CODECS and OVERHEAD
Codec:
-a device or program capable of performing encoding or decoding on some digital data stream or signal.
-transform voip media streams into another format: A to D; D to D; or D to A.
-especially important on low-speed serial links where bandwidth is very important.
Codecs supported by the Cisco IOS GWs:
-G.711
for encoding tel audio on 64-kbps channel
it is a PCM scheme operating at 8KHz sample rate, with 8 bits per sample
widely used in telecoms ind as it improves the signal-to-noise ratio without increasing the amount of data.
has two subsets:
1. mu-law
used in Nth Ame and Jap phone ntwks.
2. a-law
used in Europe and elsewhere around the world.
both subsets use compressed speech carried in 8-bit samples. Use 8KHz sampling rate with 64kbps of storage.
-G.722
wideband speech codec.
provides 7KHz of wideband audio at data rates from 48kbps to 64kbps.
tech is based on adaptive differential PCM (ADPCM).
G.722.1 - lower bit-rate compressions
G.722.2 (Adaptive Multi-Rate Wideband)- offers even more lower bit-rate compressions
-G.726
is an ITU-T ADPCM which ops at data rates of 40, 32, 24, 16 kbps.
-G.728
16kbps low-delay CELP (LD-CELP)
-G.729
uses conjugate-structure algebraic-CELP (CS-ACELP)
-G.723
describes dual-rate coder for multimedia communications for compressing speech or audio signal components at very low-bit rate as part of the H.324 family of standards.
two bit rates assoc with it:
r63: 8.3 kbps using 24 byte frames and Multipulse LPC with Maximum Likelihood Quantization (MPC-MLQ)
r53: 5.3 kbps using 20 byte frame and the ACELP algorithm.
-GSM FR
frame size of 20 ms and ops at 13 kbps bit rate.
is a Regular Pulse Excited-Linear Predictive (RPE-LTP) coder.
network must support GSM FR codecs in order to write VoiceXML scripts.
-iLBC [Internet Low Bit Rate Codec]
designed for narrowband speech
results in a payload bit rate of 13.33kbps for 30 ms frame and 15.20 kbps for 20 ms frames.
algorithm is based on block-independent linear predictive coding with the choice of data frame lengths of 20 ms and 30 ms.
There is a need to balance the need for voice quality against the cost of BW when choosing codecs. The higher the codec BW, the higher the cost of each call across the ntwk.
VOICE SAMPLE:
-voice sample size is a variable that can affect total BW used.
-voice sample is defined as the total output from a codec DSP that is encapsulated into a Protocol Data Unit (PDU)
Table of various codecs, their sample sizes and the number of pkts reqd for voip to xmit 1 second of audio:
The larger the sample size, the larger the packet and the fewer the encapsulated samples that have to be sent (wc reduces BW).
FORMULA:
ENCAPSULATED BYTES CALCULATION:
Bytes_per_Sample = (Sample_Size * Codec_Bandwidth) /8
Other factors to bear in mind when calculating overhead of voip call:
1. Layer 2 and security protocols add to pkt size significantly.
Layer 2 overhead for various protocols:
-Ethernet II protocol
carries 18 bytes of overhead
6 for source MAC
6 for dest MAC
2 for type
4 for CRC
-PPP
carries 6 bytes of overhead
1 flag byte to indicate beginning and end of a frame
1 address byte
1 control byte
1 protocol byte
2 bytes for CRC
-FRF.12
carries 6 bytes of overhead
2 bytes for DLCI header
2 for FRF.12
2 for CRC
-Multilink PPP
carries 6 bytes of overhead
1 for flag
1 for address
2 for control or type
2 for CRC
2. The IP and transport layers also contribute to overhead
IP adds a 20 byte header
UDP adds 8 byte header
RTP adds a 12 byte header
3. Security overhead
IPSec adds 50 to 57 bytes of overhead when u r using VPN.
L2TP or GRE adds 24 bytes of overhead.
if in use, MLP will add 6 bytes
MPLS adds 4 byte label to every pkt
CALCULATING TOTAL BW 4 A VOIP CALL:
Points to consider before calculating:
-if more bw is reqd for the codec, then more total bw is reqd.
-if more overhead is assoc with the data link, the more total bw is needed.
-if there is a larger sample size, then less total bw is reqd.
-if cRTP is being used then the total bw reqd is reduced significantly.
As a ntwk engineer:
-you need to calc the total bw for each voip call
-this info can then be used to calculate the total bw for the company's WAN links
FORMULA for TOTAL BW per CALL:
TBW = total packet size * packets per second (pps)
-total packet size in bytes = (Layer 2 header: MPPP, FRF.12, or Ethernet) + (IP/UDP/RTP header)+(voice payload size)
-pps = codec bit rate/voice payload size
Protocol header assumptions used for the calcs:
-40 bytes for IP(20)/UDP(8)/RTP(12) headers
-cRTP reduces IP/UDP/RTP to 2 or 4 bytes (cRTP not available over Ethernet)
-6 bytes for MPPP, or FRF.12 L2 header
-1 byte for the end-of-frame flag on MP or Frame Relay frames
-18 bytes for Ethernet L2 headers (including 4 bytes for FCS or CRC)
Example calc:
G.729 codec (8 kbps) with a 20 byte sample size and using FRF.12 without cRTP
total packet size = 6 bytes (FRF.12) + 40 bytes (IP/UDP/RTP) + 20 bytes (voice payload size) = 66 bytes
total packet size (bits) = 66 bytes * 8 bits per byte = 528 bits
PPS = 8 kbps/160 bits (20 bytes * 8 bits) = 50 p/s i.e 8000/160
BW per call = 528 bits/s * 50 p/s = 26, 400 bps = 26.4kbps.
Example calc:
G.729 codec (8 kbps) with a 20 byte sample size and using FRF.12 with cRTP
total packet size = 6 bytes + 2 bytes + 20 bytes = 28 bytes * 8 bits = 224 bits
pps = 8 kbps/160 bits (20 bytes * 8 bits)= 50 p/s
total BW per call = 224 bits/s * 50 p/s = 11200 bps = 11.2 kbps
VAD provides a max 35% BW savings. VAD should however not be taken into account for the purpose of ntwrk design and bw engineering. Features such as music on hold (MOH) and fax render VAD ineffective. VAD reduces silence on voip conversations but also provides comfort noise generation (CNG).
DSP FUNCTIONALITIES:
media resource - sw-based or hw-based entity that performs media processing functions on the data streams to wc it is connected.
-transcoding: conversion from one codec to another.
processed by DSPs on a DSP farm - sessions are initiated and managed by CUCM which refers to transcoders as hw MTPs.
-voice termination: the digitization and packetization of an analog signal on a TDM interface.
-conference bridge: a resource that joins multiple parties into a single call.
hardware conference bridges are used in two environs:
central site
remote site
-MTP: an entity that accepts two full-duplex voice streams using the same codec.
can be used for:
repacketization - transcode a-law to mu-law and vice versa
H.323 supplementary services
two types of MTPs:
1. sw MTP
2. hw MTP
CODEC COMPLEXITY
This refers to the amount of processing required to perform voice compression
Two categories:
medium complexity
high complexity
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