CS-09 – Data Communications and Networks

AGENDA (Session 4)

Quiz

Telephone System

Modem

Physical Connections

RS-232C

RS-449

Fibre in Local Loop

Multiplexing on Trunks

FDM

TDM

Quiz

1. Ethernet in Novell Netware acts as

a) Physical and Data-link Protocol

b) Transport Protocol

c) Network Protocol

d) Network and DataLink Protocol

2. Fully connected network of 12 nodes would require

a) 24 Lines b) 12 Lines c) 66 lines d) 144 lines

3. SMDS allows Broadcasting. TRUE/FALSE

4. X.25 is a connection oriented protocol. TRUE/FALSE

5. ‘PAD’ stands for

a) Protocol Assembler Disassembler

b) Packet Assembler Disassembler

c) Protocol Answering Device

d) Packet Assembler and Distributor

6. Cell Size of ATM cell is

a) 53 Bytes b) 52 Bytes c) 55 Bytes d) 48 Bytes

7. If 4 voltage levels are used to transmit the signal at a baud rate of 20 bauds, the bit rate required would be

a) 20 bps b) 40 bps c) 80 bps d) 5 bps

8. Shannon Theorem expression for maximum number of bits/second is.

a) S/N log₂ H

b) H log₁₀ (1+S/N)

c) H log₂ (1+S/N)

d) H log₂ (S/N)

Telephone System

· Computer Networks are forced to reach telephone networks due to its wide reach.

· Telephone Systems Error Rate are 1 in 100kbps as against 1 in 1 Terabit in Regular Data Networks, thus special designing is required.

Structure

· Telephone system structure grew from point-to-point fully connected system to Central Office Switch to Interconnected hierarchy of switches

· Various types of transmission media are used in between different levels.

· Twisted Pair (Analog) – Digital or Analog Trunk (Fibre Optic, Microwave) – Digital Intertoll (Fibre Optic, Satellite, Microwave)

· A/D conversion for long distance transmission (Why?=regeneration possible, Efficient Ckt Use, Economical, Easy Maintenance.

· Reverse conversion on Final local Loop at receipt.

· Modems allow digital transmission over analog local loop also.

Modem – Modulator & Demodulator

· Modulation required as DC signalling is unsuitable due to various transmission impairments like attenuation and propagation delay.

· Noise effect can also be minimised in case of digital transfer.

· Amplitude, Frequency or Phase can be the property to modulate any signal.

· Since transfer rate is limited by Nyquist Theorem, More bits are attempted per sample. 4 bits/baud are packed in case of QAM (Quadrature Amplitude Modulation). QAM is capable of transferring 9600 bps on a 2400 baud line.

· QAM based modems standard = V.32

· 6 bits/sec = 14,400 bps = V.32 bis

· 7 bits/sec = 28,800 bps = V.34

· 8 bits/sec = 33,600 bps = V.34 bis (Maxm speed as per shannon)

· Trellis coding is used to reduce errors.

· Modems may divide 3000Hz band into small bands and then reject noisy bands to reduce errors.

· Built in compression and error correction, MNP-5 (Run length Encoding), V.42 bis (Ziv-lempel) compression.

· Echo Suppression:

o Transmission path is reversed by sensing who is talking.

o Different frequency bands are used to receive and talk

· Echo Cancellation

o A simulated Echo is subtracted from the signal.

Physical Connection – RS232C

· Standard from EIA – Electronic Industries Association

· Mechanical, Electrical, Functional and procedural interface standard

· CCITT recommends an equivalent called V.24

· Interface to connect DTE – DCE

· More negative than –3volt = Bit 1, More Positive than +4 volt = Bit 0.

· 15 meters distance upt0 20 Kbps.

· Physical on 25 pin D-Type Connectors.

· Main Signals: Tx, Rx, RTS, CTS, DSR, Return, CD, DTR

· Computer-Computer connection using null modem.

· Still commonly used for Mouse and Modem connections on most PCs.

· 9 Pin variants are more common now.

· Original standard support both Synchronous and Asynchronous data. 9 Pin variant is for ASYNC data only.

Physical Connection – RS449

· Improved over RS-232C in terms of faster data rate and communication distance allowed.

· Basic standard defines mechanical, functional and procedural interface part.

· Electrical Interface Defined by

o RS-423A – Unbalanced transmission similar to RS-232C with common return path.

o RS-422A – Balanced transmission with 2 wires of differential signal without common ground.

· Upto 2Mbps over 60 meter distance.

· Mechanical specs includes 37 and 9 pin Dtype connector specification.

· Current Variants may be available on various other types of connectors.

· Often Used for Exchange to Console transmission.

· Ethernet – on UTP also use similar kind of balanced transmission between HUB and NIC.

Fibre in Local Loop

· Necessity of advanced services like On-Demand Multimedia.

· FTTH (Fibre To The Home) connects upto the home but not economically viable.

· FTTC (Fibre To The Curb) allows fibre to reach within 100 meters of Houses. This Allows UTP or Coaxial cable from Home to be easily connected to Fibre Junction Box.

· The Junction Box on the curb allows to terminate multiple users of 1 to 10Mbps bandwidth that can be sharing 1 Gbps backbone.

· FTTC supports upgrade to FTTH when fibre is economically viable.

· Coaxial Cable Media of Existing Cable operators can also be used to connect Houses to Fibre Junction Box.

Multiplexing

· Utilising same channel by multiple signals.

· Multiplexing is often required as switching offices are connected with High bandwidth while downlines are low-bandwidth customers.

· Basic Categories:

o FDM – Frequency Division Multiplexing (Different signals are modulated on different Frequencies and thus it is possible to Send them together)

o TDM – Time Division Multiplexing (channel is allocated to each signal in a time based round-robin fashion)

FDM – Frequency Division Multiplexing

· Utilises the property of Media, that it allows multiple frequency to coexist during transmission.

· Incoming signals are first filtered for Band Limitation (Say 300 to 3100 Hz)

· Each Signal is then modulated with a different band of Higher frequency.

· All Higher Frequency Bands are then Multiplexed together to utilise the common channel for transmission.

· Multiple levels of FDM hirarchy can be performed to keep merging Frequency channels at different switching levels.

· 5 nos. of 12 Voice channel Group is called a Super Group.

· 5 SuperGroups Form a MasterGroup (CCITT Standard) that can carry 300 Voice channels.

· WDM (Wavelength Division Multiplexing) is a special terminology of FDM type used for Fibre channels as Wavelengths of Light are the primary description of modulating signals.

TDM – Time Division Multiplexing

· In case of TDM - Transmission channel is shared by mutiple requestors on time sharing basis.

· FDM utilise Analog Circuity, that is not possible to implement using computers today. Thus TDM approach is more handy.

· Analog Signal is first Digitised Using CODEC (Coder-Decoder)

· Sampling Rate of 8 KHz is Standard for Voice channel (125 microsec/Sample).

· PCM (Pulse Code Modulation) technique used to prepare 7/8 bit code of Analog Voltage levels.

· Multiple Digital Channel Frame is then Prepared along with Synchronisation Information.

· 24 channel is T1 standard followed in US and Japan. 24x8 + 1 =193 bit every 125 microsec gives it a speed of 1.544 Mbps.

· 32 Channel is E1 Standard used in Europe and Asia. Speed is 2.048 Mbps.

· Differential Pulse Code Modulation = Where differential of voltage levels can be coded into less number of bits.

· Delta Modulation = Where Only the change of voltage is positive or negative is coded into just 1 bit.

· Differential and Delta can produce compressed signal but at the cost of signal quality as all the signal levels may not be coded.

· Multiplexed signal can be further Multiplexed to Higher carriers yielding 32, 128, 512, 2048 channels (various CCITT standards).