User to Network Signaling – This dictates how end user communicates to PSTN.  A user connects to the PSTN through analog, ISDN or T1 carrier.  The most common method of signaling used in this kind of communication is Dual Tone Multi-frequency, it is also known as in-band signaling because of tones carried through voice path.  When we pick up phone handset and press digits the tone passes from phone to the central office switch to which you are connected and tells the switch what number you intend to call.  ISDN uses out-of-band signaling; in this method signaling is transported on a channel separate from the voice.  The channel carrying voice is refereed as bearer and channel carrying signal is known as data channel.  Some of the benefits of out of band signaling are outlined below:

  • It is multiplexed into a common channel
  • Glare is reduced
  • A lower post dialing delay
  • Higher bandwidth is realized
  • Call completion is greatly increased because setup messages are not subject to noise as in case of DTMF tones

Network to Network Signaling – This dictates how switches in the PSTN intercommunicate.  It is normally carried across the transmission media over T1/E1 carrier over twisted pair, T3/E3 , T4 carrier over coaxial cable, T3, T4 carrier over a microwave link, Synchronous optical network across fiber media.  Network to network signaling comprises of multi-frequency and Robbed bit signaling.  MF is similar to DTMF but uses a different set of frequencies.  MF tones are sent in-band and MF signals from switch to switch.

  • Network to network signaling also uses out of band signaling knows as signaling system 7 (SS7).  SS7 is a method of sending messages between switches for basic call control and for CLASS.  These CLASS services depend on the end office switches and the SS7 network. Some of the benefits of SS7 are outlined below:
  • Reduced post dialing delay as there is no need to transmit DTMF tone on each hop of the PSTN.
  • Increased call completion as SS7 is out of band packet based signaling protocol and single packets having all information transmitted faster than tones generated one at a time across in in-band networks.
  • Connection to IN provide connection to new applications and services in a transparent manner across multiple vendors switching equipment as well as capable of creating new services and applications in a quick manner.

Figure 1 illustrates how call flow happens in case of PSTN networks.


Figure 1: PSTN call flow

 ccna voice pstn and internet

Internet calling is new and evolving technology, which has revolutionized the way voice communication happening over the IP networks.  This technology made it possible to place domestic and international calls from a personal computer to any telephone in the world.  Using a sound-equipped PC, you can place a call through one of the growing number of call providers, commonly referred to as Internet Service Telephony Providers (ITSP).

The fundamental technology for Internet telephony is dramatically different from traditional telephone calling on the public switched telephone network (PSTN).

The traditional telephone networks are circuit switched whereas internet is a packet switched network.  In packet switched network, the data / voice packets have their own address and the network routes each packet to destination address.  In internet telephony the recorded voice is transmitted in a real time.

Internet telephone networks are digital in nature and speaker voice is digitized and send as packets to the destination. These packets are transmitted at the rate of 30-50 packets per/second, creating a sound which appears to be continuous in nature.  There is a minimal delay of about 1/10 of a second between the time word is spoken and when it is heard at other end / destination.

In case of Internet telephony networks common infrastructure tools are no longer needed such as physical ports for services like voice mail.  In a circuit switched packet networks, voice mail is sold based on the number of mailboxes and the number of physical ports needed to support multiple users simultaneously.  In internet telephony networks physical circuit switched ports are optional, the voice mail server need to have only an IP connection.

Internet telephone enables voice mail systems to be put on standard based platforms like PCs and UNIX machines.  With a voice mail server same as email server we can choose to download voice mail over a telephone or use email client to peruse voice mail.

PSTN networks are circuit switched networks, which work on analog signaling; in circuit switching telephone switches establish dedicated connections from end to end for the entire duration of the voice call.

Some of the underlying advantages of Internet telephony over PSTN networks are described in the following paragraphs.

Though circuit switching is very reliable and guarantees a good quality of sound, packet switched networks are offering similar services at a lower cost.  The higher quality of sound is achieved in a more efficient manner in case of packet switched networks.  Same DS0 channel or circuit handling one traditional telephone call could have potential to handle 10 packet switched telephone calls.  Also it makes better use of bandwidth and equipment available to support packet switched networks are cheaper and flexible.  One more limitation of circuit switching is that all of the intelligence of routing and additional features resides in switches and not at the end / edge devices so it is difficult to introduce new features into a PSTN network.  This limitation is becoming an advantage for packet switched networks as they provide more flexible environment to add new features and functionality on customer demands.  Internet telephony has introduced a growing set of features such as caller ID, video, Conferencing calling, chalkboards, encryption and document sharing and so on.

Internet telephony networks offer larger cost savings along with flexibility to call anywhere.  One of the major drivers of combing voice and data network is money savings.  Price savings may vary from country to country.  For enterprise networks consolidation of voice and data networks would mean fewer circuits from the PSTN.  In IP infrastructure we can use features such as DHCP, which gives a flexibility to move phone wherever you need and still retain the same number.

In PSTN networks, circuit switching call centers enable users to work from home and take calls but the equipment is quite expensive.  With packet switching, call centers users can log in to a phone, irrespective of where they are and have access to same features as if they are at their desk at a much lower costs.

Internet telephony networks offer value added services also.  We will learn about them in the section below:

ICW or internet call waiting enables subscribers to receive notification of an incoming voice call on their PCs while connected to their ISP. Subscribers are notified of the incoming call through a screen pop up and there are options available to:

  • Send call to voice mail.
  • Receive the call on the system using H.323 software.
  • Drop the internet session and receive the call on telephone.
  • Ignore the call (busy or ringing).

V2L is a special service which enable internet users to place and receive calls through their ISP only when they are connected through their internet connection (modem, cable, digital subscriber line, and so on).  With V2L ITSPs effectively have a local loop to their customer’s through their modem access and offer long distance services through the internet.

This concludes the lesson of the CCNA Voice topic covering the differences between traditional PSTN networks and packet switched internet networks.  We have covered the basics of PSTN and how internet telephony is offering more flexible and cost effective services over PSTN networks.