Redes

INTRODUCCION
La capa de enlace de datos es responsable de la transferencia fiable de información a través de un circuito de transmisión de datos.
El nivel de enlace (del inglés data link level) es el segundo nivel del modelo OSI. Recibe peticiones del nivel de red y utiliza los servicios del nivel físico.
El objetivo del nivel de enlace es conseguir que la información fluya, libre de errores,entre dos máquinas que estén conectadas directamente (servicio orientado a conexión).
Para lograr este objetivo tiene que montar bloques de información (llamados tramas en este nivel), dotarles de una dirección de nivel de enlace, gestionar la detección o corrección de errores, y ocuparse del control de flujo entre equipos (para evitar que un equipo más rápido desborde a uno más lento).
Cuando elmedio de comunicación está compartido entre más de dos equipos es necesario arbitrar el uso del mismo. Esta tarea se realiza en el subnivel de acceso al medio.
TRAMAS
En la capa de enlace, los datos se organizan en unidades llamadas tramas. Cada trama tiene una cabecera que incluye una dirección e información de control y una cola que se usa para la detección de errores.
La cabecera de una tramade red de área local (LAN) contiene las direcciones físicas del origen y el destino de la LAN. La cabecera de una trama que se transmite por una red de área extensa (WAN) contiene un identificador de circuito en su campo de dirección.
Recuerde que un enlace es una red de área local, una línea punto a punto o alguna otra facilidad de área extensa por la que se pueden comunicar los sistemasmediante un protocolo de la capa de enlace de datos.
FUNCIONES
La capa de enlace de datos es responsable de la transferencia fiable de información a través de un Circuito eléctrico de transmisión de datos. La transmisión de datos lo realiza mediante tramas que son las unidades de información con sentido lógico para el intercambio de datos en la capa de enlace. También hay que tener en cuenta que en elmodelo TCP/IP se corresponde a la segunda capa
Sus principales funciones son:
1. Iniciación, terminación e identificación.
2. Segmentación y bloqueo.
3. Sincronización de octeto y carácter.
4. Delimitación de trama y transparencia.
5. Control de errores.
6. Control de flujo.
7. Recuperación de fallos.
8. Gestión y coordinación de la comunicación.
INICIACION,TERMINACION E IDENTIFICACION
La función de iniciación comprende los procesos necesarios para activar el enlace e implica el intercambio de tramas de control con el fin de establecer la disponibilidad de las estaciones para transmitir y recibir información.
Las funciones de terminación son de liberar los recursos ocupados hasta la recepción/envío de la última trama. También de usar tramas de control. Laidentificación es para saber a que terminal se debe de enviar una trama o para conocer quien envía la trama. Se lleva a cabo mediante la dirección del nivel de enlace.
SEGMENTACION DE BLOQUEO
La segmentación surge por la longitud de las tramas ya que si es muy extensa, se debe de realizar tramas más pequeñas con la información de esa trama excesivamente larga.
Si estas tramas son excesivamentecortas, se ha de implementar unas técnicas de bloque que mejoran la eficiencia y que consiste en concatenar varios mensajes cortos de nivel superior en una única trama de nivel de enlace más larga.
SINCRONIZACION DE OCTETO Y CARÁCTER
En las transferencias de información en el nivel de enlace es necesario identificar los bits y saber que posición les corresponde en cada carácter u octeto dentro deuna serie de bits recibidos.
Esta función de sincronización comprende los procesos necesarios para adquirir, mantener y recuperar la sincronización de carácter u octeto. Es decir, poner en fase los mecanismos de codificación del emisor con los mecanismos de decodificación del receptor.
DELIMITACION DE TRAMA
La capa de enlace debe ocuparse de la delimitación y sincronización de la trama. Para la…

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