Data link layer

The Data link layer is the protocol layer which handles the moving of data in and out across a physical link in a network. This layer accepts packets from the Network layer and packages the information into data unit in the same time they are the frames which are presented to the physical layer for transmission.

Here in data link layer protocols respond to service requests from the Network Layer and they perform their function by issuing service requests to the Physical Layer

There are two sub layers in the data link layer such as,

• Logical link control sub layer(LLC): It is the upper sub layer of the Data Link Layer which concerned with managing traffic over the physical medium and provides multiplexing and flow control mechanisms that make it possible for several network protocols to coexist within a multipoint network and to be transported over the same network media
  
• Media Access Control sub layer(MAC):This is the sub layer that determines who is allowed to access the media at any one time. Here there are forms of media access control such as distributed and centralized. However both of these may be compared to communication between people.
  

Data link layer includes,

• Devices
  

--Bridges                     --Switches

• Protocol
  

--Ethernet                    --EAPS(Ethernet Automatic Protection Switching) 

--MPLS (Multiprotocol Label Switching)     --PPP (Point to Point Protocol)

--CDP(Cisco Discovery Protocol)         --CAN(Controller Area Network )

--FDDI(Fiber Distributed Data Interface )    --Frame Relay

--HDLC(High-Level Data Link Control )    --IEEE 802.2 (provides LLC functions to IEEE 802 --MAC layers)

--IEEE 802.11 wireless LAN        --LAPD(Link Access Procedures, D channel)--Spanning tree protocol

--Token ring                    --UDLD(Unidirectional Link Detection )

 

Functions and Servicers

 

The functions and servicers that can identify in this layer as follows,

• Provides error detection capability
  
• Acknowledged Connectionless service and connection-oriented service
  
• Framing, link access, synchronization
  
• Reliable delivery between two physically connected devices
  
• Ethernet incorporates both this layer and the Physical layer
  
  
• Error Detection & correction
  
• signals sender for retransmission or drops frame