Title: COE 342: Data
1COE 342 Data Computer Communications
(T042)Dr. Marwan Abu-Amara
- Chapter 6
- Digital Data Communications Techniques
2Contents
- Asynchronous and Synchronous Transmission
- Line Configuration
- Topology point-to-point, multipoint
- Full/Half Duplex
- Interfacing
- V.24/EIA-232-F (formerly known as RS-232)
- ISDN Physical Interface
3Timing 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 Tb 1msec, total drift after 50 bit
times 50 X 0.01 0.5 msec - 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
4Asynchronous Synchronous Transmission
- Timing problems require a mechanism to
synchronize the transmitter and receiver - Two solutions
- Asynchronous
- Synchronous
5Asynchronous
- 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
6Asynchronous 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)
Receive idle signal
Receive Start bit
Receive Stop element
S1 receiver in idle state S2 receive is
receiving character
7Asynchronous (diagram)
8Asynchronous - 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)
9Synchronous - 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)
10Synchronous Frame Format
11Line 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)
12Interfacing
- 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
13Characteristics 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 examplesV.24/EIA-232-F, and ISDN physical
interface
14V.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
15Mechanical Specification
16Electrical 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 lt 20kbps
- Distance lt15m
- For control, more (-)ve than -3v is off, more
()ve than 3v is on
17Functional Specification
- (See table 6.1 in Stallings chapter 6)
18Local and Remote Loopback
19Procedural 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
20V.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)
21V.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)
22Dial 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
23Dial Up Operation (2)
Modem A confirms the connection to DTE A and also
generates a carrier for Modem B
24Dial 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
25Null Modem
Good for short distance
26ISDN 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!)
27ISDN 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