Telephone network: service types

1
Telephone network service types

• Analog
• Dialup
• Voice-grade leased lines
• Point-to-point vs. multipoint
• Digital
• Narrowband services
• CSU/DSU rather than modem e.g., digital data
  service (DDS) subrate (DS-0)
• Broadband service
• e.g., T-1, T-3 (T-2, T-4 used by carriers)
• Dialup
• e.g., ISDN, switched 56K, xDSL

2
Telephony - Digital Transmission of Voice

• Why would we do this?
• Can transmit over digital high bandwidth medium
• Digital signal concerned with content of message
• This improves clarity (not susceptible to noise)
• Can be bad, signal is either there or not. No In
  between
• Can easily integrate with the internet and
  telematics
• CIT - computer integrated telephony accepts the
  digital form of the caller phone number and
  queries it in a computer bringing up information
  about the caller and even a picture instantly
• Can use domain such as Internet as a telephone
  network
• IP Telephony allows data, voice, and video to be
  transmitted over a single network infrastructure.

3
How do we do this?

• First the voice must be digitized using a codec
• Most often the voice is then compressed before
  transmission
• ADPCM (Adaptive Differential Pulse Code
  Modulation) transmits the difference between the
  samples frequencies or amplitudes rather then the
  absolute position of the sample
• The coded voice data is put in a packet and sent
  though a network using a protocol
• The recipient decodes the packet and the data is
  translated into an analog sound wave
• Happens simultaneously with both parties so full
  duplex communication is established

4
Digital Telephony

• Also known as Voice Over IP - VoIP
• You have used it without even knowing it
• The transmission of telephone calls over data
  networks like the internet
• Most phone companies do this for phone to phone
  calls. (T-2, T-4 carrier lines for trunks)
• Does not use traditional circuit switching
  Examplehttp//www.howstuffworks.com/ip-telephony1
  .htm

5
Digital Telephony to Telephone

• In a typical phone conversation, much transmitted
  data is wasted. While you are talking, the other
  party is listening, which means that only half of
  the connection is in use at any given time.
• Based on that, we can surmise that we could cut
  the file in half, down to about 4.7 MB. Plus, a
  significant amount of the time in most
  conversations is dead air -- for seconds at a
  time, neither party is talking. If we could
  remove these silent intervals, the file would be
  even smaller.
• Instead of circuit switching, uses packet
  switching

6
Packet Switching

• The sending computer chops data into these small
  packets, with an address on each one telling the
  network where to send them. (The Internet usually
  uses 8 bit packets corresponding with ASCII
  values)
• When the receiving computer gets the packets, it
  reassembles them into the original data
• This reduces the load on the network
  significantly
• Compression techniques further decrease load
• Packet Switching Example http//www.howstuffworks
  .com/ip-telephony2.htm

7
4 main types of VoIP - 1

• Computer-to-computer - This is certainly the
  easiest way to use VoIP.
• You don't even have to pay for long-distance
  calls. There are several companies offering free
  or very low-cost software that you can use for
  this type of VoIP.
• All you need is the software, a microphone,
  speakers, a sound card and an internet
  connection, preferably a fast one like you would
  get through a cable or DSL.
• Except for your normal monthly ISP fee, there is
  usually no charge for computer-to-computer calls,
  no matter the distance.

8
4 main types of VoIP - 2

• Computer-to-telephone - This method allows you to
  call anyone (who has a phone) from your computer
• Like computer-to-computer calling, it requires a
  software client. The software is typically free,
  but the calls may have a small per-minute charge
• Charge is dictated by the phone network providers
  for using their network. Internet transmission is
  free
• In early development all calls were free

9
4 main types of VoIP - 3

• Telephone-to-computer - A few companies are
  providing special numbers or calling cards that
  allow a standard telephone user to initiate a
  call to a computer user.
• The caveat is that the computer user must have
  the vendor's software installed and running on
  his or her computer.
• The good news is that the cost of the call is
  normally much cheaper than a traditional
  long-distance call.

10
4 main types of VoIP - 4

• Telephone-to-telephone - Through the use of
  gateways, you can connect directly with any other
  standard telephone in the world.
• To use the discounted services offered by several
  companies, you must call in to one of their
  gateways. Then, you enter the number you wish to
  call, and they connect you through their IP-based
  network.
• The downside is that you have to call a special
  number first.
• The upside is that the rates are typically much
  lower than standard long distance.

11
IP Telephony advantages

• Network will never be down if using TCP/IP
  protocol and a path exists.
• Can use TDM to stack many conversations
  simultaneously with no interference or noise.
• Can send packet through the Internet and talk
  with people all over the world for free without
  sound quality degradation. (PC to PC)
• Can control telephone networks privately via LAN
  or WAN. (PBX - Private Branch Exchange)
• Can be used interchangeably with existing
  telephone networks. (PC to Phone)

12
IP Telephony disadvantages

• During high traffic times on public networks such
  as the Internet, bandwidth may be limited slowing
  down transmission.
• If modem or low bandwidth connection used, the
  digitization, compression, transmission time adds
  a noticeable delay to the sending and receiving
  of the message. (main reason it is not replacing
  the traditional circuit switching systems)

13
Text Messaging

• Often referred to as SMS, Short Message Service.
• Same concept pagers use.
• Uses digital ASCII values and transmits them via
  the internet and/or telephone networks.
• Phone receives digital frequencies via radio
  waves and assigns Ascii values to the binary.
• Telus Mobility PCS phones with messaging service
  turned on can receive messages from any digital
  source tied into the internet and/or a telephone
  network.
• http//www.telusmobility.com/sendamessage/sendames
  sage.shtml
• Can send short text to any phone anywhere in the
  world via Internet free of charge.
• Global SMS directory http//www.textmefree.com/
• SMS Czech Republic Eurotel Network
  http//www2.eurotel.cz/sms/index.html?n_langenc_
  S1RGNHNoOFdXTUF5SQPFh8BaDaCFkAAG69jbQ