Traditional Telephony Concepts

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Cisco Voice Technologies, now typically referred to as Unified Communications Technologies, not only depend on a solid network foundation, but also use conceptual building blocks from the world of traditional telephony. The reasons for this are:

  • Capability to interconnect the Unified Communications Environment to the PSTN
  • Creating “new world” equivalent logical functions that mimic “old world” methodologies

Understanding the basic elements utilized in older telephony can help in knowing how these same functions are created in Unified Communications networks. Here we will map out the basics of an analog telephone call.

Calling Party

When an individual wishes to have a voice conversation with someone at another physical location, he/she typically will initiate the conversation using an analog telephone. This device receives power from the RJ-11 wall jack, and the initiating caller (the “calling” party) picks up the handset (“off hook”), receiving a dial tone indicating the network is ready. The next step involves entering a series of digits, which indicates where the intended party is located. In the United States, the values and format of the number typically follows ten digits and is referred to as the North American Numbering Plan (NANP). These digits are transmitted to the telephone company Central Office, where the analog line terminates.

PSTN Local Central Office

When the calling party goes off- hook, the circuit between the local residence and the Central Office is energized, and a dial tone is sent to the user. Once all the digits are entered by the end-user, specialized devices in the Central Office (often referred to simply as switches) analyze the string of numbers, and perform a lookup to determine the location of the “called party” (the intended recipient of the phone call). If the called party is connected to the same Central Office, then a ring tone is sent to that phone. It is more likely, however, that the called party is served by a remote Central Office.

PSTN Remote Central Office

The PSTN is composed of an entire network of circuits, Central Offices, and subscribers/ customers, throughout the world. In reality, many services providers/telephone companies exist independently of one another, but for the sake of simplicity, this example assumes both parties are connected to the same provider. When the calling and called parties are remote from one another, specialized connections called trunks are used to pass traffic in all Central Offices along the call path. To clarify the term a bit, a typical analog phone line (typically called POTS, or Plain Old Telephone Service) carries only one voice conversation at a time. Trunks, on the other hand, carry multiple conversations simultaneously, from Central Office to Central Office, for the life of the circuit used for a particular call. The local office sends specific signaling messages along the path, and when the called party is reached, it sends a ring tone to the subscriber.

Called Party

At this point, the intended recipient of the voice conversation hears an audible ring tone on his/her telephone, and picks up the handset. In the not so distant past, the entire circuit would have been tied up even for this “call setup” phase; in later years, all of the process would just use signaling messages, and only when the called party picked up would the circuit path be established. Once the conversation is completed, either side can hang up (“on hook”) and the circuit path is terminated.

Signaling Traffic

While the example presented above is rather simplistic, in reality a number of background processes work to make it all possible; this is referred to as signaling. Three basic types of signaling exist.

  • Supervisory: Indicates the state of the telephony device (such as on/off hook)
  • Address: Digits dialed, such as those of the called party (such as digit tones)
  • Informational: Indicates the status of the call itself (such as dial tone, ringback, busy signal, etc.)

Bearer Traffic

Signaling traffic creates, maintains, and terminates the voice conversation, but is just a tool to serve that call, which goes by the term bearer traffic. Bearer traffic is the voice conversation itself and, in reality, the entire point of voice conversations. Keep in mind that this traffic is created using electrical impulses in circuit-switched calls and with data packets in the world of Unified Communications.

Reproduced from Global Knowledge White Paper: Getting Started with Cisco Unified Communications Technologies.

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VoIP Protocol Essentials: SIP
CIPT1 – Implementing Cisco Unified Communications IP Telephony Part 1 v8.0
CVOICE – Implementing Cisco Unified Communications Voice over IP and QoS v8.0

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  1. Andrew Reply

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