Data Transmission: digital/analog
We distinguish between digital and analog
- Digital transmission sends data as pulses (as inside the computer).
- Analog transmission sends data as waves with amplitudes and frequencies.
Example: ordinary calls on phone lines.
DT can be synchronous or asynchronous
- asynchronous transmission has low speed. It sends start signal at
beginning of "message" and stop signal at its end.
- synchronous transmission has high speed and is a contionuous data
stream. Have clocks of both computers synchronized at the beginning.
DT: simplex, half-duplex, duplex
simplex transmission: only one-way.
half-duplex: only one-way at a time.
(full) duplex: two-way transmission.
What: Communication media
The bandwidth measures the capacity of the link.
|wire pair (twisted pair)
||already in use (telephone lines)
||low bandwidth, suspectible to noise
||two conductors, twisted around
||higher bandwidth, more noise-resistant
||center conductor, isolation, other conductor, isolation
||no metal, high bandwidth, no noise, very secure.
||glass or plastic fibers, transmission by light,
|radio waves (like WiFi, Bluetooth, ...)
|microwave transmission (special radio waves)
||line-of-sight, small distance
||microwave transmission to and from satellite
In principle, all these media can be used for digital or analog
transmission. fiber optic cables are more suited for digital transmission, the
others have originally been used for analog transmission.
How phone lines are used:
Integrated Services Digital Networks. Digital transmission. Bandwidth of about
128 Kbit/sec. Uses twistes pairs.
Digital subscriber line. Uses ordinary copper telephone lines, by exploiting
(high) frequencies that are unused for phoning. High bandwidth , you are always online,
and have a dedicated line. But the service is not available everywhere (in rural areas)
and works fastest if you are close to the provider.
- ADSL (asymmetric DSL):
bandwidth for download much higher than for upload.
about 10 megabit/sec.
- SDSL (symmetric DSL): here you cannot phone
at the same time.
- VDSL (very high bit-rate DSL):
up to 52 megabits/sec, but only for short distances (< 1000m).
See an ad
on DSL, it contains even more
information about DSL
uses the coaxial cable (cable TV). very high bandwidth, about 10 megabit/sec.
Is always on (dedicated line). However you share the bandwidth with your
neighbors. Not all cable lines are suitable, they must be duplex.
See an ad
on Cable Internet, another
one with FAQs
Modulation is the conversion from digital to analog signals.
The converse is called demodulation For both you need a Modulatordemodulator.
- external modems, usually 56Kbit/sec.
- internal modems
- PC-cards (PCMCIA cards): modems used for notebooks.
- DSL modems (not really modems, since digital to digital)
- cable modems use the coaxial TV cable (not really modems, digital-digital)
- cellular modems: use the cellular phone system.
Software installed on all computers of the network to allow communication.
Nodes are linked by some medium and sent short messages.
Each message has an address (and also the source address). Nodes open
messages only if they are addressed to them. Examples:
- TCP/IP: Transmission Control Protocol/Internet Protocol, used in
- ethernet : transmission method CSMA/CD: Before a node sends
a message, it "listens" to the medium whether someone else is talking. A collision
occurs when two nodes start to talk at about the same time. Then both nodes
wait a random time before trying again, thereby avoiding collision. Works
only for local networks. Ethernet works at 10 or 100 Mbps and is suited for
bus or star topologies.
- token ring: A token circles around, loading and uploading messages.
This protocol works for ring or star topologies.
- WiFi (802.11), standards for a wireless LAN. They use ethernet technology,
meaning that WiFi can be combined with ethernet networks.
- star network: one central node, the "hub", all other nodes connected
- ring network: nodes in a ring.
- bus network: all nodes are attached to one single line (bus).
Wide area network. links are temporarily (switched or dial-up service) or permanently
(dedicated service) connected. The mainframe computer controlling the net is
called host computer. Between this and the WAN is the front-end processor.
Local area network.
- Ethernet networks
- Powerline cable networks use the existing power lines.
- Phoneline networks use the existing phone lines in the house.
- Wireless home networks, using radio waves, either HomeRF (up to 50 m)or
WiFi (up to 500 meters). You need a wireless network card.
Essentially ethernet or token ring protocols used.
Another distinction for networks
- Client/Server Networks: One computer, the server, controls
the network. The other computers are the clients. Usually server does
some preprocessing for the client.
- Peer-to-peer networks: No single computer in control. Usually used
for small networks of up to 10 computers. Each computer contains the network
operating system and the application programs. Internet peer-to-peer (P2P)
networks are used for file sharing over the internet.
Some more network devices
A bridge connects different similar LANs.
A router connects several computers and routers (like a hub), but also
transmits data to the correct destination. Most contain built-in firewalls.
An IP switch directs traffic if several networks are connected.
A gateway lets a node communicate to a dissimilar network, and translates