Interfaces and Synchronization Martin Weiss

EIA 232D Interface Standard u Synonymous with ITU V.24 u Asynchronous interface u Up to 19.2kbps u 50 foot maximum distance between DCE and DTE

Other Interface Standards u EIA Uses EIA-422 (balanced) or EIA-423 (unbalanced) signalling standards u EIA Same as EIA 449 except with a 25 pin connector

Signalling Standards Circuit TypeMax. Separation (m)Max. Bit Rate EIA kbps 10kbps 1kbps EIA Mbps 1Mbps 100kbps

Main EIA 232D Signals u Receive data (RxD) - Pin 2 u Transmit data (TxD) - Pin 3 u Request to Send (RTS) - Pin 4 u Clear to Send (CTS) - Pin 5

Main EIA 232D Signals u Data Set Ready (DSR) - Pin 6 u Data Terminal Ready (DTR) - Pin 20 u Ring Indicator (RI) - Pin 22 u Carrier Detect (CD) - Pin 8

EIA 232D Protocol Modem DTR DSR RTS CTS DTR DSR TxD Data DCD RxD

Questions u For data transfer, when would we want to transfer bits one at a time, and when as a group? u Why do we have to worry about synchronization? u How can we begin to quantify performance issues?

Bit Synchronization u The receiver must know when a bit starts and when it stops u Normally, synchronization sequences are necessary

Character Synchronization u When does a character start?

Asynchronous Communications u Characters are transmitted when they are generated u The receiver must be configured so that the gross transmission characteristics are identical to the transmitter

Implementation of Async u Line is normally in a logical “High” state u Preceding a character is a “Start Bit” (Low for one bit time) u Subsequent bits may be low or high u Character may be ended by stop bit(s)

Implementation of Async Start Parity =? Stop

Discussion of Async u Very simple to implement u Inefficient u Alternative: synchronous transmission

Discussion of Async

Synchronous Transmission u Transmitter and receiver are synchronized at the bit and character level prior to transmission u Messages may still arrive asynchronously u Synchronous systems are normally more complex

Performance Analysis of Communications Links u Consider the effect of propagation delay and transmission rate Recall that a = Propagation time Transmission time

Definitions u Let U = (throughput)/(capacity) = T/R –Throughput is the number of bits actually transmitted per unit time –Capacity is the number of bits that could be transmitted per unit time u R = data rate of the channel u d = maximum distance between any two stations

Definitions u V = velocity of signal propagation u L = frame length (average or fixed) u T = throughput u Assuming no overhead, = frame length transmission time + propagation delay T = Number of bits Time from source to destination

Illustration (a < 1) t0t0 Start of Transmission t 0 +a Start of Reception t 0 +1 End of Transmission t 0 +1+a End of Reception

Illustration (a > 1) t0t0 Start of Transmission t 0 +a Start of Reception t 0 +1 End of Transmission t 0 +1+a End of Reception

Utilization of a Baseband Bus Utilization = Throughput/Capacity T = frame length Propagation delay + transmission time

U vs. a U a