Voice Basics

Question 0

What is an E1?

Answer 0

Circuit built from 30 separate 64kbps channels.

Used in areas outside the US, Canada, and Japan.

Question 1

What is a T1?

Answer 1

Circuit built from 24 separate 64kbps channels.

1544 kbps data speed.

Question 2

What is CAS?

Answer 2

Digital trunk signaling where information is transmitted using the same bandwidth (channels) as voice.
Signaling uses the 8th bit of every 6th frame/sample, for each DS0 channel.
Also known as Robbed Bit Signaling (RBS).

Question 3

What is a DS0?

Answer 3

Single channel capable of transmitting 64kbps.

Single voice channel.

Question 4

What is CCS?

Answer 4

Digital signaling where information is transmitted using a separate, dedicated signaling channel.
T1 and E1 have one less channel available for voice.
Q.931 is the most common signaling protocol, it is used in ISDN circuits.
T1: 24th time slot used for signaling.
E1: 17th time slot does signaling.

Question 5

What timeslot carries signaling in CCS for T1/E1?

Answer 5

T1: 24th channel.
E1: 17th channel.

Question 6

What is Q.931?

Answer 6

Common-channel signaling protocol used in ISDN circuits. Most common CCS signaling protocol.

Question 7

What is SS7?

Answer 7

Signaling protocol used throughout the PSTN.
Responsible for routing calls, providing informational signaling, communicating call setup, billing info, etc.
CCS/out-of-band signaling.

Question 8

What is the ITU?

Answer 8

International Telephony Union. Governing body responsible for creating many of the standards of the PSTN, and voice in general.

Question 9

What is E.164?

Answer 9

International numbering plan created by the ITU.
Contains a country code, national destination code, and subscriber number.
Can be no longer than 15 digits.
Ex:1-858-555-5555

Question 10

What is G.729?

Answer 10

Codec using only 8kbps.
2 variants:
-a: lower MOS score, but more processor efficient.
-b: supports Voice Activity Detection (VAD).

Question 11

What is G.711?

Answer 11

Codec using 64kbps, with a MOS of 4.1.
“Common ground” codec that would be used between different vendor devices.
U-law: used in US/Japan, 1’s and 0’s are flipped from a-law.
A-law: used everywhere else. First bit is 1 for positive, 0 for negative.

Question 12

What is RTP?

Answer 12

Real-time Transport Protocol, used with UDP for transport of voice packets in VoIP.
Adds sequence number and time stamp, as well as type field specifying voice or video.
Devices choose even number port between 16384 and 32767.
One-way stream.

Question 13

What is RTCP?

Answer 13

Real-time Transport Control Protocol.
Protocol for statistics reporting of a call.
Uses odd UDP port number between 16385 and 32767, one port number of corresponding RTP stream.
Includes packet loss, packet delay, packet count, jitter, etc.

Question 14

What is ICCS?

Answer 14

Intra-Cluster Communication Signaling.
TCP sessions formed between CUCM servers in a cluster to exchange real-time database information.
Uses ports 8002-8004.

Question 15

What is a CUCM cluster?

Answer 15

Contains a single publisher and up to eight subscribers.
Changes are made on the publisher and pushed to the subscriber.
Subscribers do all the heavy lifting.
Publisher owns the IBM Informix SQL database, and normally services TFTP requests as well.

Question 16

What is the CME GUI?

Answer 16

Loaded into flash from a TAR file.
HTTP/Java based configuration management for CME.
Not pretty, but functional.
Just for CME, not for general router features.

Question 17

What is Cisco Configuration Professional?

Answer 17

GUI for administering CME.
Installed on local PC.
Configures many aspects of a router, including UC, FW/IPS, LAN/WAN, VPN.

Question 18

What does “max-phones” do?

Answer 18

Configures the maximum number of IP phones that the CME will support. Affects the amount of memory used by the router. This is a required configuration.

Question 19

What does “max-dn” do?

Answer 19

Configures the maximum number of directory numbers the CME will support. Directly affects the memory reserved by the router. This is a required configuration.

Question 20

What is an ephone-dn?

Answer 20

The directory number that can be assigned to one or more phones.

Question 21

What is the difference between a single-line ephone-dn and a dual-line ephone-dn?

Answer 21

Single-line only supports one call per DNS. If the line is in use, then the caller will receive a busy signal.

Dual-line can handle multiple calls, and is useful for call waiting, transfers, conference calls, etc.

Question 22

How do you assign a physical IP phone to an ephone-dn?

Answer 22

You set the MAC address of the phone in the ephone configuration mode.
ephone
mac-address

Question 23

What does the button command do?

Answer 23

It associates an ephone-dn with a button on an ephone. The first # is the button number on the IP phone. The second number is the ephone-dn. The separator in between the two numbers decides how the phone rings.

Question 24

What does “show voice port summary” do?

Answer 24

It shows the available analog voice ports on the router (FXS/FXO).