Ccna routing and Switching Complete Study Guide

Logical Link Control (LLC)    Responsible for identifying Network layer protocols 

and then encapsulating them. An LLC header tells the Data Link layer what to do 

with a packet once a frame is received. It works like this: a host receives a frame and 

looks in the LLC header to find out where the packet is destined—for instance, the IP 

protocol at the Network layer. The LLC can also provide flow control and sequencing 

of control bits.

The switches and bridges I talked about near the beginning of the chapter both work at 

the Data Link layer and filter the network using hardware (MAC) addresses. I’ll talk about 

these next.

As data is encoded with control information at each layer of the OSI model, 

the data is named with something called a protocol data unit (PDU). At the 

Transport layer, the PDU is called a segment, at the Network layer it’s a 

packet, at the Data Link a frame, and at the Physical layer it’s called bits. 

This method of naming the data at each layer is covered thoroughly in 

Chapter 2.