Tabla resumen de codecs

La comunicación de voz es analógica, mientras que la red de datos es digital. El proceso de convertir ondas analógicas a información digital se hace con un codificador-decodificador (el CODEC). Hay muchas maneras de transformar una señal de voz analógica, todas ellas gobernadas por varios estándares. El proceso de la conversión es complejo. Es suficiente decir que la mayoría de las conversiones se basan en la modulación codificada mediante pulsos (PCM) o variaciones.

Además de la ejecución de la conversión de analógico a digital, el CODEC comprime la secuencia de datos, y proporciona la cancelación del eco. La compresión de la forma de onda representada puede permitir el ahorro del ancho de banda. Esto es especialmente interesante en los enlaces de poca capacidad y permite tener un mayor numero de conexiones de VoIP simultaneamente. Otra manera de ahorrar ancho de banda es el uso de la supresión del silencio, que es el proceso de no enviar los paquetes de la voz entre silencios en conversaciones humanas.
A continuación se muestra una tabla resumen con los códecs más utilizados actualmente:

- El Bit Rate indica la cantidad de información que se manda por segundo.
- El Sampling Rate indica la frecuencia de muestreo de la señal vocal.(cada cuanto se toma una muestra de la señal analógica)
- El Frame size indica cada cuantos milisegundos se envia un paquete con la información sonora.
- El MOS indica la calidad general del códec (valor de 1 a 5)

Para entender mejor la formación de un codec y los parametros expresados en la tabla recomendamos leer el apartadofuncionamiento de un codec donde se puede aprender como funciona detallamente el codec G.711 y que significan en su caso los parametros de la tabla.

Nombre	Estandarizado	Descripción	Bit rate (kb/s)	Sampling rate (kHz)	Frame size (ms)	Observaciones	MOS (Mean Opinion Score)
G.711 *	ITU-T	Pulse code modulation (PCM)	64	8	Muestreada	Tiene dos versiones u-law (US, Japan) y a-law (Europa) para muestrear la señal	4.1
G.711.1 *	ITU-T	Pulse code modulation (PCM)	80-96Kbps	8	Muestreada	Mejora del codec G.711 para abarcar la banda de 50 Hz a 7 Khz. Mas info
G.721	ITU-T	Adaptive differential pulse code modulation (ADPCM)	32	8	Muestreada	Obsoleta. S e ha transformado en la G.726.
G.722	ITU-T	7 kHz audio-coding within 64 kbit/s	64	16	Muestreada	Divide los 16 Khz en dos bandas cada una usando ADPCM
G.722.1	ITU-T	Codificación a 24 y 32 kbit/s para sistemas sin manos con baja perdida de paquetes	24/32	16	20
G.722.2 AMR-WB	ITU-T	Adaptive Multi-Rate Wideband Codec (AMR-WB)	23.85/ 23.05/ 19.85/ 18.25/ 15.85/ 14.25/ 12.65/ 8.85/ 6.6	16	20	Se usa principalmente para compresíon de voz en tecnología movil de tercera generación. Mas info
G.723	ITU-T	Extensión de la norma G.721 a 24 y 40 kbit/s para aplicaciones en circuitos digitales.	24/40	8	Muestreada	Obsoleta por G.726. Es totalmente diferente de G.723.1.
G.723.1	ITU-T	Dual rate speech coder for multimedia communications transmitting at 5.3 and 6.3 kbit/s	5.6/6.3	8	30	Parte de H.324 video conferencing. Codifica la señal usando linear predictive analysis-by-synthesis coding. Para el codificador de high rate utiliza Multipulse Maximum Likelihood Quantization (MP-MLQ) y para el de low-rate usa Algebraic-Code-Excited Linear-Prediction (ACELP).	3.8-3.9
G.726	ITU-T	40, 32, 24, 16 kbit/s adaptive differential pulse code modulation (ADPCM)	16/24/32/40	8	Muestreada	ADPCM; reemplaza a G.721 y G.723.	3.85
G.727	ITU-T	5-, 4-, 3- and 2-bit/sample embedded adaptive differential pulse code modulation (ADPCM)	var.		Muestreada	ADPCM. Relacionada con G.726.
G.728	ITU-T	Coding of speech at 16 kbit/s using low-delay code excited linear prediction	16	8	2.5	CELP.	3.61
G.729 **	ITU-T	Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP)	8	8	10	Bajo retardo (15 ms)	3.92
G.729.1	ITU-T	Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP)	8/12/14/16/ 18/20/22/24/ 26/28/30/32	8	10	Ancho de banda desde 50Hz a 7 Khz Mas info
GSM 06.10	ETSI	Regular­Pulse Excitation Long­Term Predictor (RPE-LTP)	13	8	22.5	Usado por la tecnología celular GSM
LPC10	Gobierno de USA	Linear-predictive codec	2.4	8	22.5	10 coeficientes.La voz suena un poco "robotica"
Speex			8, 16, 32	2.15-24.6 (NB) 4-44.2 (WB)	30 ( NB ) 34 ( WB )
iLBC			8	13.3	30
DoD CELP	American Department of Defense (DoD) Gobierno de USA		4.8		30
EVRC	3GPP2	Enhanced Variable Rate CODEC	9.6/4.8/1.2	8	20	Se usa en redes CDMA
DVI	Interactive Multimedia Association (IMA)	DVI4 uses an adaptive delta pulse code modulation (ADPCM)	32	Variable	Muestreada
L16		Uncompressed audio data samples	128	Variable	Muestreada
SILK	Skype	Uncompressed audio data samples	De 6 a 40 kbit/s	Variable	20	El codec Harmony está basado en SILK

* El codec g711 tiene dos versiones conocidas como alaw (usado en Europa) y ulaw (usado en USA y Japón). U-law se corresponde con el estandar T1 usado en Estados Unidos y A-law con el estandar E1 usado en el resto del mundo. La diferencia es el método que se utiliza para muestrear la señal. La señal no se muestrea de forma lineal sino de forma logaritmica. A-law tiene un mayor rango. Para mas información de las diferencias ver G.711 Ley A vs Ley u

** existen varias versiones del codec g729 que es interesante explicar por su extendido uso
G729: es el códec original 
G729A o anexo A: es una simplificación de G729 y es compatible con G729. Es menos complejo pero tiene algo menos de calidad. 
G729B o anexo B: Es G729 pero con supresion de silencios y no es compatible con las anteriores. 
G729AB: Es g729A con supresión de silencios y sería compatible solo con G729B.
Aparte de esto G729 (todas las versiones) en general tienen un bit rate de 8Kbps pero existen versiones de 6.4 kbps (anexo D) y 11.4 Kbps (anexo E). 

How to configure LACP on our Smart/ Managed Switch?

Introduction

LACP (Link Aggregation Control Protocol) is defined in IEEE802.3ad and enables the dynamic link aggregation and disaggregation by exchanging the LACP packets with its partner. The switch can dynamically group similar ports into a single logical link, which will highly extend the bandwidth and flexibly balance the load.

Application scenario

As is shown in the picture above, we take TL-SG5428 and T3700 as example and we will configure ports 26,27,28 on TL-SG5428 and ports 21,22,2 on T3700 correspondingly as LACP.

Note: For the member ports in an aggregation group, their basic configurations must be the same. The basic configuration includes STP, QOS, GVRP, VLAN, port attributes, MAC Address Learning mode and other associated settings. If you are not sure about these basic configurations, you can export the config file and reset the device and then try it again. We highly suggest you configure the LAG function before configuring any other function for member ports.

Configuration

Step1: login to the management interface of the TL-SG5428, and go to Switch--LAG--LACP, you can designate a System Priority for the TL-SG5428 or leave as default value, The default value is 32768. And then choose the ports 26/27/28, designate an Admin Key 2 for these ports. The ports with the same Admin Key will be aggregated to the same group. Choose the Mode as Active and Status as Enable. And then click Apply to take the setting into effect. The result will show as below:

Note:

1.A lower system priority value indicates a higher system priority. When exchanging information between systems, the system with higher priority determines which link aggregation a port belongs to, and the system with lower priority adds the proper ports to the link aggregation according to the selection of its partner. If the System Priority value is the same, the device with smaller MAC address owns higher priority.

2.There are two mode of the port, Active and Passive. In Active mode, the port can send LACP packets actively while in Passive mode, the port can only send LACP packets after it has received a LACP packet. It is suggested that set one side as Active mode and the other side as Passive mode.

Step2: almost the same settings as the T3700. Login to the management interface of the T3700, and go to Switch--LAG--LACP, you can designate a System Priority for the T3700 or leave as default value, The default value is 32768. And then choose the ports 21/22/23, designate an Admin Key 3 for these ports. Choose the Mode as Passive and Statusas Enable. And then click Apply to take the setting into effect. The configuration will show as below:

Now, the configuration is done, and we can check the result in the LAG Table. Since the lines are aggregated or disaggregated dynamically into the group, we can only see the ports that are added to the group at this moment in the Member.

Configuring multiple LACP groups on a TP-LINK switch

I realize this is a very specific topic and situation, but this solution needs to exist on the internet. In the past I've written about Link Aggregation and what to expect performance-wise and since then I've run into an issue as our network has expanded. We're using a TP-LINK TL-SG2424 switch for our SAN network (budget, I know) but this applies to many TP-LINK switches which offer LACP. 

Here's the setup, we have 2 iSCSI SAN appliances connected to our switch. One of the appliances is connected via 4 network interfaces which are aggregated using LACP 802.3ad with Jumbo Frames enabled. This is our clustered shared volume for our virtual machines. This was the first LAG group that we configured using LACP and it was straight forward: enable LACP on the appliance bonded NICs, then choose the ports on the switch and enable LACP in Active mode:

Cypress North

Cypress North

Once this was completed, the appliance and the switch negotiated the link aggregation and we were off and running. 

A few weeks later we migrated our backup appliance to the SAN. The backup appliance is connected via 2 network interfaces which we also want to enable LACP on. Following the same steps as in our first setup, we bonded the NICs on the appliance and enabled LACP, then we went onto the switch and set the two ports to Enabled and Active.

Cypress North

This is where the trouble began. Upon setting the new port group (11+12) to Active/Enabled in LACP, the switch added the ports to the initial port group under LAG 1. Obviously these are different groups, connections, and appliances so this wreaked havoc for a few minutes until we disabled the new LACP ports.

After banging around in the switch settings, we emailed TP-LINK support to confirm that multiple LAG groups were possible using LACP. They assured us that they were supported but their budget support could not explain to us how to configure it. Likewise, the manual doesn't describe how to enable multiple groups either. Google search failed us as well. 

The solution, it turns out, is a column idiotically labeled "Admin Key". On a whim, we changed the value in this column to "2" when enabling LACP on the two ports. This time, the two ports were placed into a new group LAG 2 and LACP was properly negotiated. 

Cypress North

The Admin Key column is not talked about anywhere in the documentation as having an affect on the link aggregation grouping and I hadn't seen anybody mention it online. While the audience of this how-to is fairly limited, someone, somewhere, someday is going to be saved a massive headache by reading it.