CH 1 Traditional vs Unified Voice REVIEW Flashcards
1
Q
Analog connections
A
- Analog phone lines use the properties of electricity to convey changes in voice over ca-
bling. - Signaling includes messages such as dial tone, dialed digits, busy signals.
- Each analog circuit is composed of a pair of wires. One wire is the ground, or positive
side of the connection (often called the tip). The other wire is the battery, or negative side
of the connection (often called the ring).
2
Q
Loop Start Signaling
A
- When the phone is lifted off hook, the phone con-
nects the two wires (tip and ring), causing an electrical signal (48V DC voltage) to flow from the phone
company central office (CO) into the phone. - Home Environments use loop start signaling.
3
Q
Glare Issue
A
- Glare occurs when you pick up the
phone to make an outgoing call at the same time as a call comes in on the phone line be-
fore the phone has a chance to ring. - Not so much a big issue for Home Environments but causes issue in Business Environments.
4
Q
Ground Start Signaling
A
- Because of glare issue, Business Enviroments use Ground Start Signaling.
- This type of signaling in PBX system allows the PBX to separate an answering phone from an incoming phone line, reducing the problem of glare.
- To receive a dial tone from CO, the PBX must send a ground signal on the wires; this intentionally signals to the CO tht an outgoing call is going to happen.
5
Q
Two issues with Analog
A
- Problem: Analog Signals problem of signal fading over long distances.
- Solution: Phone Company installed Repeater devices to regenerate the signals as it became weak.
- Side Effect: Unfortunately, as the analog signal was regenerated, the repeater device was unable to differentiate between the voice traveling over the wire and the line noises.
- Problem: With analog connections, the sheer number of wires the phone company had to run to support a large geographical area. With a large number of phones and each phone requiring two wires, the bundles of wire became massive and difficult to maintain.
* Solution: Digital Signals were developed to send multiple calls over a single wire.
6
Q
Digital Signals
A
- Digital signals use numbers to represent levels of voice instead of a combination of electrical signals.
- Digitizing voice – the process of changing analog voice signals into a series of numbers that you can use to put the voice back together at the other end of the line. (each number sent represents a sound that someone made while speaking into a telephone)
- Today’s network devices can easily transmit a numeric value any distance a cable can run without any degradation or line noise.
7
Q
Time-Division Multiplexing
(TDM)
A
- Digital voice uses a technology known as Time-Division Multiplexing (TDM).
- TDM allows voice networks to carry multiple conversations at the same time over a single, 4-wire path.
- The multiple conversations have been digitized (the multiple conversations have been digitized, the numeric values are transmitted in specific time slots that differentiate the separate conversations), the numeric values are transmitted in specific time slots that differentiate the separate conversations.
8
Q
Issue with Digital Signaling
A
- Problem: Phone companies generate informational and address signals through specific frequencies of electricity. By solving the problems associated with analog signaling, digital signaling also removed the typical signaling capabilities.
- Solution: Two Primary Styles of Signaling were created:
- CAS, and CCS
9
Q
Channel Associated Signaling (CAS)
A
Signaling information is transmitted using the same bandwidth as the voice.
- Uses Robbbed-bits -“steals” binary bits that would typically have been used to communicate voice information and use them for signaling.
- The 24 channels of the digital T1 circuit carry only voice data for the first five frames that they send. On the sixth frame (marked with an S in Figure), the eighth bit (also called the least significant bit) is stolen for the voice devices to transmit signaling information.
10
Q
Common Channel Signaling (CCS)
A
Signaling information is transmitted using a separate, dedicated signaling channel.
- Dedicates 1 of 24 the DS0 channels from a T1 or E1 link for signaling info. This is called out-of-band signaling because the signaling traffic is sent completely separate from the voice traffic. As a result, a T1 connection using CCS has only 23 usable DS0s for voice.
- T1 lines, the 24th time slot (channel) is always the signaling channel.
- E1 lines, the 17th time slot (channel) is always the signaling channel.
- The most popular protocol used is Q.931, which is the signaling protocol used for ISDN circuits.
11
Q
Q.931
A
- The most popular signaling protocol used for ISDN circuits.
- A signaling protocol used by ISDN CCS implementation - call setup, tear down in CCS.
12
Q
Public Switched Telephone Network (PSTN)
A
- Its primary purpose is to establish worldwide pathways to allow people to easily connect, converse, and disconnect.
- The modern PSTN is now a worldwide network (much like the Internet).
13
Q
Pieces that make up the PSTN
A
- Analog telephone: Able to connect directly to the PSTN and is the most common device on the PSTN. Converts audio into electrical signals.
- Local loop: The link between the customer premises (such as a home or business) and the telecommunications service provider.
- CO switch: Provides services to the devices on the local loop. These services include signaling, digit collection, call routing, setup, and teardown.
- Trunk: Provides a connection between switches. These switches could be CO or private.
- Private switch: Allows a business to operate a “miniature PSTN” inside its company. This provides efficiency and cost savings because each phone in the company does not require a direct connection to the CO switch.
- Digital telephone: Typically connects to a PBX system. Converts audio into binary 1s and 0s, which allows more efficient communication than analog.
14
Q
Why use PBX or Key system in business environments
A
Many businesses have hundreds or even thousands of phones they support in the organization. If the company purchases a direct PSTN connection for each one of these phones, the cost would be astronomical. Instead, most organizations choose to use a PBX or key system internally to manage in-house phones. These systems allow internal users to make phone calls inside the office without using any PSTN resources. Calls to the PSTN forward out the company’s PSTN trunk link.
15
Q
Key System
A
- Geared around Small Business Environments (less than 50 users)
- Typically support fewere features and have a “shared line” - meaning if user1 were to use line 1 of the 4 phone lines, line 1 of 4 would appear busy for all users in that office.
