Cvoice "voip Fundamentals"

Front 1

Define ALI

Back 1

Automatic Location Identification

The ALI database contains the records that map telephone numbers to geographic locations.

Front 2

Define ANI

Back 2

Automatic Number Identification

ANI is the calling-party number that is included with each call to identify where it came from.

Front 3

Define PSAP

Back 3

Public Safety Answering Point

The PSAP is the answering point where an emergency call is terminated.

Front 4

Define ERL

Back 4

Emergency Response Location

ERL is the location from which an emergency call is placed.

Front 5

Define ELIN

Back 5

Emergency Location Identification Number
An ELIN is a NANP telephone number used for routing an emerg call to the appropriate PSAP

Front 6

Define MSAG

Back 6

Master Street Address Guide

MSAG is a database maintained by a government agency. Maps geographic locations to PSAP.

Front 7

Define "Selective Router"

Back 7

A selective Router is a dedicated 911 switch in the service provider network that routes 911 calls to the appropriate PSAP based on "calling" (ANI)number. Note: Normal call routing uses the "called" number.

Front 8

Define CAMA

Back 8

Centralized Automated Message Accounting
A CAMA is an analog trunk that connects a customer switch directly to a selective router. It carries 911 traffic only.

Front 9

Define CER

Back 9

Cisco Emergency Responder

CER is an app the automatically tracks and updates moves, add, changes,to equipment in real time to keep track of locations for 911 purposes.

Front 10

G.711 codec uses what coding scheme? What is total bandwidth excluding overhead?

Back 10

PCM (Pulse Code Modulation)
Bandwith of 64kbps

Front 11

G.726 codes uses what coding scheme? What is total bandwidth excluding overhead?

Back 11

ADPCM (Adaptive Pulse Code Modulation)
Bandwith varies with sample size uses 4,3 or 2 bits/sample. Therefore bandwidth is 32Kbps, 24kbps, or 16 kbps

Front 12

G.728 codes uses what coding scheme? What is total bandwidth excluding overhead?

Back 12

LDCELP (Low-Delay Code Excited Linear Prediction)
Bandwidth of 16kbps

Front 13

G.729 and G.729A codes uses what coding scheme? What is total bandwidth excluding overhead?

Back 13

CS-ACELP (Conjugate Structure Algebraic Code Excited Linear Prediction)
Bandwidth of 8kbps

Front 14

G.723 and G.729A codes uses what coding scheme? What is total bandwidth excluding overhead?

Back 14

MPMLQ (Multipulse Maximum Likelihood Quantization)
Bandwidth of 6.3Kbps or 5.3kbps

Front 15

Cisco voice equipment encapsulates ___ worth of audio in each PDU by default regardless of the codec used

Back 15

20ms

Front 16

Cisco uses DSP's that output samples based on digitization of ___ of audio.

Back 16

10ms

Front 17

State the formula used to determine the number of bytes encapsulated in a PDU given the CODEC bandwidth and sample size.

Back 17

Bytes_per_Sample = (Sample_Size * CODEC_Bandwidth) / 8

Ex: 160 bytes per PDU = (.020sec * 64000 bits per sec) / 8 bits per byte

(assuming G.711 codec)

Front 18

VoIP alone without cRTP uses a ___ byte IP/UDP/RTP header

Back 18

40 byte header

Front 19

List the Layer 2 overhead for EthernetII

Back 19

18 bytes of overhead: 6 bytes for source MAC, 6 bytes for destination MAC, 2 bytes for type, and 4 bytes for CRC

Front 20

List the Layer 2 overhead for Multilink Point-to-Point Protocol (MLP)

Back 20

6 bytes of overhead: 1 byte for flag, 1 byte for address, 2 bytes for control (or type), and 2 bytes for CRC

Front 21

List the Layer 2 overhead for Frame Relay Forum Standard 12 (FRF.12)

Back 21

6 bytes of overhead: 2 bytes for data-link connection identifier (DLCI) header, 2 bytes for FRF.12 header, and 2 bytes for CRC

Front 22

Describe overhead associated with IPSec per packet

Back 22

IPSec will typically add 50-57 bytes of overhead per packet

Front 23

Describe overhead associated with Layer 2 Tunneling Protocol / Generic Router Encapsulation (L2TP/GRE) per packet

Back 23

L2TP/GRE will add 24 bytes of overhead per packet

Front 24

Describe overhead associated with Multi Link Protocol (MLP) per packer

Back 24

MLP will add 6 bytes of overhead per packet

Front 25

Describe overhead associated with MPLS per packet

Back 25

MPLS will add a 4 byte label to every packet

Front 26

cRTP changes 40 byte overhead of IP/UDP/RTP header down to what?

Back 26

4 bytes if CRC is used.
2 bytes if no CRC

Front 27

Give the formula to calculate total bandwidth per call

Back 27

Total_Bandwidth = ([Layer_2_Overhead + IP_UDP_RTP Overhead + Sample_Size] / Sample_Size) * CODEC_Speed

Front 28

Define VAD and give average bandwidth savings associated with using it

Back 28

Voice Activity Detection. Save average of 35 percent bandwidth assuming english language and other factors.

Front 29

True or False... VAD is enabled by default for all VoIP calls?

Back 29

True

Front 30

What does GOS(.01) mean?

Back 30

GOS (.01) means that one call is blocked in 100 attempts.

Front 31

Define GOS

Back 31

Grade of Service is a unit that measures the chance that a call will be blocked. Usually defined with traffic is at it peak. (ie: busiest time of the day)

Front 32

State the equation to calculate traffic flow

Back 32

A=C*T (A is the offered traffic, C is the number of call originating in a 1 hour period, T is the average holding time of the call)

Front 33

Define an "Erlang"

Back 33

An Erlang is a nondimensional unit that defines the amount of traffic one truck can handle in one hour.

Front 34

State other measurement units equivalent to 1 Erlang

Back 34

1 Erlang = 60 call minutes = 3600 call seconds = 36 centum call seconds (CCS)

Front 35

List two example to peer-to-peer signaling protocols

Back 35

SIP and H.323

Front 36

List two examples of signaling protocols with client/server architectures

Back 36

MGCP and Megaco/H.248

Front 37

True or False: Codecs only compress the voice stream

Back 37

True

Front 38

List the major VoIP protocols

Back 38

H.323, MGCP, Megaco/H.248, SIP, RTP, RTCP

Front 39

When a voice stream is assigned UDP port numbers, RTP is typicaly assigned an ____ port number and RTCP is assigned the next ___ port number. Each voice call has four ports assigned: RTP plus RTCP in the transmit direction and RTP plus RTCP in the recieve direction.

Back 39

even, odd

Front 40

State the command to enable cRTP compression on an interface.

Back 40

ip rtp header-compression [passive]

Note: Command must be defined on both ends. Use "passive" option on one end only.

Front 41

In a WAN Model with centralized call processing define the role of the "Central Gateway Controller" (Call Agent)

Back 41

The call agent handles the switching logic and call control for all sites under the central controller.

Front 42

Which codec is typically used on WAN links?

Back 42

G.729 (8kbps)

Front 43

List typical features of centralized call processing deployments

Back 43

- MCGP or Megaco/H.248 for call control,
- Cisco Call Manager at central site for managing call control
- Centralized apps (voicemail, etc)
- Up to 30000 phones per cluster
- Call Admission Control (CAC) to limit calls per site
- SRST for remote branches

Front 44

Although not typical you can build centralized deployements with SIP or H.323 networks using what?

Back 44

This is done using back-to-back user agents (B2BUA) or gatekeeper-routed call signaling (GKRCS)