Data Transmission

Here’s a quick look at moving bits.

Unit Goals

To gain exposure to the different kinds of media that can transmit data and their relative speeds, and to understand some inherent differences between local and long-distance communication.

Physical Transmission Media

Recall that nodes are devices such as computers, printers, routers, terminals, mobile devices, copiers, vending machines, speakers, and household appliances. The data is carried between node via connections such as wires, glass fiber, radio, infrared, microwave, staellite, laser. What are some characteristics of the different transmission media?

Wire

Copper is the nearly universal choice for a wire since it is a good conductor, but there is a potential for interference, which is usually solved either by using twisted pairs or coax.

wire.gif

Exercise: Read an article about coax. How does it differ from twisted pair? Consider whether either or both is bidirection and can support multiple channels.

Glass fiber

You can also use media not subject to interference.

opticalfiber.gif

Radio

Radio is an unguided media: no physical wire, no physical connections.

Microwave

Also unguided. Differs from radio in that:

Infrared

I’m not too familiar with these. I’ve heard:

Satellite

Similar to RF. Transponders used to amplify signal. A single satellite will often have multiple transponders and share channels too. Up to around 45Mbps tops. Has noticeable end-to-end delay (270ms). Kinds of satellites:

Laser

Pretty similar, feature-wise, to microwave communication.

Speed

What’s the maximum (stated) rate that bits can actually flow through certain media? (There is a table at WhatIs.com that I grabbed some of this information from.)

Medium Max bit rate
Analog Modems 56 Kbps
ISDN Phone Line 128 Kbps
AppleTalk 230.4 Kbps
Satellite 400 Kbps
Frame Relay 1.544 Mbps
T1 1.544 Mbps
IBM Token Ring 4 Mbps, 16Mbps
DSL Phone Line 6 Mbps
(Plain) Ethernet 10 Mbps
T3 44.736 Mbps
OC-1 51.84 Mbps
Cable Modem 52 Mbps
Fast Ethernet 100 Mbps
OC3/STM1 155.52 Mbps
OC12 622.08 Mbps
Gigabit Ethernet 1 Gbps
OC48/STM16 2.488 Gbps
OC192/STM64 10 Gbps
OC256 13.271 Gbps
Exercise: You will sometimes see max data rates listed for different “categories” of ethernet cables. What are the max transmission speeds for Cat 5 and Cat 6 cables? There is a qualifier on the Cat 6 maximum speed. What is it?

How useful is this data anyway?

The speed of a line doesn’t mean you actually get data at this rate. For example a line may be shared, like in a Cable Modem so you may only be getting, say, 1 MB/sec to your modem. Then the data has to go to your network card where it is processed and shoved to your memory, which may be slow! Also the actual data you are downloading is only part of what is traveling though the net, as you are getting a lot of packets with a lot of header and checksum information, that must be stripped and processed, taking even more time. And some setup are such that upstream data rates might be much slower.

Local vs. Long Distance Communication

Some technologies are way better for local communication and some are better for long distance. Local communication:

Long distance communication, on the other hand:

Exercise: Why do we say long-distance communication requires multiplexing?

Summary

We’ve covered:

  • Various ways that data can be transmitted
  • Theoretical maximum throughput for different technologies
  • The difference between local and long distance communication