Friday, November 29, 2013

principles of communications, 2013/2014, end of week #7 to L22

This week, we covered shared media & ad hoc capacity, and started on traffic engineering.

A sharp question on proportional fairness in earlier material prompted me to notice that that isn't well contrasted with max-min fair sharing -- It turns out (as often with technical areas) Wikipedia has a nice explanation - see this article on
proportionally fair w.r.t weighted (max/min) fair queues

Next week will finish traffic engineering and wrap up with summary of course.

Friday, November 22, 2013

principles of communications, 2013/2014, end of week #6 to L19

This week have done scheduling, queue management and switching
and just about to start on shared media

One interesting point historically -the colossus computer at Bletcheley Park built for code breaking was not a von Neumann classical architecture computer but was a "switched programme" machine  - this made it incredibly fast (for a 1940s design) although incredibly inflexible -- and it took a very long time for people to catch up on a standard desktop (about 50 years) - amusingly, about as long as the Dr Who series has run on BBC TV:)

Friday, November 15, 2013

principles of communications, 2013/2014, end of week #5 to L16

This week, control theory and optimization...

Some minor inaccuracies in slides have been corrected in the online copies linked from the course materials page...[or will be as soon as I can get powerpoint with the right fonts:) - the key error is in the calculation of the steady state error of the proportional controller - for some reason, there's a subtraction of the two terms for U(s) where it should be +
(KUs + Rc) / (s(s+K)
I think {need to check this:) it kind of makes sense (if the completion rate increases, the admission rate should increase....)

then when we take the limit of s(U(s), as s->0, we'll get Us + Rc/K
so ess (error in steady state) is Us - (Us + Rc/K) which gives us -Rc/K
(i.e. the answer is right, but the system response wasn't...will check and correct soon...

again, to note, the chapter on control theory in Keshav's book is very clear if you want alternative source + some nice example problems.

Friday, November 08, 2013

principles of communications, 2013/2014, end of week #4 to L13

Error, Flow and Congestion Control done (99.9%)

further reading - maybe - on Network Coding (see Digital Fountains)
and on what's in Linux (CUBIC) and Windows (Compound) for congestion control, and what real traffic actually looks like - see CAIDA

next week: control theory...and optimzation:)

Friday, November 01, 2013

principles of communications, 2013/2014, end of week #3 to L10

This week we covered routing -

there's one egregious error on the slide explaining Dijkstra's algorithm in Link State where the sign on the comparison is the wrong way round (well spotted students!) - I leave it as an exercise for you to find, as it makes for careful reading:-)

In Sparse Mode, we use Rendezvous Points to coordinate a single RPF tree around a designated/configured router (maybe one for each of a different block or subset of multicast addresses) - there's no guarantee the RP is in a sensible place, although the switch from RP centric tree to source based tree after an traffic flows helps reduce latency -  automatic placement of an RP to be in the "centre" of the group would be a solution to the Steiner Tree (Min spanning tree) problem which is NP-Hard, although there are polynomial time approximation algorithms for it (but you probably wouldn't deploy them in routers, but in a network management system for e.g. a gamer or trader network, this might be sensible)

One other note - consistency, symmetry of routes, and so on - IP and IP routing make no guarantees about this at all! BGP (inter-AS routes) are often asymmetric...recent computer science work on building new protocols that provide global consistency during route update and computation does exist, but is still research, largely....although the techniques are promising!

Next week, errors, then flow and congestion control.

Monday, October 21, 2013

principles of communications, 2013/2014, end of week #2, to L7

To note for today- the slide on graphs, with Edge and Node list has a list of all edges, alongside o nthe right list of nodes  - the list of nodes isn't meant to line up with the list on the left - its just a list for node i=1-5, what other nodes, in the directed graph,  are adjacent (look at arrows on edges - note in 2 cases (1<->2 and 5<->4, they are bi-directional)....

fun references today:-
Ghost Maps




Small Worlds...

DDOS visualised

Couple more corrigenda/errata
1. in the alpha/beta models of random graphs, there's k used for average degree of the net (e.g. pN in the alpha model), but also used for the toal number of edges (N*(N-1)/2) - so take care with k
2. there's an expression in the slides about max-flow in DAR (the "Sticky Random Routing" for the telephoen net) for using Erlang's call blocking probability for a given link, then work out what the toal capacity will be for 1 hop and 2-hop/tandem routes - this has n, which is number of calls you get through, then mentioned a technique called LP  to solve the maximisation problem given in terms of sum of calls that get through (or are blocked) over all direct and tandem routes- we are'nt covering that technique this year, but LP stands for Linear Programming, and is fairly straightforward if you want to look it up - it is commonly used in optimisation and shows up in Operations research/Logistics (freight etc) and so on all the time.

Friday, October 18, 2013

principles of communications, 2013/2014, end of week #1, to L4

We have now covered Systems, and Layers (lawyers)...

Next week, Graphs, and Routes!