Lower OSI Layer Facts

Lower OSI Layer Facts  

The following table summarizes basic characteristics of the lower OSI model layers.

Layer		Description
Physical		The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. It describes how digital data (bits) are converted to electric pulses, radio waves, or pulses of lights. Devices that operate at the physical layer send and receive a stream of bits.
Data Link	Media Access Control (MAC)	The Media Access Control (MAC) layer defines specifications for controlling access to the media. The MAC sublayer is responsible for: Adding frame start and stop information to the packet Adding Cyclical Redundancy Check (CRC) for error checking Converting frames into bits to be sent across the network Identifying network devices and network topologies in preparation for media transmission Defining an address (such as the MAC address) for each physical device on the network Controlling access to the transmission medium
	Logical Link Control (LLC)	The Logical Link Control (LLC) layer provides an interface between the MAC layer and upper-layer protocols. LLC protocols are defined by the IEEE 802.2 committee. The LLC sublayer is responsible for: Maintaining orderly delivery of frames through sequencing Controlling the flow or rate of transmissions using the following: Acknowledgements Buffering Windowing Ensuring error-free reception of messages by retransmitting Converting data into an acceptable form for the upper layers Removing framing information from the packet and forwarding the message to the Network layer Provide a way for upper layers of the OSI model to use any MAC layer protocol Defining Service Access Points (SAPs) by tracking and managing different protocols
Network		The Network layer describes how data is routed across networks and on to the destination. Network layer functions include: Maintaining addresses of neighboring routers. Maintaining a list of known networks. Determining the next network point to which data should be sent. Routers use a routing protocol to take into account various factors such as the number of hops in the path, link speed, and link reliability to select the optimal path for data. Packets forwarded from the Transport to the Network layer become datagrams and network-specific (routing) information is added. Network layer protocols then ensure that the data arrives at the intended destinations.
Transport		The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other. Upper layers format and process data without regard for delivery Lower layers prepare the data for delivery by fragmenting and attaching transport required information Transport layer uses the following: Port (or socket) numbers are used to identify distinct applications running on the same system. This allows each host to provide multiple services. The Transport layer receives large packets of information from higher layers and breaks them into smaller packets called segments. Segmentation is necessary to enable the data to meet network size and format restrictions. The receiving Transport layer uses packet sequence numbers to reassemble segments into the original message. Connection-oriented protocols perform error detection and correction and identify lost packets for retransmission. A connection-oriented protocol is a good choice where: Reliable, error-free communications are more important than speed Larger chunks of data are being sent Connectionless services assume an existing link between devices and allow transmission without extensive session establishment. Connectionless communications use no error checking, session establishment, or acknowledgements. Connectionless protocols allow quick, efficient communication at the risk of data errors and packet loss. Connectionless protocols are a good choice where: Speed is important Smaller chunks of data are being sent

 

 

 

 

Credit: Testout 640-802 CCNA Notes