Interactive communication concerns the number of bits that must be transmitted when a sender wants to convey information to a receiver who has related data, and the potential transmission savings afforded when the two communicators are allowed to interact.
This outline describes some of the basic concepts, main results, and most pressing open problems concerning worst-case transmission for general communication problems. Among them:
Additional outlines describing balanced distributions and average-case communication, where stronger results can be proven, will be added soon.
Interactive communication is best introduced via examples, especially the following example.
There are n teams in the n-BA. Over breakfast, Bob reads that the Celtics will play the Lakers. On the evening news, Alice hears that the Lakers won, but not whom they beat. As a result, Bob knows the two teams that played (Celtics and Lakers), but not who won (Lakers); Alice knows the winning team, but not who they played against.
How many bits of information must Alice and Bob exchange in the worst case for Bob to find out who won?
We first consider the simplest case where only Alice can transmit. We show that in this one-way communication scenario she must send exactly log n bits in the worst case.
log n bits suffice. As we remarked above, log n bits suffice even without using Bob's information: Alice and Bob agree in advance on a log n-bit indexing (say lexicographic) of the teams and Alice transmits the winner's index.
log n bits are necessary. To see that log n bits are needed in the worst case even with Bob's information, note that Alice's message to Bob is determined by the winner, for that is all she knows. For example, she may transmit 011 if she hears that the Celtics won and 1100 if she hears that the Pistons won. If for two different teams she transmits the same message, say 101, then in the event that these two teams played each other, Bob will not know who won. It follows that Alice must transmit a different message for every team. The picky will also note that if one message is a prefix of another then if the corresponding teams play, Bob may not know when a message ends. Hence no message can prefix another. There are therefore n different messages, none a prefix of another. At least one requires log n bits.
The preceding analysis assumed that only Alice can transmit so, for example, if two teams are assigned the same message Bob can't indicate he's missing information.
What if interaction is allowed? Can the total number of bits be reduced?
Interaction can reduce communication significantly. We first describe a simple interactive protocol where Alice and Bob exchange just two messages and transmit a total of log log n+1 bits. Considerably less than the log n bits required with one-way communication.
Alice and Bob agree in advance on a log n-bit representation of each of the n teams. Bob, who knows the two players, considers their binary representations. He transmits to Alice the location of the first bit where the two teams differ. Alice replies with the winner's bit in that location.
To count the number of bits, observe that Bob describes a number between 1 and log n, hence transmits loglog n= log log n bits. Alice transmits 1 bit for a total of log log n+1 bits.
Is that the best we can do? Is there a protocol that requires fewer bits in the worst case?
In the most general communication problem a sender knows a random value X while a receiver knows a random value Y and wants to determine X. We are interested in the number of bits required in the worst case to ensure that the receiver can always determine X correctly.
We assume that X and Y are distributed according to a known probability distribution p(x,y) and that the sender and the receiver agree in advance on a protocol that determines who transmits what, based on the values they hold and previous transmissions.
One can consider various scenarios depending on the number of messages the communicators can exchange. We let W_{m} denote the least number of bits transmitted by both communicators in the worst case when they are allowed to exchange at most m messages. W_{1} is therefore the number of bits required with one message, namely only the sender can transmit, and W_{2} is the total number of bits required with at most two messages: the receiver transmits, and the sender replies.
Allowing additional messages can only reduce the number of bits (we allow, not require, more messages), hence W_{1}, W_{2}, W_{3}, ... is a non-increasing sequence of nonnegative integers and therefore converges to a limit W_{∞} which is the number of bits that must be transmitted in the worst case when there are no restrictions on the number of messages exchanged.
We would like to determine the W_{m}'s for specific problems and the largest possible discrepancies between them for arbitrary problems.
The following segments describe general results that hold for all communication problems. In particular they apply to the League for which we still want to determine W_{2} and W_{∞}.
The League Example shows that interaction can significantly reduce communication over the one-way number of bits. Can interaction achieve arbitrary savings? Is there for example a communication problem that requires a trillion bits one-way but only two bits with interaction?
No. The maximum reduction via interaction is logarithmic. For all communication problems:
W_{∞}logW_{1}+1. | (1) |
Bound (1) can be used to completely determine the number of bits
needed for the League Example.
We showed that for League
W_{1}
=
log n.
Therefore,
But we also saw that for League
Hence for League all the inequalities above hold with equality and
This shows that the two-message protocol we described is optimal. There's no better league protocol, regardless of the number of messages.
Bound (1) cannot be strengthened. It is achieved for all communication problems where (like league), there are two possible X value for every Y value. For all these communication problems,
W_{∞}=logW_{1}+1. | (2) |
How should Equation (2) be interpreted?
If multiple messages are acceptable, it is a positive result saying that
interaction can save a lot.
However if, as in many scenarios, fewer messages are preferable,
then, rewriting Equation (2) as
It is natural to ask whether a small number (albeit >1) of messages suffices to reduce communication to almost the best possible.
Perhaps the most interesting result applying to all communication problems is that just two messages always suffice to reduce communication to at most four times the optimal. For all communication problems,
W_{2} 4W_{∞}+2. | (3) |
Recall that for some problems (like league) one message is exponentially wasteful. Bound (3) says that for all communication problems, two messages are almost optimal. Hence while there may be a large incentive to move from one to two messages, moving from two to more messages has a limited benefit.
In certain cases, Bound (3) also points us to efficient two-message protocols where they are not a priori obvious. This is illustrated by the following example.
As if one league wasn't enough, imagine a tournament with l leagues, each with n teams. As in the (soccer) world cup, two teams from each league proceed to the playoffs and one of these 2l teams wins the playoffs.
Bob know the 2l teams that proceeded to the playoffs. Alice knows the winning team and wants to convey that information to Bob.
If only one message is allowed, Alice must completely describe the winning
team.
If the message she assigns to one team is the same as, or a prefix of, a
message she assigns to another team,
then if both teams participate in the playoffs, Bob will not know who won.
Hence each team must be assigned a different message from prefix-free set.
Therefore,
It is also easy to find an efficient three-message protocol. Alice can transmit log l bits describing the winning team's league. Thereafter, Alice and Bob can use the League problem's protocol. Bob considers the two teams that proceeded to the playoffs from that league, and uses log log n bits to describe a location where the two teams differ. Alice transmits one bit describing the winning team's bit in that location. It follows that
Efficient two-message protocols are more elusive. Next we use Bound (3) to upper bound W_{2}. Later on, we'll mention a lower bound.
Consider the playoffs problem when l is constant and n increases.
We saw that
A low-rate 2-message protocol is not immediately obvious.
Yet Bound (3) implies that
Two messages are the first achieving near optimality. It is natural to wonder about their precise efficiency. Can bound (3) be improved? Are two messages exactly optimal?
Two messages are not optimal. They may require twice the minimum number of bits.
More specifically,
for every α, however large,
there is a communication problem such that
W_{3}α
and
This discrepancy is manifested for the Playoffs problem.
We saw that for the Playoffs problem with l leagues, each with n teams,
Therefore, as l increases and n=2l^{2},
We saw that two messages are always within a factor of four of the optimal and sometimes requires twice the optimal. What is the largest possible discrepancy between W_{2} and W_{∞}?
Namely, what is the largest constant c
such that for all communication problems
One message can require exponentially more than the minimal number of bits. Two messages are always within a factor of four from the optimal, and sometimes require twice the optimal. The most interesting theoretical open problem is whether a fixed number m of messages is always asymptotically optimal:
Is there an m such that for all communication problems,
An affirmative answer would mean that there is never a strong reason to exchange more than m messages.
The answer to this question is not known, but the rest of the overview describes some partial results.
If an optimal number of messages exists it is not three either. For every α, however large, there is a communication problem such that W_{4}α and
W_{3}2W_{4}- o(W_{4}). | (4) |
The lowest number of messages that could potentially be optimal is four. But the only bound know is that four messages are within a factor of three of the optimal. For all communication problems,
W_{4} 3W_{∞} +o(W_{∞}). | (5) |
We showed that for the worst case number of bits:
Among the more interesting open problems, we mentioned that it is not known:
To find out more about interactive communication you can read
To better understand the material and see the proofs, you can read the following papers. Most of the results are described in: