telephone are other such cases, as is electrical power generation and transmission. Cells are extremely expensive to install and operate even if they carry no calls at all, but you have to have them before you can even offer your service and start recruiting a customer base.
The problem is that the kind of robust excess capacity Glenn thinks should be there for the once-every-blue-moon massive failure would have to be paid for during normal times. Let's take two competing cell phone companies, and let one of them do what Glenn says it should do.
It puts in all the extra equipment which would be needed during a massive emergency, most of which isn't utilized during normal periods. It makes a huge capital investment in all that, while its competitor doesn't. The competitor builds a smaller system, with the same normal capacity but much less provision for dealing with emergencies.
The first company has to charge higher rates. After all, it's spent more money, and it has to recoup its investment, somehow. The second company has invested less, which means that it can offer the same service (in normal times) at a lower cost. Which company gets your business?
Historically speaking, price is a really major factor for most customers who choose among competing cell phone companies. And because of that, companies which don't do what Glenn recommends have a commercial advantage.
Update: Dean writes:
This may be naive of me but would it be possible to have an emergency mode in a cell system that would permit more traffic during emergency situations. I've noticed that wideband services are being offered now, pictures being sent from/to cell phones, internet access, etc. Could this "extra" bandwidth be redeployed during an emergency by restricting service to basic voice and/or text messaging? Would this allow a more robust basic services network?
The mechanisms to which you refer don't represent new bandwidth. Rather, what they're doing is to permit a single user to use a larger percentage of the existing bandwidth.
In AMPS and all existing TDMA systems (including GSM) the bandwidth allocated per user is constant and inherent in the system design. (In IS-136 it's 10 kbps per call, and in GSM it's 25 kbps. Some of that is used up by overhead; it's effectively somewhat less.) When a cell is deployed in a given area, then based on the technology and the amount of spectrum assigned to it there is a hard upper limit on the number of simultaneous calls which can be carried, and nothing can change that. GSM is allocated in 200 khz channels each of which can carry 8 calls. If a cell has 5 of them, it's limited to 40 calls simultaneously.
In CDMA systems (older IS-95, new CDMA2K and the upcoming GSM UMTS system) the bandwidth used by any given call is always been dynamic; it uses as much as it needs, though there were upper limits. CDMA systems use what are called variable rate codecs which means that the actual amount of data sent for any frame (20 milliseconds of sound transmitted as a digital packet) depends on how complex the sound was. It can be "full rate", "half rate", "quarter rate" or "eighth rate", using respectively the full specified bandwidth of the codec, half, a quarter or an eighth of it, and this choice is made each 20 milliseconds based on the sound being sent. And because of the way CDMA works, there are no allocations per phone at the level of bandwidth. They're assigned "channels", but all that means is that they're assigned a Walsh code no one else will be using in that sector. It's not like in GSM where they're assigned one of the 8 slots on a given carrier frequency. Whatever a given CDMA phone isn't using at any given instant is available for the other phones in the sector.
Which is one of the big reasons why CDMA systems are referred to as having "soft capacity". The overall bandwidth available isn't really exact for a number of complicated technical reasons, and in any case there's no way of knowing how much bandwidth any given call will be using at any given instant. CDMA was designed from the beginning to let each active call dynamically change how much data it sent and received, with the ability to change every 20 milliseconds. That was done so that the system as a whole could take advantage of the unneeded bandwidth during the majority of the frames where a phone's codec doesn't need to send a full-rate packet. But they later took advantage of it to support different codec rates, and t
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