Mayank Desai's CCNA Notes

OSI Layer 2 Protocols

CSMA /CD - Carrier Sense Multiple Access / Collision Detection local area network
A broadcast network - all frames that cross a segment can be heard by all machines on the segment
All machines have equal access, they wait for a quiet time to transmit, collisions occur when two machines transmit at the same time. When a collision occurs both the machines apply a back off time that prevents them from colliding again
Services both the Physical and Data Link layers

Ethernet frame (read right to left)
FSC	Data	Type	Source	Destination	SOF	Preamble
Frame Check Sequence	Data to be handed to the upper layer protocol	Defines which upper layer protocol will receive the packet	MAC Address	MAC Address	Start of Frame	Beginning of the frame
4 bytes	46-1500 bytes	2 bytes	6 bytes	6 bytes	1 byte	7 bytes

IEEE 802.3

802.3 frame (read right to left)
FSC	Data	Length	Source	Destination	SOF	Preamble
Frame Check Sequence	Contains 802.2 header and the encapsulated data to be passed to an upper layer protocol that is defined in data field	Defines how many bytes the 802.2 header and data field is	MAC Address	MAC Address	Used to synchronize the frame reception portions of all the machines on the segment	Beginning of the frame
4 bytes	46-1500 bytes	2 bytes	6 bytes	6 bytes	1 byte	7 bytes

Specifies baseband signaling, token passing, and data rates
Equal time is allotted to each station. achieved by passing a token around the ring

Tokens

A small frame, a station cannot transmit data unless in possession of the token
The Token is passed around the ring, each station waits for the token, if the station doesn't have anything to transmit, it passes the token along. If it does have something to transmit, it change the frame to a start of frame identifier and appends data to the frame. When the token is changed, no other station on the ring can use the token to transmit data. This eliminates collisions. The altered frame is passed along the ring until it reaches the destination address. The destination station copies the data and the original frame continues around the token until it reaches the source station. At the source, the token is removed and a new token is sent out onto the network. This way each station has an equal opportunity to transmit data.
Fault Recovery - an Active Monitor is designated by the workstations on the ring
Active Monitor - responsible for token monitoring, token generation when token disappears, purging recycled frames, and other ring maintenance

Token Frame (read right to left)
End Delimiter	Access control	Start Delimiter
Indicates end of frame	Defines frame type	Indicate start of frame
1 byte	1 byte	1 byte

Data / Control Frame (read right to left)
Frame Status	End Delimiter	FCS	Data	Source	Destination	Frame Control	Access Control	Start Delimiter
Can relay 2 statuses Frame copied or address recognized	Indicates end of frame	Error checking, used in CRC calculations	All data that is to be encapsulated. length is determined by how long the station can hold a token	Packet's address	Packet's address	Info Regarding Frame Control	Assigns Priority to the frame, distinguishes frame type	Alerts each station that the frame is Data/Control Frame
1 byte	1 byte	1 byte	varies	6 bytes	6 bytes	1 byte	1 byte	1 byte

Specifications

Physical Layer Medium Independent (PMD) - defines physical transmission characteristics - light levels, allowable bit errors, decibel loss, actual physical equipment
Physical Layer Protocol (PHY) - encodes and decodes data, before and after transmission over the physical medium
MAC - bridges FDDI with layer 2 of OSI model, responsible for how FDDI runs as layer 2 technology, the physical device, and the way data is presented to the physical layer
Station Management (SM) - responsible for station and ring configuration, fault recovery and other management duties

PPP

Used to transfer data over serial point-to-point links, accomplishes this by using a layer 2 serial encapsulation HDLC ( High Level Data Link Compression)
Uses LCP (Link Control Protocol) to manage the serial connection
Network Control Protocols are used to allow PPP to use other protocols from layer 3, thus enables PPP to assign IP addresses dynamically

PPP frame (read right to left)
FCS	Data	Protocol	Control	Address	Flag
Used for CRC calculations	Information handled by the upper layer protocols	Indicates the encapsulated data's protocol	Request a transmission of user data	Holds a broadcast address	Specifies beginning of frame
1 byte	varies	2 bytes	1 byte	1 byte	1 byte