We described several technologies that provide receive-constrained channels for use in secure
spontaneous device association. Which of those technologies also provide send-constrained
channels?

What will be an ideal response?

Physical contact. Clearly this also provides send-constrained channels.
Infrared. By default, this provides us with a channel that is send-constrained only up to the boundary of a room,
say, since an infrared receiver on, say, a PDA or mobile phone could receive data from many possible sources.
However, it is possible to mount a receiver behind a lens to prevent signals from sources outside a required
direction.
Audio. This provides us with a channel that is send-constrained only up to the boundary of an insulated room,
say, since received audio could emanate from many possible sources.
Laser. By default, this provides us with a channel that is send-constrained only up to the nearest optical barrier.
However, by mounting a receiver at the end of a narrow, opaque, non-reflective tube one could limit sources
to lie in a given direction.
Barcode and camera. This combination can provide useful send-constrained channels. A camera can be
focused and positioned so as to read data only from symbols presented from a given orientation and distance.
For example, a user can place her camera phone so as to be sure of reading just the symbol presented by a given
device.