TDC 461Basic Communications

Systems

8 May, 2001

Agenda

• Midterm Exam Review

• Six Steps to Data Communications

• OSI 7-Layer Model

• Big Concepts:– Standards

– Architectures

– Protocols

– Error Detection / Correction

– Simplex / Half Duplex / Full Duplex

– Multiplexing

– Baseband v Broadband

Data Communications

6 Steps to Data Com

1. Select and Deal with transmission media

2. Use communicating devices that will

present the proper signal to the line

3. Set up or abide by already-accepted rules

(protocols)

6 Steps, cont.

4. Use an alphabet (character set) that the

devices can understand

5. Ensure the integrity of information before,

during, and after transmission (through

error detection and correction)

6. Deliver information to the receiving device

Big Concept #1: Standards

• Standards: a definition or description of a technology

• 2 Types:– –

Big Concept #2: Architectures

• Architectures=>

Big Concept #3: Protocols

• Protocols=>

•

Asynchronous Transmission Protocols

• Data is clocked or synchronized one byte at a time

•

•

•

Asynchronous - Cont.• Speed calculation =>

• Clocking

• Serial (RS232) Interface

Asynchronous - Cont.

Synchronous Transmission Protocols

• Data clocked on blocks of bytes rather than

a single byte

•

•

Synchronous Transmission Protocols

•Flag - Fixed bit pattern to start and end frame - set to 01111110

•Address - Address of destination device

•Control - Contains frame sequence number and acknowledgement bits

•CRC - Allows error detection

Synchronous Transmission Protocols, cont.

• Clocking

•

Synchronicity

• Modems communicating at 2400+bps are synchronous “over the wire”– –

•

Big Concept #4: Error Detection

• Relates to step 5 of the Data Comm Basics

• 3 modes of error control– Recognition and Flagging– Recognition and Retransmission– Recognition and Forward Error Correction

Recognition and Flagging

• – –

Recognition and Retransmission

• –

Recognition and Retransmission

• – – –

Recognition and Forward Error Correction

• –

Parity Checking

•

• – Odd Parity => – Even Parity =>

Parity Checking

• Example

Parity Checking

• Example:–

Longitudinal Redundancy Checking (LRC)

• Adds a parity bit to each row and each column of a block of data

• Detects a greater range of errors than simple parity.

• Can also be used to correct errors (Forward Error Correction)

Longitudinal Redundancy Checking (LRC)

Parity

1 0 1 0 1 0 1 0 1 1 1 1 0 0 0 0 0 0 1 1 0 0 1 1 1 1 0 1 1 1 0 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0

arity 1 1 0 0 1 1 1 1

LRC Error Correction1-bit error

Parity

1 0 1 0 1 0 1 0 1 1 1 1 0 0 0 0 0 0 1 1 0 0 1 1 1 1 0 1 0 1 0 1 1 0 0 0 0 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0

arity 1 1 0 0 1 1 1 1

Cyclic Redundancy Checking

•

•

Big Concept #5: Plexing

• Simplex

• Half Duplex

• Full Duplex

Simplex

Data flows one way only

Half Duplex

Data flows only one way at any particular time, but can flow either direction.

Full Duplex

Data can flow both ways simultaneously

Big Concept #6: Multiplexing (Review)

• Space Division Multiplexing

• Frequency Division Multiplexing

• Time-division Multiplexing

Big Concept #7: Baseband vs. Broadband

• Baseband:

• Broadband:

Baseband vs. Broadband

• History—3 Factions– Voice people

– Data people

– Radio people

Motivations for Baseband LANs

• Wiring schema.

• Date rates.

• Internconnection.

• Compatibility.

• Cost.

Baseband Cable Systems

• Signaling:

• Plug-n-Play:

Baseband Claims

• Baseband is cheaper.

• Baseband is scalable.

• Baseband is efficient.

Broadband Cable Systems

• D=>any of a specific set of services offerings from common carriers (e.g., T1, T3, ATM, Frame Relay, SONET). The specific technique of using coaxial cable to carry multiple channels over LAN distances

• Signaling.

• Components

• Plug-n-play?

• Target market:

Broadband Network Configurations

• Head end

• Tuners

• Modulation techniques

Trends

OSI• Open Systems Interconnect model (1977)

–

–

–

OSI-7 Layers

• Layer 7-Application Layer– – Examples: File Transfer Protocol (FTP),

Telnet, Simple Mail Transport Protocol (SMTP)

• Layer 6-Presentation Layer–

OSI-7 Layers cont.• Layer 5 - Session Control Layer

– – Example: NetBIOS Extended User Interface

(NETBEUI) (Microsoft)

• Layer 4 -Transport Protocol– – Example: Transmission Control Protocol (TCP)

OSI-7 Layers, cont.

• Layer 3-Network Layer–

–

– Examples: IP (Internet Protocol), Appletalk, Novell IPX

OSI-7 Layers, cont.

• Layer 2 - Data Link Control– – – – Examples: HDLC, Ethernet, Token Ring

OSI-7 Layers, cont.

• Layer 1 - Physical Layer–

•

•

•

– – Examples: Ethernet, Token Ring, T-1, SONET

OSI-7 Layers, cont.• Example: A Windows 95 PC might use:

– Layer 7: Microsoft Network Protocol (SMB)

– Layer 6: ASCII

– Layer 5: NetBEUI

– Layer 4: TCP

– Layer 3: IP

– Layer 2: Ethernet Framing

– Layer 1: Ethernet Physical Network