Post on 19-Dec-2015
transcript
Introduction to key concepts
Physical media Analogue & Digital transmission Multiplexing Circuit & Packet Switching Bandwidth & Latency
It will be at a broad principle level. We need to get these definitions clear before we can proceed. Some might find it simplistic.
Wide Area Network
Local Area Network
Home ISP
ISP & Telco (IBP)
The last kilometre
Business
WiFiNetwork
Media (electromagnetic spectrum)
PowerVoice
RadioInfrared
lightUV
lightX, Gamma
rays
Visible light
0 3khz 300ghz
Copper cable
Wireless 802.11 & 802.162-60ghz range
Optic fibre
Twisted pair
Colour coded pairs of insulated copper wires twisted around each other
One carries the signal, the other is grounded and absorbs interference
Electrical noise is a big issue – the tighter the twist the more resistant the cable is to noise
Lots of variations STP shielded twisted pair UTP unshielded twisted pair Quality of copper, number of twists, length of segment, devices
on the line
Limit of 90 metre lengths
UTP Categories
CAT 3 used for Ethernet segments to 10mbps,
CAT 5 – higher grade copper, more twists, used for fast Ethernet up to 100mbps – 4 wire pairs
CAT 5E and CAT 6 – current standards, up to 1000mbps
Fibre optic cable
Consists of One or more glass fibre at the core Clad by a glass layer that acts as a mirror Layer of plastic Braiding of Kevlar Plastic jacket
Data is transmitted by a pulsating light generated by a laser or LED
Fibre characteristics Currently used as a LAN backbone or in the WAN for
high capacity links It is marginally more expensive – starting to replace
copper to the desktop Basically unlimited bandwidth – up to 1gbps at
present Cannot be easily tapped Does not need repeaters or amplifiers Does not transmit in both directions –2 strands NIC and Hubs more expensive
Wireless
Big range of complex products being introduced here
We will look at Mobile telephones Packet Switched Data Networks Point to point links Wireless LANs - 802.11 - WiFi Last kilometer – 802.16 - WiMax Satellites
Mobile Network Generations 1G – Analogue – phased out in Oz 2G – Digital
GSM in Europe & Asia – other technologies in the US Data to 9.6kbps, SMS messages to 160 characters
2.5G - Digital Higher data rates at lower intro cost than 3G, GPRS (General Packet Radio Service) can go up to
115kbps, Oz intro first GPRS network at 24kbps to go to 48kbps
Mobile Network Generations (cont.) 3G –
Expected to offer data rates up to 2mbps Frequencies sold recently by auction in Europe &
Oz Three competing protocols (CDMA) each
backwards compatible with the 3 main 2G technologies
4G Some small implementations have been made Expected to go up to 10mbps Maybe data only
Other wireless products Packet Switched Data Networks
Used in the US and some other areas at low data rates
One new US carrier expects to offer up to 128kbps
Point to point links Usually line of sight from rooftops Micro wave links have been in use for many
years Laser links now offer up to 1gbps for 2km
Other wireless products (cont.)
Last kilometer – give access to local exchanges – may be very significant to countries without wired infrastructure 802.16 WiMax
Wireless LANs – allow a LAN to operate within a building without wires up to 11mbps WiFi
Wireless Personal Area Networks – Bluetooth – allow devices to be connected within 10 metres without wires
Satellites GEO – Geo-synchronous Earth Orbit MEO – Medium Earth Orbit LEO – Low Earth Orbit In use downstream in OZ now Main problem is latency
250 mille-seconds for GEO 10 to 100 mille-seconds for LEO
Analogue Transmission Until recently telephone networks were analogue Sound is carried along the wire in sine wave
form Put simply, there are three attributes, the height
and length of the wave, known as amplitude, frequency and phase.
Loudness varies the amplitude and pitch varies the frequency
Data can be carried by varying – Amplitude, Frequency or Phase
PhaseShiftKeying
PSK
ASK – susceptible to noise degradationFSK – has band width limitationsPSK – can transmit multiple bits per cycle
Digital Transmission Digital signals do not use the sign wave It turns the electrical signal on and off.
“On” representing 1 and “Off” Zero Usually represented as a square wave
form Not as clear as On or Off because of
noise and voltage variation
- Problem of synchronisation if a long row of zeros or ones sent- This is one of a number of solutions – Return to Zero
Movement to Digital Telephone converting to digital ISDN – Integrated Services Digital Network
available for some years at 64kbps ADSL – Asymmetric Digital Subscriber Line Asymmetric means that downstream is
faster than upstream, mirroring the usual Internet pattern
ADSL Downstream up to 1.5mbps Upstream up to 256kbps Telephone on the same line “Always on” Internet connection Availability & bandwidth dependent on
Quality of copper line Length of segment Existence of devices on the line to the
exchange
Multiplexing Transmission capacity of the media is often
much greater than the needs of any one user
This capacity can be shared by allowing simultaneous transmission of multiple signals on a single data link
This technique is known as multiplexing There is one device to combine the signals
– a mux and one to separate them again – a demux – at each end of the link
Approaches There are a range of approaches
FDM – Frequency Division Multiplexing divides the frequency into sub frequencies – used in ADSL to divide channel into Up and Down stream and Telephone channels
TDM – Divides the time into fixed sized frames and then into slots. A connection is then allocated a slot within the frame. Gives a fixed bandwidth whether used or not
Multiplexing (Cont.) Statistical Multiplexing allocates capacity to
those tasks that have data to transmit WDM – Wave Length Modulation is used in
fibre-optic cable, which allows multiple signals with different wavelengths to be transmitted simultaneously
CDMA – Code Division Multiple Access is used in mobile phone networks. Allocates a code to each bit and sends them at a higher rate – called a chip rate. See Kurose page 436 for details.
Serial & Parallel In Serial transmission the bits follow
each other down the one path In parallel multiple paths of wires are
used to transmit all the bits in a byte at the same time or in parallel
Parallel is now largely limited to printer cables and they are now being replaced by the USB (Universal Serial Bus)