Day: 2010-06-29

Optical Flow Switching

| David Levinson

Chan, Weichenberg, and Medard Optical Flow Switching. A proposed communications protocol, but one which has some insights for transportation (if not immediate applicability)

Motivated by the minimization of network management and switch complexity in the network core, flows are serviced as indivisible entities. That is, data cells comprising a flow traverse the network contiguously in time, along the same wavelength channel, and along the same spatial network path. This is in contrast to packet switched networks, where transactions are broken up into constituent cells, and these cells are switched and routed through the network independently. Note that in OFS networks, unlike packet switched networks, all queuing of data occurs at the end users, thereby obviating the need for buffering in the network core. Core nodes are thus equipped with bufferless optical cross-connects (OXCs). OFS is a centralized transport architecture in that coordination is required for logical topology reconfiguration. However, OFS traffic in the core will likely be efficiently aggregated and sufficiently intense to warrant a quasi-static logical topology that changes on coarse time scales. Hence, the centralized management and control required for OFS is not expected to be onerous. The network management and control carried out on finer time-scales will be distributed in nature in that only the relevant ingress and egress access networks will need to communicate.

(emphasis added)
So instead of storing and buffering (on-road queueing), they allocate the whole path between origin and destination to the flow (a trip).
Of course, since this is communications, there are some things they can do that transportation cannot:

If any errors are detected, a request for retransmission of the whole file is done via feedback to the transmitter.

(Sorry, your trip didn’t work out, please have your clone repeat the trip).

Intercity Bus: Jefferson Lines

| David Levinson

Strib has an article on Jefferson Lines, a local Intercity Bus company: Jefferson Lines: Flying low

Traffic on daily roundtrips has risen from 16 passengers a day between Duluth and Minneapolis when Jefferson took over from Greyhound in 2004 to about 90 a day. And that’s before Jefferson added two more express routes last week.

Compare with the proposed so-called High Speed Rail (110mph) Northern Lights Express which will cost at least $615 M (and probably closer to $1B) to build, serving the same market.

The American Bus Association Foundation commissioned a study by DePaul University Prof. Joseph Schwieterman that found that motorcoaches are the most fuel-efficient transportation mode. They provide, on average, 207 passenger miles per gallon, compared with 27 miles per gallon for single-occupant automobiles, 44 miles per gallon for airlines and 92 miles per gallon for commuter railroads, based on typical passenger loads.
What’s more, the airlines need something more than 80 percent of seats full to make money. [Jefferson Lines] needs fewer than half the seats filled on a 55-passenger coach to make a profit.

So the NLX will travel 155 miles in 2 hours (averaging 77 MPH), while buses, which are slightly slower (we could say they should be able to average 60 MPH on this route, which is almost entirely interstate), but more fuel efficient. So the difference is 0.58 hours. Is 17 MPH for 2 hours (i.e. 34 minutes savings for 90 people a day worth a $1B capital investment and a higher environmental cost? 90 people * 34 minutes/person * 365 days/year * 30 years = 33 507 000 person minutes saved. (or $30/minute or $1800/hour). That is a high value of time required to justify this HSR line over its most similar alternative, commuter bus. (I.e. car is not the right comparison, since most people are going point to point, not station to station, and will need transportation on the other end. People going to/from Duluth to the MSP airport to change planes, will probably still want to fly rather than transfer in downtown Minneapolis at significantly extra time)
(This back of the envelope analysis ignores discounting (which makes the value of time higher) and induced demand (which makes the value of time smaller))
OK, so bus is not marketed the way HSR would be, and HSR might attract more passengers. But surely this problem is solvable for significantly less cost than the capital required to construct a new HSR line.