COE 342: Data & Computer Communications (T042) Dr. Marwan Abu-Amara Chapter 6: Digital Data Communications Techniques

COE 342 (T042) – Dr. Marwan Abu-Amara 2 Contents 1. Asynchronous and Synchronous Transmission 2. Line Configuration a. Topology: point-to-point, multipoint b. Full/Half Duplex 3. Interfacing a. V.24/EIA-232-F (formerly known as RS-232) b. ISDN Physical Interface

COE 342 (T042) – Dr. Marwan Abu-Amara 3 Timing Requirements Reception of digital data requires sampling of received signal at receiver  Sampling time should be known Clock drift (example):  If a receiver clock drifts by 1% every sample time,  Then for T b = 1  sec, total drift after 50 bit times = 50 X 0.01 = 0.5  sec  Hence, instead of sampling at the middle of the bit time, the receiver will sample at the edge of the bit (i.e. receiver is out-of-synch with transmitter clock) For correct reception, receiver clock/carrier should be synchronized with transmitter

COE 342 (T042) – Dr. Marwan Abu-Amara 4 Asynchronous & Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions  Asynchronous  Synchronous

COE 342 (T042) – Dr. Marwan Abu-Amara 5 Asynchronous To avoid the timing problem by NOT sending long, uninterrupted streams of bits. For that, data transmitted one character at a time  5 to 8 bits Timing only needs maintaining within each character The receiver has the opportunity to resynchronize at the beginning of each new character. Used for short stream of bits – data transmitted one character (5 ~ 8 bits) at a time Synchronization is needed to be maintained for the length of short transmission Character is delimited (start & end) by known signal elements: start bit – stop element

COE 342 (T042) – Dr. Marwan Abu-Amara 6 Asynchronous: Efficiency Efficiency: transmit 1 start bit + 8 bits of data + 2 stop bits  Efficiency = 8/11 = 72% (or overhead = 3/11 = 28%) Good for data with large gaps (e.g. keyboard, etc) S1S2 Receive idle signal Receive Start bit Receive Stop element S1: receiver in idle state S2: receive is receiving character

COE 342 (T042) – Dr. Marwan Abu-Amara 7 Asynchronous (diagram)

COE 342 (T042) – Dr. Marwan Abu-Amara 8 Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard)

COE 342 (T042) – Dr. Marwan Abu-Amara 9 Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line  Good over short distances  Subject to impairments Embed clock signal in data  Manchester encoding  Carrier frequency (analog)

COE 342 (T042) – Dr. Marwan Abu-Amara 10 Synchronous Frame Format Typical Frame Structure Preamble bit pattern: Indicates start of frame Postamble bit pattern: Indicates end of frame Control fields: convey control information between Tx-er and Rx-er Data field: data to be exchanged data field 8-bit flag 8-bit flag control field

COE 342 (T042) – Dr. Marwan Abu-Amara 11 Line Configuration Topology  Physical arrangement of stations on medium  Point to point  Multi point Computer and terminals, local area network Half duplex  Only one station may transmit at a time  Requires one data path Full duplex  Simultaneous transmission and reception between two stations  Requires two data paths (or echo canceling)

COE 342 (T042) – Dr. Marwan Abu-Amara 12 Interfacing Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE)  e.g. modem DCE transmits bits on medium DCE communicates data and control info with DTE  Done over interchange circuits  Clear interface standards required

COE 342 (T042) – Dr. Marwan Abu-Amara 13 Characteristics of Interface Mechanical  physical specification of connection type, dimensions, location of pins, etc Electrical  Voltage level, timing, encoding Functional  Data, control, timing, grounding  specify functions that are performed such as Rx and Tx circuits Procedural  Specification of sequence of event for transmitting data based on functional specification Sequence of events Two examples:V.24/EIA-232-F, and ISDN physical interface

COE 342 (T042) – Dr. Marwan Abu-Amara 14 V.24/EIA-232-F ITU-T V.24 Only specifies functional and procedural  References other standards for electrical and mechanical EIA-232-F (USA)  RS-232  Mechanical ISO 2110  Electrical v.28  Functional v.24  Procedural v.24

COE 342 (T042) – Dr. Marwan Abu-Amara 15 Mechanical Specification

COE 342 (T042) – Dr. Marwan Abu-Amara 16 Electrical Specification Digital signals Values interpreted as data or control, depending on circuit More (-)ve than -3v is binary 1, more (+)ve than +3v is binary 0 (NRZ-L) Signal rate < 20kbps Distance <15m For control, more (-)ve than -3v is off, more (+)ve than +3v is on

COE 342 (T042) – Dr. Marwan Abu-Amara 17 Functional Specification (See table 6.1 in Stallings chapter 6)

COE 342 (T042) – Dr. Marwan Abu-Amara 18 Local and Remote Loopback

COE 342 (T042) – Dr. Marwan Abu-Amara 19 Procedural Specification E.g. Asynchronous private line modem When turned on and ready, modem (DCE) asserts DCE ready When DTE ready to send data, it asserts Request to Send  Also inhibits receive mode in half duplex Modem responds when ready by asserting Clear to send DTE sends data When data arrives, local modem asserts Receive Line Signal Detector and delivers data

COE 342 (T042) – Dr. Marwan Abu-Amara 20 V.24/EIA-232-F : Procedural Specification (1) Two terminals (computers) connected back to back through modems (no telephone network) as shown bellow Interchange Circuits Needed:  Signal Ground (102)  Transmitted Data (103)  Received Data (104)  Request to Send (105)  Clear to Send (106)  DCE Ready (107)  Received Line Signal Detector (109) two-way DTE A DTE B DCE A DCE B

COE 342 (T042) – Dr. Marwan Abu-Amara 21 V.24/EIA-232-F : Procedural Specification (2) Sequence: (DTE A sends a character to DTE B)  When DCE A is ready (e.g. turned on)  DCE Ready (107) ON  When DTE A has data to send  Request to Send (105) ON  DCE A responds  Clear to Send (106) ON  DTE A now sends data  Transmitted Data (103) When DCE B receives data  Received Line Signal Detector (109) DCE B deliver data to DTE B  Received Data (104)

COE 342 (T042) – Dr. Marwan Abu-Amara 22 Dial Up Operation (1) 1. For dial up operation we will also need DTE Ready (108.2) Ring Indicator (125) 2. Example: Modem A rings Modem B, Modem B accepts the call

COE 342 (T042) – Dr. Marwan Abu-Amara 23 Dial Up Operation (2) Modem A confirms the connection to DTE A and also generates a carrier for Modem B

COE 342 (T042) – Dr. Marwan Abu-Amara 24 Dial Up Operation (3) Data exchange phase – DTE sends data to Modem A – Modem A modulate and transmit to Modem B – Modem B recovers data and sends to DTE B

COE 342 (T042) – Dr. Marwan Abu-Amara 25 Null Modem DTE A DTE B two-way Good for short distance

COE 342 (T042) – Dr. Marwan Abu-Amara 26 ISDN Physical Interface ISDN Terminology:  TE: Terminal equipment (equivalent to DTE)  NE: Network equipment (equivalent to DCE or point of connection to network) Two pins for exchanging data and control for each direction (Note TE and NE need to make the right interpretation of signal!)

COE 342 (T042) – Dr. Marwan Abu-Amara 27 ISDN Physical Interface Connection between terminal equipment TE (corresponds to DTE) and network terminating equipment NE (corresponds to DCE) Physical connection defined in ISO 8877  Which define cables terminate in matching connectors with 8 contacts Transmit/receive carry both data and control