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OSI Networking Model Layer 2


2 - DATA LINK - Physical addressing (data frames to bits)


Function: Takes care of moving data from one network node to another which are all within the same Local Area Network LAN. Used with Layer 2 switches and can run other protocols such as Siemens SinecHl.


Data unit: Frames and Packets.


Protocols (examples): IEEE 802.2, IEEE 802.3, 802.5 - Token Ring, HDLC, Frame Relay, FDDI, ATM, PPP.


Devices: Bridge, Switch, ISDN Router, Intelligent Hub, NIC, Advanced Cable Tester


Topics & Methods Explained

- DATA LINK - MAC (Media Access Control)


Logical Topology

Every signal is received by all devices.


Each device only receives signals that have been specifically sent to it.

Media access

Contention:   (802.2 & 802.3)
Protocols: CSMA/CD (Carrier Sense, Multiple Access/Collision Detection) Sense the cable prior to transmission. Detects collisions & initiates retransmission. CSMA/CA (Collision Avoidance) Time-Sliced accessing or request to send


Token passing:  (802.4 & 802.5)
A small data frame (token) is passed from 1 device to another. A token is a special message that temporarily gives media control to the device holding the token. Passing the token around distributes access control among the devices.


One device (called a controller, primary, or master) queries each of the other devices (secondaries) to see whether they have information to transmit. Ideal for networking time-sensitive devices, such as automation equipment.


Physical device:
The Data Link layer is only concerned with physical device addresses (MAC addresses). Physical device addresses are unique hardware addresses typically assigned by hardware vendors.

- DATA LINK - LLC (Logical Link Control)


Rely upon the transmitting and receiving devices to maintain their own internal clocks. The two devices use similar timing but do not synchronize their clocks. Each frame is sent separately, using the asynchronous bit sync. method.


Require that the communicating devices take responsibility for providing a transmission (or framing) clock. This can be accomplished by -Using a separate channel for the clock or -Sending special bit string or 1 or more control char.


Use a constant fixed-frequency transmission clock to create set time slots. A clock signal is generated by a designated network device and is passed to all devices on the network. Data is inserted in free space within the time slots.

Connection services

LLC-level flow control:
Regulate how much data can be transmitted within a specified time. Allows slow devices to turn down faster ones. Window Flow Control: static or dynamic buffer (window) size, Guaranteed Rate Flow Control.


Error control:
Notification of lost or scrambled frames. 1-When the intended receiver fails to receive a packet: If sender fails to receive an ACK. 2-When checksums do not mach: If the calculated checksum do not match the one passed by sender.



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