Tài liệu Chapter 6 The Data Communications Interface pdf

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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)

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)

Synchronous - Block Level
❚
Need to indicate start and end of block
❚
Use preamble and postamble
❙
e.g. series of SYN (hex 16) characters
❙
e.g. block of 11111111 patterns ending in 11111110
❚
More efficient (lower overhead) than async

Synchronous (diagram)

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)

Traditional Configurations

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, NIC
❚
DCE transmits bits on medium
❚
DCE communicates data and control info with DTE
❙
Done over interchange circuits
❙
Clear interface standards required

Characteristics of Interface
❚
Mechanical
❙
Connection plugs
❚
Electrical
❙
Voltage, timing, encoding
❚
Functional
❙
Data, control, timing, grounding
❚
Procedural
❙
Sequence of events

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

Mechanical Specification