Friday, November 26, 2010

Week 7 - to Nov 26 - Got to Traffic Management.

Looks like we won't make it to the
Optimisation Theory and LP material this year -

I will wrap up on Monday 29th Nov
with last part of Traffic Management,
and an overview of what I've covered.

That will be the last lecture for Principles of Communications.

Students that are very keen can read the slide-ware on Optimisation and on LP - I am happy to answer questions on it too.

Students interested in the lower levels of physical/link layer may want to take the Digital SIgnal Processing course by Markus Kuhn next term (see here. Students interested in networking performance (and systems in general) may well want to go to Richard GIbbens' course on Computer Systems Modelling which covers a number of these topics in more theoretical depth. If really keen, please sign up for the new Part III, which will be running next year.

Note that next year, Information theory will be taught separately (again), which may make the amount of theory material in PrincComm slightly more tractable.

Friday, November 19, 2010

week 6 - to Nov 19 - Got to end of Switching

Next week, to cover
Shared Media
Capacity of Multihop net
Traffic Management

Then final week, will ust get to do optimisation and LP hopefully:)

n.b. to supervisors and students:- i've put a couple more links to some online information about
control theory, graph theory and some of the sources have worked problems...
see slides page for course

Friday, November 12, 2010

Week 5 - Nov 12 - end with Control Theory

Today, noticed that the wikipedia article on this is pretty good, but most especially nice is that it cites an 1868 Royal Society paper from the Royal Society by James Clerk Maxwell, which not only mentions Mr Watt's Steam Engine, but Mr J Thomson's experiments (noting that our building is between JJ Thomson Avenue and James Clerk Maxwell Road:)

Next week, we should cover
Shared Media Access

[The paper above also mentions a Mr Siemens!]

Friday, November 05, 2010

Week 4 Friday November 5th - Principles of Communications Progress

Today, we'll cover Queueing Theory. So that completes network layer stuff
(graphs, routing, errors, queueing)

Note in the printed (and old online pdf) queuing theory slides, there was a font error on some slides with \rho being rendered as ~n.
I've fixed it on the PDFs online (its ok in the ppt). apologies (again).

Next week we start on Flow Control, and hopefully get up to control theory.

Saturday, October 30, 2010

Principles of Communications - End of Week 3

I have just about got to the end of routing
(having fixed, i think, some bugs in the distance vector worked example) -

next week
monday, wrap up multicast/mobile routing

error control
and maybe start flow control

Friday, October 22, 2010

Principles of Communications end of (full) week 2.

Today, I'll finish the graph theory lectures, covering social networks, small world nets, random graphs, alpha/beta and spreading/search.

So we'll have done:
# Introduction1
# Systems
# Layering
# Information Theory
On this topic, John Daugman's notes are great
# Channel Capacity
(not Modulation - this is on hold to end in case we have time)
# Graph Theory
# Social Networks+

On the last topic, this book on Small Worlds by Duncan Watts is a nice read. Another good book on the topic covers more about flows over such networks (information or diseases for example) is
Connected, by Nicholas A. Christakis and James H. Fowler

That means from monday (and most of next week, oct 25,27,29) we're doing Routing.
If things go to schedule, then subsequent week (nov 1,3,5) will be Error Contol, Queueing Theory and Flow Control.

Wednesday, October 13, 2010

Principles of Communications...end of first week (15.10.2010)

Should just have got up to first slide set on Information Theory (Entropy)
Monday 18th will start on Shannon - by 22.10.10 hope to get to Graph Theory.

Have just updated online slides (1up and 6up should all print ok now, fingers crossed:)

Friday, October 08, 2010

information theory - live example

powerpoint for lecture on information theory+colour printer -> slides without equations:(

powerpoint for lecture on information theory+mono printer -> slides with equations:)

ergo, colour printer driver is an erasure channel with memory and rather non random behaviour and information rate is massively reduced :-(

pushing this as an example for explaining shannon is a bit of a the sense that the "physical channel" is the printer and the colour printer should have more capacity in some sense, although I suppose the point is that the "noise" process" is an erasure channel that removes (say) bytes that code yellow but not bits that code white/black...

Tuesday, September 14, 2010

In schloss dagstuhl again - 99/100 (1 small nit)

I really like dagstuhl - the organisation is wonderful - lots of lovely quietly smart tricks (for example, the librarian puts out copies of all the books by attendees for display and they are invited to sign them!) - the bar (wine) is fantastic (makes up for the so-so food - although the cheese platter is superb !) - the visitors book and other information for wider awareness of what is going on and who is there is excellent - the area is great - even the "interesting" challenge of getting here from the "nearest" airport or railway station is a fun problem

only 1 minor complaint - the website (in almost all ways very very good) has a facility for attendees to upload materials for their seminar/talk/wiki/discussion etc - but the site as a design flaw which makes it very counter intuitive to getting the first thing uploaded (when you navigate to My Documents, you MUST have a title and click on the very small "save all" button to avoid weird apache error messages:)

otherwise 999/100 for the best seminar/retreat/workshop site in Europe by miles

fine snooker room ,very nice music room, great cellar, good walks/hikes, pleasant and efficient (and mostly invisible!) staff....

Friday, June 25, 2010

2 weeks of workshops taking its toll!

I was at IMDEA for this workshop on energy in networks, then at this one at Microsoft on Network Economics, then another one at UKERNA on SuperJANET 6 research requirements planning at the very nice Royal Society Chichelery Hall Kavli center then there's this one at the Isaac Newton Institute on networkmeasurements, and one on new internet architectures and incentives rn by the Eiffel thinktank, then one at Telefonica I+D on a vision for 2020 networking, then this one run by the Trilogy Project on resource pooling

Boy, now I need a vacation:)

Thursday, June 10, 2010

here we go again with government interference

without waiting for Lord Brown of Madingley's report, the new government make proncouncements about UK Universities - the BBC is reporting David Willett's saying he needs 700M savings and the coalition's evidence-free arguments that
a) we (the uk universities) are in a financial mess
b) that we have to find ways to be cheaper and more flexible.

Well we don't actually. We are quite cheap (which is because we have to serve the UK government) compared to a lot of EU and US universities AND we are evidentially quite flexible and quite good (compred to most EU universities) - the government should check the figures on health and education in the UK and compare them with similar quality systems in the world. We attract a lot of overseas students of very high quality preceisely because we strike both a bargain, and a high standard. We don';t have to serve the UK - we could just turn into private institutions for rich kids from here and overeas. Or we could be rubbish. But we are not inflexible or expensive.

Frankly, I'd held out a bit of hope that this new government would actually base its policy on facts, but it looks like I was hopelessly naive and sentimental.

What a shame

Thursday, May 27, 2010

schroeding & turing

Schroeding's Cat is the typical name for a thought experiment about the oddness of the Quantum model of the Universe. It is an artificially constructed scenario where the normally microscopic quantum level effect selects a macroscopic change in the state of the Universe - i.e. the decay of 1 single atom triggers the life or death of a large creature (a cat is presumably on the order of 10^24 molecules) - Life or Death states of the cat are
quite visibly distinguishable to a human observer. Of course a simpler trick would be to have a critical mass of fissile material, less 1 atom, and then just add that 1 atom and then see if it decays or not and does so triggering the chain reaction or not....that would also be quite visible and not very distorted by emotive weird observable things (when is the cat dead? what is life? :-)

Turing's test of intelligence decides the matter based on a human observer's ability to distinguish the responses of a black box to a set of questions (sent over some teletype line to avoid any complex non verbal comms). It can be generalized.

Some confusion arises when thinking about Schroedinger's Cat ("paradox") and observers.
Note that the state of the cat is actually created by an observer (the geiger counter described in the wiki article above is the thing that detects the decaying atom, and triggers the hammer to break the flask of cyanide that kils the cat (probably). The uncertainty is purely about when the decay happens. An observer of the dead cat is a secondary thing - we can assume fairly certainly the cyanide kills the cat (all nine lives) so in fact all we need is a red light on the box that says the geiger counter detected the atom decaying. The confusion arises because two words are misused from natural language:

  • observer
  • description

    The observer does not have to be intelligent.
    The description is not obvservable.

    Hence we can't use this to talk about consciousness, and there is no paradox (descriptions of intermediate unobservable states are figments of maths - only if actually observable are they interesting (e.g. remote entaglement).

    The point of mentioning the turing test is that Schroedinger also conflates two things:
  • simple observation (detect decay from emitted particle
  • complex observation (is the cat alive).

    Is detecting life, perhaps a nice turing test?

  • Sunday, May 23, 2010

    remarkable engineers

    this is a fun read if you like these sort of mini-bio things :-
    Remarkable Engineers follows similar excellent books on maths&science. But it is an interesting book for its lack of people that I would regard as engineers from Ancient Greece, Arabia, and China - however, within its well-defined model of what an engineer is, it is interesting (seems like you not only have to build something, you have to patent and sell it, and preferably be Scottish:)

    Thursday, May 20, 2010

    1 mole of stars. Avagadro, state and human perception of scale

    so reading
    The Eerie Silence by Paul Davies,
    he points out that there are estimated to be around 10^23 starts in the universe around now....this is an interesting number (like all numbers) which is around Avagadro's number which is the number of atoms in 12 grams of carbon 12
    or roughly 24 liters of air at STP.

    one amusing number to play with is the number of molecules in a glass of water, which is MUCH more than the number of glasses of water in all the oceans of the Earth.

    So the number of stars is in between.

    maybe, they are the same thing.

    Monday, April 19, 2010

    Conferences, Journals & unrealistic levels of Academic labour taken for granted

    I was talking to some PhD students here recently about the relative merits of submitting their work to Conferences or Journals - a typical chapter of a thesis makes a nice paper

    before you write the dissertation, it is nice to have it published in paper form, but to get the process done and dusted before the PhD examination comes along, you almost certainly have to send it to a conference rather than a journal because of the turnaround times - its 6 months from submission to presentation for typical top-league conferences, and more like 2 years for the equivalent conferences in Computer Science.

    Reason for publications: confidence boosting (for student AND for examiners:)
    and feedback is useful to improve work even if it doesn't get in...

    Plan B (if things work out) is to submit a 9-12 page version to a top conference, get your PhD and submit the 12-20 page version to a journal after....reason for submission: archival version (maybe) and certain academic communities' career paths still value journal higher than conference (mostly, though, citation is lower...)

    Some points to consider though
    1. academics or industry research lab workers do all the work for conference and journals (run submission sites, do reviewing, do editorial work, do selection, do a large part of conference organisation etc etc)
    2. a typical top class journal or conference paper represents about 1MY effort - typically if you include all the effort by a PhD student for a year, plus their advisor and any co-authors - in systems, it can be as much as 2-3 people's work
    3. a typical conference reviewer might look at 20 papers in 30 days - so if they are really really insanely fast at reading, and on top of all the latest material (and not handing it out to their poorly as yet calibrated own students to review, or at least only doin this to help their students get calibrated or give them up-to-date feedback, but still doing the review themselves) if you are lucky, your paper gets 4 hours work on it at a conference
    4. a journal submission might get a rainy Sunday first submission, and then several hours more over revisions, so you get a lot more detailed care and feeding in the process.
    5. In my experience, examining a PhD takes 3 days work - if you consider the naive equation, that 1 dissertation is == 3 good papers, then this is about twice the level of attention a paper gets (at best)
    6. Finally, a lot of researchers use long journeys to read a lot of papers (transoceanic flights are good) - this appears to have stalled for the last few days around this part of the world - I wonder what impact this will have on conference review quality?
    On the other hand, maybe the mortorium on flying will reduce the number of weak or nearly pointless workshops ...who knows...

    Thursday, March 18, 2010

    two mixed reality dystopias.

    1. Gravity 3.0 didn't turn out to be such a good idea, did it Professor Sheckley - I mean superficially, the notion of an inverse cube law force so that people on smaller planets, but nearby, get a strong attraction and don't drift off into space, looks OK - but then look at the terminal velocity - not good for the robot ships landing delivering out food....gasp gasp
    Of course, it wasn't such a terrible mistake when you compare with Gravity 2.0, the result of the Campaign for Real Gravity - attaching everyone to the surface of an apparently zero mass planet, by colored elastic looked cool - but we didn't reckon with people customizing their g-strings, to have different h-indexes - Hookes' law is amusing, but then when they started playing paddle-ball with the alien, and we had to tell them to turn down the space-trance remixes of Phil Collins and eventually called in the Cruel and Unusual Ludic Police, it took a turn for the worse.

    2. The entropic viral pandemic of 2012 has run its race, and now we cannot rely on the value of bits to be discrete any longer. Unfortunately, it was detected too late to do anything about backups, and so we cannot state the meaning of any program or data with any certainty any more. Such diseases of meaning were unanticipated in the early days of the semantic web, so that elementary precautions, like repeated recall and re-enforcement of what we were saving
    were not taken. Now we have to rely on humans to memorize entire sections of the Internet, including music, live performance of movies, and physical versions of VR games.

    The cause of the emergent entropic virus will never truly be known (just like every other piece of what use to pass for online human knowledge) but it is suspected that it was the mean temperature of the time series of arrivals of Youtube videos, exceed Centigrade 451, the point at which binary systems move to a higher, ternary state.

    Cooling the system down will do no good now. It would be like telling Schroedinger to open the box and finding one and a half cats, one half dead and the other half alive.

    This has been a Horizon special

    Tuesday, March 16, 2010

    progress bars and stress

    so i just have had the 3rd experience of the week when I want to carry out a routine task on some Win* box and it decides it needs to run a massive update and
    to make matters worse, this locks out other activities
    but the straw that broke this camel's back is the
    progress bar

    the example of the service pack update for vista will suffice to show what a load of stress this causes - the bar is in %age complete - but there are 3 seperate phases and you are given no indication of their relative longevities nor does the bar actually
    move linearly w.r.t time in any fashion (not even relativistic) so if you go away for what ought to be enough time (e.g. cup of tea or watch a movie) you come back and inevitably either something finishes 10 times earlier, so you now have to go to the next step, or else it still has a full 3-day test match (cricket) to go ....

    this is like being in a travel warzone when the train/plane is late and the announcements are intermittent and noisy (or in a language you don't speak)...

    Friday, March 05, 2010

    The Law in 1995

    I found this proposal I wrote in 1995 for a programming language for a sort of active web called The LAW - A Language for Agents in the Web

    It kind of presages stuff like map/reduce and declarative networks, n'est ce pas?

    Monday, March 01, 2010

    I am a gadget, you are a widget, they fidget, bridget - Its the Long Gadget, Gromit

    Jaron Lanier's fine new book,
    You are not a Gadget has a lot of things to say that I agree with.

    Turning it up to 11, there's a nice chapter suggesting some new models, which might very much fit the Horizon Digital Economy project - there are three notions, which I have re-branded as follows:-

    1. songlets - his idea is more Ubicomp than mine - basically, create a
    "songle" - this is a physical key to a right to play a song - could be an Rfid, or a Barcode on a can of coke - it creates an "artificial scarcity" for songs so that instead of people trading them for free on filesharing nets, people trade them for
    micropayments (possibly bundled with other things)...its a nice idea
    2. giglets - telegigging is the idea of inviting a live band to your party over the net - giglets would be my old version, which is that all live gigs should be webcast and recorded (a la bbc iplayer, NOT a la youtube) with low cost DRM - possibly using a model where its pay per view up to some deadline, and then switches to free if not moving out of the long tail, but staying pay per view for longer if it gets popular and moves up the zipfian...
    3. marklets - these are instruments (in the trader sense) which can be formally checked for sanity against a policy + safety rule-base - this is a good idea and resembles some of the things George Soros- wrote in his fine work on the Crash of 2008 and What it Means - he put in some meta-rules (some are basically stability conditions in feedback systems with many actuators - some are what a Programming Languages person would recognize as reflection - all nicely and intuitively explained).

    Things I disagree with:-

    1. music and midi

    - kids share music - they don't share midi, but they do share
    recordings, Youtube videos, tab and formal classical notation
    so that they can learn stuff - kids I see doing this
    do play amazingly better than kids when I was their age - they
    don't know they aren't supposed to be that good yet, so they just do it

    The real problem is that any abstract representation of music (i.e. notation) degrades it - classical notation, tab etc

    using printed symbols for notes is (to reuse an Elvis Costello phrase) like
    dancing about architecture

    kids i know (in the 12-18 current cohort) share the recordings of the results
    not midi - maybe its coz uk is guitar-live oriented, or maybe i am ignorant of dance culture (well, 20 years out of date on it)

    midi is so 70s/80s/ synth/drum machine - agreed - but that's only a trap
    someone who is synth/keyboard/tech obsessed would fall into

    2. code and inflexibility -
    i) I was amazed not to see Larry Lessig cited...
    ii) I don't see files as lock-in - that is like saying
    bits are lock-in - files are just ways to frame a set of data - you can have sets of sets , etc - just like bits. there's a load of theory why this is general....

    this is a granularity/sampling error, which i think is a mistake as a metaphor for a cognitive framing error - the notion of Unix files as a problem is much less weak, but still not as good an example as midi, even if I believe midi is a red herring...

    3. on music
    Doh, how dumb, the internet
    Ray, a blade of golden sun
    me, an apple trademark bet
    far, site, neat place to keep one
    sew, a cpu scheduling threads
    la, a loony tune land of cgi fun
    t, a pipe with 3 ends
    and that brings us recursively back, doh

    good, bad or midi?

    4. singularity (c.f. Singularity Sky, Accelerando etc) and
    techno-totalitarianism (none so bad as Extropian!)

    The idea goes back a long way - Indian and pretty much
    every other early myths have avatars and nirvana,
    although, perhaps less blue and with less grunge...
    to sublime, to get to cloud nine, to reach Twoness (oops, sorry, Woody Allen:)
    or oneness - read Lord of Light....

    5. upload and the Turing test - yes, its reductionist - Turing
    wanted something akin to what Shannon had for information theory of a channel - so
    he constrained the situation - of course it then lacks context, state, emotion, society etc - yes its sad.

    6. Some more thoughts on this as I read about his critique of the impact of the "free to air, paid by advertising" business model - he uses examples from
    journalism and music and berates the long tail argument

    However, the death of journalism long pre-dates the Internet, although it is a technocentric death - the centralisation of press by barons such as Murdoch (and before, Maxwell - c.f. Wapping etc etc) destroyed most of the community of local newspapers and full time journalists who gave detail and colour - this is well documented - the book "Flat Earth News" shows the fraction of news that is actually written anew and analytically by journalists and the vast majority that is simply a verbatim reproduction of press releases. The complaints by media that Google gets revenue from avertisers simply for "indexing" this stuff is rediculous, since the majority of the material isn't even copyrightable by the so-called "publishers" in the first place - it is effectively plagiarised anyhow. (often literally without citing the original "source").

    secondly, the music business has concentrated for 20 years on back catalog - this is why it has seen a demise faster than film - the problem is that this is the exact OPPOSITE of the long tail - the claim that A&R men invest a significant fraction of profit in finding new talent is pathetic - it is about as pathetic as most drugs company claims that they spend a large fraction of their profits on new drug development - they don't - they spend more on advertising. One obvious consequence of this is that kids are overwealmed with the quantitiy of high quality material they can see - if you are setting out to learn to play (remember first 3 issues of the first and greatest punk fanzine, Sniffin Glue covers: "here's a chord". "here's another chord". "here's a third chord - now go form a band") - this is just not going to occur in the retro world....on the other and, myspace is full of some cool crazy dudes - i would comapre the clunky wonderful amateurism of, say, sulek, with the madness that is harmelodic metal tapping, sweeping and shredding of Buckethead...

    The Internet is a corrective to these poor trends - that doesn't mean it is a perfect replacement, but it sure is better than the alternative.

    I love the idea that language might have started as swearing about smells - that is really cool.

    otherwise 11/9:)

    Sunday, February 28, 2010

    research is wasteful

    reading about the government's reaction to the recession (well at least in its own (our) coffers), we can see that it is highly likely they will cut funding to research over the next year(s).

    there are two ways to do this - both wrong - one is to spread the reduced funding thinner, the other is to concentrate funding in fewer (elite) places (actualyl 50% of the money goes to the top 5 institutes anyhow).

    But what they should do is spend more. Research is wasteful, very very much like evolution - but necessity is the mother of invention and (just like in evolutonary systems) the impact of a big recession might make people more creative - more creativity does lead to more potential outputs, but it also leads to LESS predictability - we don't know what will fit the future landscape (i.e. what is "selected" as most fit, is not something you can do a priori, but a posteriori, only).

    No doubt they will get this wrong, which is sad, as doubling the UK's governemt research funding would still leave it at under 2% of the bank bailout last year, and would be likely to yield more profit by about an order of magnitude.

    Unfortunately, we are not ruled by people that have a clue.

    Wednesday, February 17, 2010

    clown computing

    its so much more fun, and easier to market than the cloud

    send in the clowns

    i've looked at clowns from both sides now

    clown atlas, using the Grimaldi internet coordination system based on
    empirical data from Big Foot

    Sunday, February 14, 2010

    CRU/Dr Phil Jones are a bigger pain than maybe they realize

    I' reasonably well trained in natural sciences and computing. So I usually trust scientists to do their job as well as they can. In medicine, there's a whole bunch of due process before they loose a new procedure or drug on the world, and even if the big Pharma might be a bit dodgy (and most are fine), the process stops rediculous things happening too often (unless politicians get in the loop).

    WHen I or a close family member or friend get ill, I do read the literature (the tech. literature, not just online freebie dodgy internet health sites) to figure out what is what - when i spent a week in hospital with metal in my leg, a couple of years after my father died in a hospital from an MRSA infection, I read all the latest papers on that - I didn't find it too hard to catch up and it mattered to me - but I assume, most of the time , that the latest practice is I don't have to read the 1 paper published per scientist per month in the world (it.d make me, on average, the 2nd reader only on average).

    This is true in most areas where there's a large scale deployment of some piece of engineering (planes, trains, automobiles) too.

    So even though I could get my head around a lot of the work, I assume I don't have to.

    So the consensus on climate change has been fairly overwealmingly behind the anthropgenic global warming explanation, leading to many government initiatives and international schemes to try to get people, society, industry, nations etc to reduce carbon emmissions. Seemed ok to me - the bit I like was the sane stuff which looks at being, simply, more sustainable, anyhow, as that seems like a good way to conduct oneself - not having had a car for 45 years and having got 3 kids to/thru school on bikes- having an efficient house, etc - I feel ok - then I feel like I should carbon offset my flights (I used to do more, but I still, as an academic, go all over the place for project meetings, conferences, PC meetings etc etc etc)....

    so now, because of the CRU fiasco (not their fault the email was leaked, but it is there fault the data isn't avaialble and in good clean shape) I now have to read through the mountains of literature on this topic to try to figure out what is sane and what isn't.

    You know what is scary? In the 21st century, there is absolutely no way to tell what is authoritative .

    I know what/who is serious in my own area. In biomedical area, I can usually guess to some extent. But in this area, it is completely impossible to determine

    so not only do I have to read the literature, it looks like I might need to go out, and audit where the data comes from, then get ahold of some of the data, and then do my own analysis.

    that is rediculous, but without doing it, I really don't see a way to have any confidence in the statements by AGW proponents, or by climate skeptics.
    I am sure most of them are genunine. But none of them has a clue how to instill a sense of public understanding of why we should listen

    this does not make me a climate skeptic (not at all) - it just means that for the first time, I'm looking at an entire body of so-called knowledge, and while I can grok the concepts, until I can grep the raw data files, I am basically saying that this is a complete crock!

    Monday, February 08, 2010

    mandy cuts his knows to spite his feys

    Mandelson said: "We know that universities have a vital contribution to our economic growth, so we are not going to undermine them. We are asking for savings of less than 5% and we expect universities to make these in a way that minimises the impact on teaching and students. I am confident they will."

    Mandelson said: "I am an arts graduate myself. We don't dictate to universities which courses they put on. They tailor courses to meet demand. We want universities to play to their strengths, but we also want to keep this country civilised."

    since the universities contribute net profit to the UK, cutting them decreases the UKs GDP. so cutting them ANY percent will have an impact both on students and on everyone else.

    of course, being an Arts graduate, Mandelson is probably not numerate to understand this.


    Friday, January 29, 2010

    4 colour map problem solved....

    ...automatically -
    turns out just to be a
    bunch of
    corner cases


    now on to fermat's.................................................................................

    Tuesday, January 19, 2010

    Big Scary Software - python runtime, filesystems, etc

    Mort (now at Nottingham) pointed us at some educational videos in the last couple of days

    1.The Giant Interpreter Loop that python runtime uses is not really something that admits of concurrency, but also has a lot of weirdnes about it...
    (sorry - as per comment - GIL = Global Interpreter Lock, not Giant Incense Lamp)

    2. Making sure things really are written to stable storage or disk, as we used to call it, is really not quite as simple (or obvious) as you might think

    3. not even on windows

    I guess these should be like public health warnings, a bit like the German and French governments recent IE warnings