Digital Signals and Digital Encoding Digital Encoding unipolar

Digital Signals and Digital Encoding

Digital Encoding • unipolar digital signal – high = +v – low = 0 • bipolar digital signal – high = +v – low = -v • time period – duration of signal • NRZ – non return to zero

Manchester • Manchester encoding – the transitions & direction of matter – up(1) – down (0) • Differential Manchester encoding – the presence (1) – or absence (0) of transition matters

Synchronizing • synchronous transmission – reference signal used for timing • asynchronous transmission – no reference signal is used – special pattern of bits used • indicate beginning/end of frames • bit stuffing – what if the special pattern is part of data? – Insert a bit to avoid pattern, ignore bit

Data Packaging and Transmission • Segments / packets hold chunks of the intended message. • Info is added to aid re-assembly • Why segment?

Parity Checking • extra bit is added to ensure significant bits are (odd/even) –Odd Parity –Even Parity

Cyclic Redundancy Checks (CRC) • CRC codes offer better parity check analysis

Segmentation and Encapsulation • Segment, frame, data-gram, protocol data-unit (PDU) packet –blocks of data

Connection-Oriented and Connectionless Communications • Connection oriented communications – connection established then data sent – connection may "reserve" channel • Connectionless communications – data is sent without establishing a connection

Circuit Switching / Packet Switching • Circuit Switching – permanent connection established between two points for communication • Packet Switching – no permanent connection established between two points for communication – each packet switched independently • virtual circuit

Routing • Packet Routing - each packet routed independently • Message Routing - all packets of a communique routed the same

Data Codes • ASCII – originally a 7 bit code • BCD/EBCDIC – IBM Mainframe - COBOL • Unicode – 16 bit extended ASCII includes Cherokee, Mongolian, Hebrew, Latin, Klingon, etc. . . • HTML – Color Codes - Hexadecimal

Protocol Frame Structures • UDP • Ethernet

UDP Frame Structure • User Datagram Protocol 4 bit header and data • Simple header – – source port – destination port – Length – Checksum • 16 bit 1's Complement

Ethernet Frame Structure • evolving • Xerox 1970 developed first iteration • IEEE 1985 802. 3 CSMA/CD • Ethernet SNAP and Ethernet II

Common Structure • • • Preamble 64 bits Destination Address 48 bits Source Address 48 bits Type/Length 16 bits Data 368 - 12000 bits (1500 B) • Frame Check Sequence 32 bits