Data link
layer
Main functions of data link layer
are
•Defining the Media Access Control (MAC) or
hardware addresses
•Defining the physical or hardware topology for
connections
•Defining how the network layer protocol is
encapsulated in the data link layer frame
•Providing both connection-less and
connection-oriented services
•Defines hardware (MAC) addresses as well as the
communication process that occurs within a media.
MAC Address
MAC address is a 48 bit long
layer two address. It is also known as hardware address. This address is burnt
with device by manufacturing company.
•The first six hexadecimal digits of a MAC address
represent its manufacture company.
•MAC addresses only need to be unique in a
broadcast domain.
•You can have the same MAC address in different
broadcast domains.
Frame
Data link layer receive packet
from network layer and wrap it with layer two header that is known as frame.
There are two specifications of Ethernet frame.
1. Ethernet II
2.802
Key points to remember:-
o Ethernet II does not have any sub
layers, while IEEE 802.2/3 has two: LLC and MAC.
o Ethernet II has a type field
instead of a length field (used in 802.3).
o 802.2 use a SAP or SNAP field to
differentiate between encapsulated layer-3 payloads.
o With a SNAP frame, the SAP fields
are set to 0xAA and the type field is used to indicate the layer-3 protocol.
o 802.2 SAP frame is eight bits in
length and only the first six bits are used for identifying upper-layer
protocols, which allows up to 64 protocols.
o 802.2 SNAP frame support of up to
65,536 protocols.
