• network is 2 or more linked computers
• telecommunication is long-distance electronic communication of any sort
• LAN (Local Area Network) network confined to an organization
• MAN (Metropolitan Area Network) network available throughout a small region
• WAN (Wide Area Network) network available "anywhere"

Channels

• A channel is a data transmission pathway
• let's explore different aspects of establishing and maintaining channels
• pumping data
• connections
• interconnections
• media
• software

Pumping data: digital, analog, bandwidth

A channel can use either Digital or Analog signaling

• Digital is two values, 0 or 1
• Analog is continuum of values

Bandwidth is the rate at which channel can transmit data

• for data transmission, usually expressed in bps: bits per second
• Bandwidth is baud rate multiplied by channel width
• Baud Rate is a measure of how fast bits can be sent through the channel sequentially.
• Channel Width is the number of bits that can be simultaneously transmitted, and can be increased to N by:
  • using N wires or
  • defining 2^N distinct signals or
  • using multiple frequencies (for wireless) or
  • some combination of the above

Getting Connected to the Network

• There are numerous options for getting a physical network connection
• See http://www.high-speed-internet-access-guide.com for an overview of high-speed options
• We will first look at wired connections
• Then we look at wireless connections

Wired Connections to Network

Dialup Modem:

• Uses telephone network.
• Requires only a cheap circuit board for your computer universally supplied with PCs now
• Requires analog-to-digital (download) and digital-to-analog (upload) conversions.
• Explain difference between analog and digital here
• Limited to 56 Kbps (kilo bits per second) download (less than that for upload)
• Download means from ISP to you
• Limit is because of limited BANDWIDTH of a single telephone “line” about 3000 Hz.
• Uses telephone voice channel.  It dials number to your ISP and ties up the line.

• Uses telephone network, an example of broadband.
• Requires special equipment at telephone switching stations and special modem usually external to your PC and connected to it through 10Mbps Ethernet port (looks like wide telephone jack).
• All digital
• Here’s the asymmetric part: You get from 1 to 8 Mbps download (1 to 2 is typical), and less than 1 Mbps upload.
• Download speed depends on how close you are to telephone switching station.  Further away means slower. Normally you have to be within a few miles of the station for it to work.
• Does not use telephone voice channel (but does generate some noise so requires filter on phones); service is always “on”.

• Uses television cable network, an example of broadband.
• Requires special modem usually external to your PC and connected to it through 10Mbps Ethernet port (looks like wide telephone jack).
• Unless you have digital cable service, requires analog-to-digital (download) and digital-to-analog (upload) conversions.
• Download can be very fast, possibly limited by cable company’s connection to Internet.  Typical is 1-2 Mbps, similar to DSL.
• Upload is usually much slower.
• Problem is structure of cable network, which is inherently asymmetric:  it is geared to send signals toward your home, not away from it.
• Another factor is that cable is a "tree" network topology in your neighborhood and so you have to share the cable capacity with your neighbors.
• Like cable TV service (and DSL) , the cable modem service is "always on".

• Normally within a building or campus, an example of broadband.
• Dominant network is Ethernet, a "bus" network topology.
• All digital
• Requires Ethernet card (as do DSL and cable modem).
• This is what dorm room, computer lab, classroom, office connections at Otterbein are.
• Maximum throughput 10 or 100 or 1000 Mbps (mega bits per second).
• Maximum cannot be achieved because channel is shared and messages can collide!
• A less popular LAN technology is Token Ring in which computers are connected in a "ring" and a computer has to have possession of a circulating "token" before it can transmit.

Wireless Connections to Network

Satellite

• Requires outdoor transceiver (transmitter/receiver)
• Download speeds comparable to DSL or Cable
• Somewhat more expensive than DSL or Cable
• Available "everywhere", only limit is line-of-sight to satellite
• Like DSL and Cable, is "always on"
• There is additional lag time, as each signal has to travel over 22,000 miles each way!
• This will add about 0.5 seconds to each Web browser request (2 round trips: 1 for request, 1 for response).

Cell or cell-like wireless

• Sometimes referred to as WWAN (Wireless Wide Area Network)
• Distinguished from Wi-Fi (below), which is wireless access to LAN (Local Area Network)
• Performance comparable to DSL/Cable/Satellite
• Typically available from cell-phone service provider
• Somewhat more expensive than DSL/Cable

Wi-Fi (802.11)

• used in conjunction with one of the previous connections
• in other words, WI-FI does not connect you directly to the ISP, it connects you to a DSL/Cable connection
• has limited range of 100-200 feet.
• commonly used for in-house networks (private)
• commonly used by organizations to provide "hot spots" (private or public)
• full-service wireless networking available from ISPs through satellite or cell phone networks; that's different from Wi-Fi

WiMax

• A cross between cell and wi-fi services
• Similar technology to wi-fi but much wider range
• Being employed for MAN (Metropolitan Area Networks)
• Still in its infancy; costs are high and there are still technical issues to resolve

Inter-connections

In a wired network, it is not feasible to connect every node (station, endpoint) directly to every other node

• in a LAN
  • most LANs use Ethernet, which has a bus configuration.
  • bus network: There is a central highway, and every station has an on-off ramp
  • the illusion of a central highway is implemented with hubs and switches
  • hub: signal-propagating device that several stations (or other hubs) can tap into. Incoming signal from one station is broadcast to all the others
  • switch: connection point like a hub, but is smart enough to redirect incoming signal only to the outgoing channel that connects to its intended destination
  • As on a central highway, collisions can occur on an Ethernet LAN
• in a WAN
• necessarily needs to be hierarchical and heterogeneous
• this is the "last mile" problem
• illustration: suppose you wanted to take a vacation to Rome, Italy -- explain the process of getting to the Coliseum from your house
• LAN connects to another LAN via bridge
• LAN connects to WAN via gateway (aka gateway router)
• gateways connect to routers, switches that interconnect different sections of the WAN
• router determines which line to forward each incoming message to, so it will be delivered properly
• the spine of the WAN is called its backbone, very fast and very high capacity fiber optic cabling
• analogy to your trip to Rome. What is its backbone? How do you get to it?

Media

raw material from which the channel is constructed.

The two main physical media have been

• copper wire (twisted pair, coaxial, etc.), which transmit electrons - electricity
• copper is analog but can carry either analog or digital signal
• fiber optic cable, which transmits photons - light
• fiber optic is digital (light is off-on)

Non-physical media include:

• radio waves (long distance),
• microwaves (long distance, line of sight, used between towers),
• infrared light waves (short distance, line of sight, e.g. remote control)
• these are analog but can carry digital signals

Software

• much of the logic that controls network devices is designed into the hardware itself
• for more flexibility, it makes sense to implement some functions in software
• Examples are network operating systems (NOS) and server software
• At the application (end user) level, everything is software controlled
• applications for asynchronous communication include: email, listserv, newsgroup, wiki, groupware
• applications for synchronous communication include: instant messaging, talk, video conferencing, VoIP
• We'll look at Internet software more closely in the next chapter