BS2911 Week 11 1

Networking and the Internet (11)

Last Week» Finishing Communication protocols (Willis, chapter 14)» Standards (EBCDIC, ASCII, Unicode, integer order)» Data formats for transmission» Introduction to exam structure

Week 11 – final theory session » The future of Voice in networking» Reflection on the assignment» Exam preparation» Module evaluation

BS2911 Week 11 2

Integrating Voice and Data

The future of telephony

BS2911 Week 11 3

Voice over IP Telephone Exchange development

» Operator connection» Strowger step-by-step switching» Crossbar – electro-mechanical and electronic» Time-division multiplexing» Digital switching – arrived with cheap AD/DA converters

Today’s Public Network» All Digital except “local loop” to subscribers» Trunks (connections to PABXs) largely digital

Private networks mixed » University has digital with some analogue

Cabling: Optical fibre long-distance; twisted pair local» Little difference between twisted pair for analogue & digital

BS2911 Week 11 4

Digital Telephone Switching

Entirely electronic – no need for mechanical switching

What happens when you call on a digital system» Handset connects to Analogue-Digital Converter» Spoken waveform converted to bit-stream and divided

into packets for transmission down wire» Each packet given header to identify called party» Switch or PABX (Private Automatic Branch eXchange)

routes this down appropriate outgoing cable» Signal converted back to analogue so person can hear it» If ADC/DAC is built into phone, it can be self-identifying –

great for a hot-desking environment » It can be built into a laptop computer (as with Skype)

BS2911 Week 11 5

Impact on the Public Network

So what’s the difference between a PABX and a Router?» Not much apart from expected quality of service» Phone must be reliable & fast; needn’t be totally accurate

Can see this in configuration of home ADSL:» Virtual circuit for point-to-point (PPP) connection to ISP» PPP over ATM (the protocol of phone company’s network –

effectively the exchange switches as an ATM router)

Same considerations apply to cellular radio network» Except it’s already digital and “routed”» Only class of service limitations keep data rate down to 9600» 3rd generation can handle much faster data

BS2911 Week 11 6

How to Integrate Voice & Data

Key requirement for data is 100% accuracy:» 0000 0000 1000 0000 could mean £1.28 taken from a/c» 0100 0000 1000 0000 means you’ve spent £165.12!» Can tolerate a slight delay in transmission

Voice is the opposite:» It must arrive in a timely way» Digital sound is inherently approximate:

– CD samples 44,100 times a second to 16-bit precision– MP3 uses compression on less precise sampling– 8-bit at 8kHz is good enough for a phone call (64kbps)– We can probably understand even if it’s a bit worse

This difference is a menace – or a saving grace

BS2911 Week 11 7

Classes of Service

If a circuit is lightly-loaded, we can mix voice and data without any problem» This allows companies to run Voice-over-IP internally

When it gets busy, something has to give» For data, we simply delay things, and rate of arrival slows» For voice, we [painful silence] can’t tolerate delay

Potential Solution – introduce Classes of Service:» Flag data packets as suitable for delay, while maintaining

integrity checks, retransmission after error, etc.» Flag voice packets as imprecise but urgent –

– Don’t delay them, but don’t retransmit on error– Could even compress them further in extremis– Better still, get sender to increase its compression (flow control)

BS2911 Week 11 8

The Future of Voice over IP

Adoption needs protocol with class-of-service information» Internetworking Protocol V.6 defines this» Also extends IP addressing from 4-byte to 8-byte

Alternatively, we can just have copious capacity» Probably the situation today» Could become a problem if we all start using full ADSL

bandwidth, for example, on– Streaming audio and video– Downloading movies (and Podcasts)– Telephony, particularly with video

» However, development of ADSL is adding to capacity at home and in the office

BS2911 Week 11 9

In Businesses: The Reliability Issue

Historically, we have set lower availability standards for computers than telephones» Network infrastructure needs to reach standard of phones

before we can let our business depend on it» But firms in e-Commerce are already in that state» and many are so IS-dependent that loss of IT is a killer

Less of an issue between sites – » If the tie-line fails, use the PSTN as fallback» So it makes sense to mix voice and data on leased lines» Or even to rely on the Internet and eliminate inter-site lines

Phone companies also have digital and VoIP technology» Can offer high reliability and low tariff on dialled calls

BS2911 Week 11 10

At Home: Twisted pair reigns supreme

Cable companies haven’t pushed phone companies aside» But many houses now have ADSL on their twisted pair

ADSL has created opportunities for services like » http://www.skype.com/ (2004 onwards)» Many imitators (e.g. Tesco from 2006)

VoIP offers» Cheap or free international calls» Multiple concurrent phone calls over the one line» Downward pressure on all telco tariffs

(so you don’t need to use it to benefit from it)» However: there’s limited interoperability between vendors» Handsets rarely as user-friendly as ordinary DECT phones

BS2911 Week 11 11

Likely progress is as follows:

Now: Services like Skype will expand» Dual phones (VoIP + PSTN) will get cheaper and better» Offers cheap telephony over the Internet» Never mind the quality, talk to Australia at 1.3p a minute

(and the quality isn’t bad, either) Now and soon: Used within single organizations

» Inter-site lines integrate voice and data or switch to Internet» Smaller sites attach only through ADSL connection » Increasing cheap and flexible use of existing LANs» By end-2008, a third of UK companies will be using VoIP

Finally: PSTN will migrate» Only when reliability is good enough

BS2911 Week 11 12

Hot Issues:

e-commerce

B2C – human user buying from the computer» What can you sell this way?» How do you retain or lose

customers?

B2B – usually computer-to-computer» Traditionally EDI over direct

connections» Now largely switched to the

Internet

Voice networking

Currently done by switching Key requirements are

» Very high availability» Urgent (phone call is interactive)

Opportunities for integration with data» Digital except for local loop» Even that moving to ADSL» Tolerant of errors

(lots of redundancy in speech)» Packet-switching far cheaper

than physical switching» Data usage has overtaken voice

BS2911 Week 11 13

Assessments

Reflection on the Assignment

Preparing for the ExamWe’ll have a revision session next week

BS2911 Week 11 14

Assignment

I’ve read in all your USB sticks, and done a cursory check of the sites

All but two worked» One may work when put on a machine with Flash installed» The other had absolute references to the student’s My Docs» Some of the sites looked interesting and professional

I took in two assignments last week:» the other is part one of a pair, and students need feedback

before doing second part, so it has to be given priority» Result is that yours won’t be ready for at least 3 weeks» Don’t worry, nothing in the exam needs this feedback

BS2911 Week 11 15

How was the Assignment?

Tested your practical understanding of HTML

From my perception; some good sites» Clear evidence of learning» Even “ringers” did something to extend their knowledge

What were your experiences with:» Writing HTML by hand» Other methods you’ve tried such as FrontPage (full or

freebie), Dreamweaver, Geocities site-builder Did you learn any research skills in the process?

» Finding on-line primers and examples of good practice» Integrating conflicting advice from various books» Trial and error

BS2911 Week 11 16

Goal of Exam

To test the learning outcomes not measured by the practical assignment. You should…» have built on earlier concepts relating to computer systems» understand the theory underpinning network technology» have studied current network configurations applicable to

business » understand Internet technologies» have accumulated sufficient practical experience of Internet

services to understand how to apply them to business (tested by the HTML assignment)

» be able to review innovations in network technology in a critical manner, without succumbing to fashion

BS2911 Week 11 17

Preparation for Exam

You may bring to the examination:» One reference book or textbook of your choice;» the notes handed out during the course;» any notes you have made

Please make sure you come next week – goal is to» Ensure you understand the topics that may come up» Answer your questions about technical side of the module

Books» Module handouts are intended to cover the content, but:» If you have Coope or Willis, so much the better» John O’Gorman Operating Systems (Macmillan)

covers File I/O and memory well

BS2911 Week 11 18

Business Final Year Projects Kick-off session – 1215 Wed 19 May at West Downs

» What is a Business FYP? » When should I do anything toward my FYP?

Needn’t prepare for this meeting – Just come along for: » Advice how to pick a topic and get started» Guidance on research and referencing» Booklets and pointers to helpful web-sites

Make sure you» Attend scheduled FYP tutorial sessions next semester» Start building the headings so you know what to chase up

– As you develop more ideas, insert sub-headings– Put throw-away text after sub-headings to remind you

what you were planning to do

BS2911 Week 11 19

Eric’s two-pennyworth

Build a skeleton of your document as soon as you can; even if you don’t use use skeleton.doc » Get the headings right and you’ll know what to chase up» As you develop more ideas, insert sub-headings» Put throw-away text after sub-headings to remind you

what you were planning to do

Table of Contents is a good display of the structure» It requires no effort if you use proper heading styles» I find it just as useful as outlining tools, and easier

Keep a “scrapped” file for stuff you decide to remove» If you change focus as you develop, you may want it again» Also, it’ll make you less scared of being critical of your work