FILES, DATA, AND SIGNALS FILE HAMLET. TXT USER DATA INTERNAL y e a , b REPRESENTATION 01011001 01100101 01100001 00101100 00100000 OF USER DATA f r o m b 01100110 01110010 01101111 01101101 00100000 Internal representation of information stored in n-bit entities corresponding to a particular character code (e.g., ASCII is shown here). Yea, from the table of my memory I’ll wipe away all trivial fond records

FILES, DATA, AND SIGNALS (cont.) 1 0 0 1 1 0 1 0 1 0 1 0 0 1 1 0 1 0 0 0 1 10 0 0 1 10 1 0 0 LSB LSB LSB LSB SIGNAL – External representation of user data -- Mapping of bits to a signal depends only on choice of implementer. NonReturn to Zero-Level (NRZ-L) shown, but other signaling types are possible.

PARALLEL TRANSMISSION , y e a b f r o m Receiver Transmitter MSB LSB CLOCK DTE DTE t0 t1 t2 t3 t4 t5 t6 t7 T8

SKEW t0 t1 1 SKEW: The arrival of bits from the same character at 1 SKEW: The arrival of bits from the same character at different times may lead to erroneous reception.

SERIAL VS. PARALLEL TRANSMISSION CPU-peripheral, internal to computer. Relative short distance DTE – DTE. Longer distances. APPLICATIONS Long distances possible. Easier to implement. Device-independent. Very high throughput. Matches internal architecture. ADVANTAGES Extra cable, connectors, Amplifiers, and Clock add expense. Skew limits distance. Relatively low speeds. Parallel-to-Serial Conversion required. DISADVANTAGE

ASYNCHRONOUS TRANSMISSION STOP BIT y e START BIT 1 1 1 1 1 1 1 1 FRAMING LSB MSB FRAMING Asynchronous transmission: Timing of each bit is specified within a character Inter-character time is non-uniform Each character must be framed

SYNCHRONOUS TRANSMISSION LSB MSB 1 y e a t Characters, within a block, are sent contiguously Each block is framed

ASYNCHRONOUS CHARACTER FORMAT LEAST SIGNIFICANT BIT SPACE MARK START BIT STOP INTERVAL TIME DATA BITS LINE IDLE Transmitter and receiver must agree on: Number of data bits per character Length of a bit time Length of the stop interval Signal level Order of bit transmission Character code

SYNCHRONOUS MESSAGE FORMAT CHAR (s) MESSAGE TRAILER MESSAGE HEADER DATA BLOCK OR FRAME All data characters sent as part of a continuous transmission block (frame) Transmitter and receiver must agree on: -- Transmission format -- Length of a bit time -- Signal levels -- SYNC characters -- Size and content of header and trailer

ASYNCHRONOUS VS. SYNCHRONOUS TRANSMISSION Between a terminal and computer or between low-cost computer – and - computer High-speed Device interconnection. APPLICATIONS Simple to implement. Low cost. Clock resynchronizes with each character. High throughput. High line efficiency ADVANTAGES Error detection protocol required. Self-clocking signaling Technique required. High cost. Low line efficiency Low speed (<20,000 bps) DISADVANTAGES

A FILE TRANSFER Host B Host A HAMLET.TXT Host A Host B TRANSMISSION LINK  FILE TRANSFER: THE STEPS IDENTIFIED IN THIS UNIT SYNCHRONOUS HAMLET.TXT CREATED BY USER AT HOST A CHARACTER CODE INTERNAL REPRESENTATION OF FILE CHOICE OF SIGNAL EXTERNAL REPRESENTATION OF DATA PARALLEL TRANS. SERIAL TRANS. ASYNCHRONOUS  ADDITIONAL AGREEMENTS STILL REQUIRED