Outline the characteristics of different transmission media.


Teaching Note:

Characteristics include: speed, reliability, cost and security.

Transmission media include: metal conductor, fibre optic, wireless.


Sample Question:

From Sample Paper 1 2014:


JSR Notes:

First, a great video about how fiber optics works.

  Use Speed Reliability Cost Security
Analog telephone cables (copper metal) For home Internet access, via ISDN service and MODEM - necessary because existing, traditional telephone lines carry analog signal which is converted at both ends to digital signal. Also referred to as "dial-up" 56 Kbps- 144 Kbps. Potential issues with signal conversion from digital to analog and back by the Modems. Already existing, so no additional cost for cable. But MODEM needed, pay per minute, and cannot be used for both calls and network access at same time. Can be easily intercepted.
Digital telephone cables (metal) For home Internet access via DSL service (Digital Subscriber Line) and cable MODEM. Uses dedicated digital telephone lines.


256 Kbps to 100 Mbps.

More reliable, since a dedicated digital line. More expensive than dial up because using dedicated lines, but that means you are not necessarily paying per minute. Better than analog if dedicated lines.
Cable (metal) For home Internet access using existing TV cable lines and cable modem. Typically 10 to 100 or more Mbps. Residential shared bandwidth and bundled services are not as reliable as dedicated lines and services. Already existing, so no additional costs for cables, and cost of Internet access often bundled with TV++ delivery. Not great if shared lines through a residential community.
Ethernet cable (metal) Used in LANS, and only good up to 100 metres. Depending on category of cable (CAT5, CAT6 etc.) Ethernet card of computer and server. But usually 100 Mbps, or 1 Gbps, though can be up to 10 Gbps or more. Mature technology, quite reliable, though only up to 100 metres. Sometimes physical issues with adapters fitting well in plugs. A lot cheaper than fibre optic, but not already existing, like cable or telephone cables. Signals can be intercepted without user knowing. Security really depends more on other features of the LAN such as authentication and encryption.
Fibre optic (glass) Mainly for the backbone of LANS, but increasingly for the whole network itself. 1 Gbps up to 50 Gbps. Reliable. Expensive - $70 USD per meter. Much harder for signals to be intercepted without user's knowledge of tampering. Though fiber tapping does exist.
Wireless For places where people use mobile devices - which is just about everywhere today.

3G: up to 200 Kbps

4G: up to 20 Mbps

Increasingly reliable, with technological advances from 2G to 3G to 4G, but still no where near as reliable as wired networks. Increasingly inexpensive with improvements in technology and access Fairly insecure since signals going through the open air, though security getting better with advances in technology.

Do note that the speed of network activity is measures in bits per second, not bytes per second. So 50 Gbps is 50 Gigabits per second. This is tradition since most often data is transferred serially, one bit at a time. Whereas when referring to storage amounts, we use Byte, so MegaBytes, GigaBytes and so on.

For additional information, read through page 77 of the Networks handout. On the St Julian's site, "twisted pair" and "coaxial" cables are also mentioned. Both telephone cables and Ethernet cables are twisted pair. Coaxial cables are much older technology used in older networks.

Jose: https://www.youtube.com/watch?v=0TZwiUwZwIE - wonderful video about laying a trans-Atlantic internet cable.