OSI Flashcards
MAC
Media Access Control
Sub layer that controls how a computer on the network gains access to the data and permission to transmit it.
What’s session layer responsible for?
Initiating, maintaining and terminating a session, supervising dialogue, facilitating recovery after failure, monitoring costs of services used.
(Set up, authorization, and authentication)
Where does IP address get attached? And what is this layer?
Network layer
Provides switching and routing technologies, creating logical paths known as virtual circuits used to transmit from node to node.
Routing and forwarding, addressing, internetworking, error handling, congestion control, packet sequencing.
TCP
Transmission Control Protocol
Occurs at transport level (4)
20 bytes + options
Connection-oriented (established connection before sending data),
reliable/error checking,
flow and congestion control.
IP
Internet Protocol
Network level (3)
Used now is IPv6 which contains: Version, traffic class, flow label (used for priority queueing), payload length, next header, hop limit, source and seat addresses
UDP
User Datagram Protocol
Transport Layer (4)
8 bytes long header + the data which is a max of 64k bytes
Connectionless
Unreliable
Minimal overhead (faster)
Contains checksum
Data Link Control
Data Link level (2)
Transaction delivers a given Protocol Data Units PDU from primary to secondary stations. In order to control flow of information a backward transmission path is required in addition to the forward path (duplex). The actual paths are in the physical layer but the Data Link controls these.
HDLC High-level Data Link Control
Most basic form of Data Link (2) protocols.
Bit-oriented, not fixed (variable) in length and doesn’t have to be multiples of bytes.
Bit-synchronous.
Control field is one of these Frames: Information (sent when data needs to be transmitted. contains data payload and associated flow control parameters),
Supervisory (sent in response to I-frames. control information to set up link and execute ARQ error control, and flow control if no returning frames present),
Unnumbered (sent as needed for management purposes).
Contains Frame Check Sequence FCS field for error control
S-Frame:
Receive Ready RR
Receive not Ready RNR
Reject REJ
Selective Reject SREJ
Bit stuffing
Used when sending frames with variable lengths. The start symbol that HDLC uses is 01111110.
To avoid this symbol appearing naturally in data and falsely stopping the frame, but stuffing occurs as a 0 is added after any 5 ones and it is ignored later by the receiver.
Transmission of TCP, IP and HDLC
HDLC (2) can carry payloads that contain IP (3) and IP and carry payloads that contain TCP (4)
nth HDLC packet has Flag, Address, and Control (FDC) before the data, and checkSum Flag after the data.
-> FAC data SF
Parity check
Example of error detection and correction.
LSB added to either make the number represented by data bits even or odd and should be consistent for all data.
An odd number of errors is detectable by this method but even numbers of errors are not.
Cyclic Redundancy Check (CRC)
Does a mathematical operation with the bits and uses the remainder as the check.
The data is comsidered to represent a binary number M. M is divided by G which is a fixed known binary number with length c. Remainder is the check.
