计算机通信与网络 课程资料 (5)

The system is very similar to using a modem on a PSTN (Public Switched Telephone Network) line except the carrier may be 100’s MHz with a B/W of MHz’s
CCN: Lecture Notes
Computer Comm. & Networking 3
Ring
• Reduces cabling costs • One node or line down and network dies • Cable rout must be circular
CCN: Lecture Notes
CCN: Lecture Notes
Computer Comm. & Networking 16
Broadband Systems (Cont’d)
Forward and reverse directions may be accomplished by different frequencies on a single cable or by using dual cables
CCN: Lecture Notes
Computer Comm. & Networking 11
Thin wire
• ≈ 0.25″ coaxial cable , i.e., RG58 • Less flexible • High data rates • Used as bus or ring cable • Used as drop* from network
CCN: Lecture Notes
Computer Comm. & Networking 7
A Typical Baseband System
Digital Information
Source
Format
Analog
Sampler
Information
Quantizer
Analog Information
CCN: Lecture Notes
Computer Comm. & Networking 13
Optical Fibre
• Rigid • Very high data r backbone
Wireless
• Infrared at 850 and 950nm • Spread spectrum modulated on 2.4 GHz ISM band • Mainly office networks, store inventory etc. • Generally moderate bit rates but portable
• Cable ends must be terminated
¾ Attach resistor across last node or loose cable end
• Tees must be avoided
¾ Problem when DTE is not close to cable
¾ Use a transceiver to connect onto the cable
4. Bandwidth Communication channels are usually band limited. One has to consider the bandwidth efficiency of the coding scheme.
5. Noise immunity Some schemes are more immune to noise than others.
Module 2
Computer Networking
A specific and very important application of data communications is computer networking. We need to examine what topologies and protocols a computer network may employ.
Computer Comm. & Networking 5
Hub/Tree
• Hub is bus/ring with all wiring inside one box • Cross between star and bus/ring • Can be combined into trees
CCN: Lecture Notes
CCN: Lecture Notes
Computer Comm. & Networking 10
Twisted wire
• Shielded and unshielded • Flexible, i.e., easy to install • Normally used for
• Low grade star networks • Hub-DTE connections • Network drop* • Low to medium data rates
Computer Comm. & Networking 4
Bus
• Further reduced cabling costs • Non circular rout • One node down, rest still work • Interconnected via bridges
CCN: Lecture Notes
CCN: Lecture Notes
Computer Comm. & Networking 12
Thick wire
• ≈ 0.5″ coaxial cable • Rigid, i.e., hard to run • High data rates • Used as bus, ring or backbone cable
¾ Receives and transmits signal onto cable directly
¾ Converts signal into a form suitable for drop cable and DTE connection (Ethernet card in PC)
¾ Transceivers and DTE connections may connect via same signals as main cable or via twisted pairs through an Attachment Unit Interface (AUI)
-V
+V
0
RZ-AMI
-V
+V
Bi-φ -L
-V
+V
Bi-φ -M
-V
+V
Bi-φ -S
-V
+V
-V
DM
Computer Comm. & Networking 9
When selecting a coding scheme, one should consider the following:
1. DC component System which requires AC coupling cannot use those schemes that contain DC component. E.g. Unipolar NRZ
Using a single cable
Headend amplifiers retransmit the received signal onto the transmit frequency or onto the transmit cable
CCN: Lecture Notes
Computer Comm. & Networking 17
Receiver
CCN: Lecture Notes
Computer Comm. & Networking 8
Baseband waveforms
CCN: Lecture Notes
1011000110
+V
-V
NRZ-L
+V
-V
NRZ-M
+V
-V
NRZ-S
+V
Unipolar RZ
0
+V
0
Bipolar RZ
• Star • Ring • Bus • Hub/Tree
CCN: Lecture Notes
Computer Comm. & Networking 2
Star
• Uses large amount of cable • Outage of node connection does not affect others
Computer Comm. & Networking 6
LAN-Media
Baseband systems
A baseband system sends digital signals straight to line. They may be encoded (e.g. Manchester code) but they are not modulated.
What is Baseband?
The original frequency band of the message signal, usually from (near) dc to a few megahertz.
Examples of Baseband transmission systems ⇒ LANS (ethernet, token ring) ⇒ RS232C ⇒ Magnetic discs and tapes