Module 2 Computer Communications and … · CCN: Lecture Notes Computer Comm. & Networking 1 Module 2 Computer Networking. A specific and very important application of data communications

CCN: Lecture Notes Computer Comm. & Networking 1

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.


Local Area Networks (LAN)-Topologies

Many nodes usually require high data transfer rates, and ability to connect to any other node within close proximity of each other.

How do we connect the nodes together?

What kinds of topologies we can use?

• Star

• Ring

• Bus

• Hub/Tree


Star

• Uses large amount of cable

• Outage of node connection does not affect others


Ring

• Reduces cabling costs

• One node or line down and network dies

• Cable rout must be circular


Bus

• Further reduced cabling costs

• Non circular rout

• One node down, rest still work

• Interconnected via bridges


Hub/Tree

• Hub is bus/ring with all wiring inside one box

• Cross between star and bus/ring

• Can be combined into trees


LAN-MediaBaseband systems

A baseband system sends digital signals straight to line. They may be encoded (e.g. Manchester code) but they are notmodulated.

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


A Typical Baseband System

Sampler CoderQuantizer

Format

AnalogInformation

WaveformEncoder(modulator)

DigitalInformation

Low-passfilter Decoder Waveform

Detector

AnalogInformation

DigitalInformation

Transmitter

Channel

Receiver

Format

BinaryDigits

PulseWaveforms

Source

Destination


1 0 1 1 0 0 0 01 1

NRZ-L

NRZ-M

NRZ-S

Unipolar RZ

Bipolar RZ

RZ-AMI

Bi- -Lφ

Bi- -Mφ

Bi- -Sφ

DM

+V-V

+V-V

+V-V

+V0

-V

+V0

-V

+V0

+V

-V

+V

-V

+V

-V

+V

-V

Baseband waveforms


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

2. Self-clockingSome schemes have inherent clocking features which permit clock recovery at the receiving end. E.g. Manchester, bipolar-RZ

3. Error detectionSome schemes provide error correction capabilities without additional parity bits.

4. BandwidthCommunication channels are usually band limited. One has to consider the bandwidth efficiency of the coding scheme.

5. Noise immunitySome schemes are more immune to noise than others.


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


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


Thick wire

• ≈ 0.5″ coaxial cable

• Rigid, i.e., hard to run

• High data rates

• Used as bus, ring or backbone cable


Optical Fibre• Rigid

• Very high data rates

• Used as bus, ring or 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


*Network dropsBecause of the frequency of the signals used, care must be taken to avoid reflections due to impedance mismatches.

• Cable ends must be terminated

Attach resistor across last node or loose cable end

• Tees must be avoided

Problem when DTE is not close to cable

Use a transceiver to connect onto the cable

Receives and transmits signal onto cable directly

Converts signal into a form suitable for drop cable and DTE connection (Ethernet card in PC)

Transceivers and DTE connections may connect via same signals as main cable or via twisted pairs through an Attachment Unit Interface (AUI)


Broadband SystemsAlmost exclusively coaxial cable systems.

A popular system is an adaptation of the Community Antenna Television System (CATV).

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


Broadband Systems (Cont’d)Forward and reverse directions may be accomplished by different frequencies on a single cable or by using dual cables

Using a single cable

Headend amplifiers retransmit the received signal onto the transmit frequency or onto the transmit cable


Broadband Systems (Cont’d)

Using dual cables

Data rates are normally between 0.25 and 1 bps/Hz of cable B/W


Cable B/W’s of up to 400MHz are common.

Connection are either

• DedicatedFrequency band assigned to 2 nodes permanently≈ 20 Mbps

• SwitchedFrequency agile modems switch to a designated frequencyFrequencies assigned from a common frequency≈ 56 Mbps

• Multiple AccessTime division accessEach node gets the entire B/W for a short time and addresses a packet to another node.Most popular


How do the different mediums perform under the different topologies?Ring


How do the different mediums perform under the different topologies?Bus/Tree


How do the different mediums perform under the different topologies?Star


How is access of each node to the media controlled?

Common Medium Access Control (MAC) protocols

Will see later that MAC forms bottom half of Link Layer

• Carrier Sense Multiple Access/Collision Detect (CSMA/CD)

• Control token


CSMA/CD• Sole application to bus networks

• All nodes connected together onto bus

Multiple access (MA)

• Data is broadcast onto the bus in a packet with a header containing the destination address

• Packet also contains source address

• Before a frame is sent, the node listens to see if the bus is busy (Carrier Sense CS)

• When the bus is quiet, the node will transmit

• Even though the bus is quiet at the listening node, a data framemay still be on its way

• When it arrives a collision will occur


CSMA/CD (Cont’d) • A collision may be detected by listening to your own

transmission (Collision detect CD).

• When what you hear on the bus is not the same as what you are sending, a collision may be in progress.

• When a collision occurs, the transmitting node sends a jam sequence of some bit sequence to ensure the other node involved in the collision detects the collision.

• Both nodes then cease transmission and back-off for a random time.

• Either one of these nodes or another node may attempt to transmit again.

• Collision rate is purely probabilistic.


CSMA/CD (Cont’d)

Concept of collision window


How is access of each node to the media controlled?

Common Medium Access Control (MAC) protocols

Will see later that MAC forms bottom half of Link Layer

• Carrier Sense Multiple Access/Collision Detect (CSMA/CD)

• Control token


Control token• The token is just a special frame that gives a node permission to

transmit when it receives it.

• All nodes are places in a logical ring• Not necessarily a physical ring

• All nodes are places in a sequence

• The token is passed from node to node around the logical ring

• A node may transmit a frame when it receives the token.

• The node then sends the token to the next logical node

• One node is generally designated to attend to initialisation of the token and regeneration of a lost token

• A physical ring may be employed in which case the nodes will be 2 port devices.


Slotted ring• A physical ring contains normal nodes plus a monitor node.

• The monitor node ensures there are a fixed number of empty frames being circulated around the ring.

• The frames contain a full/empty bit to indicate their status.


Slotted ring (cont’d)

• When a node wishes to transmit, it fills an empty frame with source and destination address and data, and it switches the full/empty bit.

• The now full frame will circulate around the ring and will be absorbed by the destination node who will modify the response bits to indicate it has read the frame.

• The frame will continue around the ring until it reaches the source node who will check the response bits and mark the frame as empty (full/empty bit).

• The monitor also checks a passed bit which it resets each time a full frame passes.

• If a node fails to mark an old frame as empty, the monitor will detect this and mark the frame empty.


Slotted ring (cont’d)

• Normally these frames are smaller than a link layer frame, therefore many are required to transmit a full LL frame.

• A special node (monitor) is required which may bring the whole network down due to failure.

• Each node may only have one frame on the ring at any one time. Sharing is therefore even.


Ring problem• Timing jitter

The received signal will contain clocking information (e.g., Manchester encoding)

Effects such as delay distortion, noise and receiver imperfections will cause the clock extraction to be less than perfect.

The node may received the data correctly but it will retransmit the data with the imperfections.

The next node will add more imperfections.

Errors may eventually occur due to this additive timing jitter

• Robustness and installation

If the cable is broken, the whole network fails.

Adjacent nodes may need to be physically identified during installation.

A physical ring must be cabled.


Bus vs. Ring • For large installations an high capacity, a bus system will be most suited.

• For moderate capacity and size, the particular application must be examined.

Bus is simple, i.e., passive taps.

Ring uses point to point communications

— Data regenerated at each node

— Therefore grater distances at lower Pε

Bus has cascaded amps

— Can do same distance at greater Pε

Ring may use optical fibre

— Very high data rates

— Low electromagnetic interference (EMI)


Network Standards in Use Today

The closed system standards were examined by IEEE who developed a set of standards which were adopted by ISO.

The common networking standards are IEEE 802 series.


IEEE 802.3 CSMA/CD (up to 10 Mbps)


IEEE 802.3 CSMA/CD (Fast Ethernet)

NRZI NRZ but transition at beginning of bit time indicates a 1

4B5B 4 bits are encoded as 5 bit code to ensure transitions at least every 2 bits

8B6T 8 bits 6 trits, each 8 binary bits are converted into 6 trinary bits (3 levels)

100BASET4 Transmits on 3 pairs and receives on 4 (33.3 Mbps data rate)


IEEE 802.4 Token Bus


IEEE 802.5 Token Ring

Fibre Distributed Data Interface-FDDI (variation on 802.5)


IEEE 802.3 CSMA/CDCommonly called Ethernet (Thick wire) or Cheapernet(Thin wire).

• A tap connects the DTE onto the cable.

• Transceiver is integrated with tap for thick wire.

• Transceiver is found in DTE for thin wire.

• A twisted wire drop interface connects to the DTE controller board.

• A transceiverSends and receives the baseband signal.Detects collisionsProvides electrical isolationPrevents transmission malfunctions (jabber)


IEEE 802.3 CSMA/CD (cont’d)

Jabber control prevents the transmitter from transmitting onto the cable due to a fault condition.


IEEE 802.3 CSMA/CD (cont’d)A CSMA/CD coaxial Ethernet may be accessed via a twisted wire hub (10BASE-T). [Experiment]

The hub combines a number of DTE’s connected with 10BASE-T (2 twisted wire pairs, IN and OUT) to the coaxial cable.

Collisions now occur at the point where the hub connects to the coax. The collision is detected still at the DTE.

Adaptive echo cancellation techniques are required to prevent crosstalk problems due to the unshielded twisted pair.

MAC→


Frame formatThe MAC unit encapsulates the data into a frame.

Transmission of frames is as per CSMA/CD

• Listen prior to transmission

• Jam after collision detected and back-off

46~1500


IEEE 802.3 frame (cont’d)802.3 uses truncated binary exponential backoff

• A certain number of retries are allowed, called the attempt limit (16)

• Collisions occur in the collision window which is the period for a frame to reach the farthest part of the network and return.

• This period (TP) has a safety margin added and becomes the slot time.

Slot time = 2×TP + safety margin

• The backoff time for the nth retry is calculated as R where

0≤R≤2k and k = min(n, backoff limit)

(backoff limit = 10)


IEEE 802.5 Token RingA token ring network is a LAN technique based on a token passingprotocol for media access control

It can be a logical ring using a physical ring or a physical star topology


IEEE 802.5 Token RingOperation has been examined already, two variations exist.

Normal token release• When a node transmits a frame it waits until the frame comes back

around the ring and removes the frame from the ring. It then transmit the token—pass it to the next node.

Early token release• Instead of waiting for the frame to return and be stripped from the ring,

the node retransmits the token straight after the frame is transmitted. This allows much greater throughput.

Connection to the medium is via a Trunk Coupling Network (TCN) by either direct connection or through a concentrator.


IEEE 802.5 Token Ring (cont’d)Connection to the medium is via a Trunk Coupling Network (TCN) by either direct connection or through a concentrator.

Concentrator (Repeater) regenerate and propagate a signal. It is used to extend the length of the LAN


IEEE 802.5 Token Ring (cont’d)

The TCU contains relays to allow the unit to go into bypass modewhen the DTE is off. This allows the remaining network to function.



When the node is connected, it may be in either listen or transmit mode. The former will normally produce a one bit delay through the device.

MAC functions• Frame encapsulation and de-encapsulation• FCS generation• Error detection• Implementation of MAC algorithm• Clock extraction



Latency of the ring is the time for a token to do one lap of an idle ring.

The ring must have a minimum latency of at least the time to transmit the token (24 bits). Otherwise the head of the token will overlap the tail.

The monitor node has a buffer to ensure this. The node may very the size of this buffer to increase or decrease bit rates due to erroredclock extraction.

For 24 bit token, the buffer is set 27 so it may very up and down.


IEEE 802.5 Token Ring Frame formats

Token frame

At the data link layer, a token frame circulates around the ring when all nodes are idle

• SD and ED have bit violations (J,K) in Manchester code to make them unique as delimiter.

• In token, I and E are both 0.• AC contains

— P priority bits describing tokens priority— T token identification bit— M monitor bit so monitor can detect old frames— R reservation bits so node may request a high priority token

• E set by DTE if it detects an error• I indicates first or intermediate frame (1=first/inter, 0=last)


IEEE 802.5 Token Ring Frame formatsData frame If the node possesses the token has data to transmit, it set AC’s T=1 and augments the token frame by including FC, DA, SA, INFO,FCS and FS

• FC contains frame control information. All DTE’s perform function as per Z bits.

• SA Source and DA destination addresses.• FCS frame check CRC• FS status, DTE acknowledges receipts


IEEE 802.5 Token Ring (priority)Token and frames are assigned 8 levels of priority to speed up important frame transmission (P bits in AC)

Pr = received priorityPm = priority of message to be transmittedRr = received reservation

• A node wish to transmit must wait for a free token with Pr≤ Pm.• While waiting, a node may reserve a future token at level Pm• To do this, it sets Rr ← Pm in a passing data frame if Rr < Pm, or sets

Rr ← Pm in a passing token frame if Rr < Pm and Pm < Pr• After transmission, the token is passed on with priority set the max of

Rr, Pm and Pm. Reservation is set to the max of Rr, Pm.• This forces priority higher and higher. To cure this the node that

upgraded the priority must downgrade it when high priority traffic is finished.


IEEE 802.4 Token BusA token bus network has a logical ring on a physical bus

• A token bus network physically resembles a bus topology; logically, it is a ring.

• A token is transmitted from lobe to lobe using network addresses and occurs in descending order.

• The lobe that possesses the token is permitted to transmit data.The lobe order in the figure is 70-59-44-32-23-16-8.

Example


Protocol Performance (CSMA/CD) Utilization of the network

U = Throughput/Data Rate

Propagation time (Tp): The time it takes for a bit to travel from sender to receiver

Transmission time (Tix): The time it takes for a station to transmit a frame

a = (Propagation time)/(Transmission time)

A – Probability that only 1 station wishes to transmit.

AAaU

/)1(211−+

= 1N)11( −−=N

Awhere


Protocol Performance (Token Ring)

When a < 1• A packet is sent at t0• It receives the leading edge of its own packet at t0 +a• Transmission is completed at t0 +1• A token is emitted which then takes a/N to reach the next

node. (N is the number of stations)• One cycle takes 1+a/N and transmission time is 1, so

NaU

/11

+=


Protocol Performance (Token Ring)

When a > 1• A packet is sent at t0• Transmission is completed at t0 +1• Receives the leading edge of this packet at t0 +a• A token is emitted which takes a/N to reach the next node. (N

is the number of stations)• One cycle takes a+a/N and transmission time is 1, so

NaaU

/1

+=


Protocol Performance (Comparison) From the equations for throughput, we see they are all dependenton a

Throughput declines for both protocols as “a” increases


Protocol Performance (Comparison)

a = Tp/Tix

An increase in Tp, i.e., separate the nodes further

Or a decrease in Tix, i.e., smaller packet

will decrease throughput



The throughput expressions are also dependant on N

The performance of a token ring improves as N increase as less time is spend passing tokens.

The performance of a CSMA/CD decreases as N increase as the likelihood of a collisions



The Asymptotes for each curve for large N are

><

=∞→ 11

11lim a

a

aUringToken

N

aUCDCSMA

N 44.311lim/

+=

∞→

Examination of a real network is much more complex and requires the use of a simulation system

Let us examine the 3 protocols for variations in packet size, active stations and data rate to ascertain the effective data transfer rate.

In all cases, the network is being fully occupied by the stated number of stations.



2000 bits/packet – 100 active stations out of 100 station total

• Ideal is strait line

• As data rate increases, Tix decreases, thus ‘a’ increase (a = Tp/Tix)

• CSMA/CD is very dependent on ‘a’

• Token control is not as dependant on ‘a’ (see equation)


• As pocket size decreases, Tix decreases, thus ‘a’ increase (a = Tp/Tix)• Overall efficiency is reduced when max pocket size is small• CSMA/CD is very dependant on ‘a’ and is virtually useless for small

packet sizes when a large number of stations are trying to transmit.• Throughput actually decrease when the load gets very high• Token ring is not as dependant on ‘a’, but has much reduced throughput

due to small packets (see equation)






• As the load is reduced CSMA/CD jumps up in efficiency• Token bus performs very poorly when load is low• For token bus, the time to pass a token becomes much greater than the

propagation delay. And when the load is low, many empty tokens are passed

• Token ring and CSMA/CD perform equally although the delay may bedifferent




• If the packet size is reduced, CSMA/CD declines• Token bus performs even more poorly due to more packets to received

and retransmit.


Protocol Performance (Summary) • If frames get small, more must be sent and the probability of

collision is greater, thus CSMA/CD throughput falls.

• Small frames also increase the overheads.

• CSMA/CD is good for low to medium loads

— Most LAN’s operate predominantly with low to medium loads

• CSMA/CD is not suited to very high bit rates

— Problems with ‘a’

— If Tix<2Tp, initial station will not detect collision

• Token ring is good for high load

• CSMA/CD is easily implemented

• Token ring is harder to cable and prone to disruption.


LAN link layerIn LAN’s, the link layer is normally subdivided into 2 sub layers

• Logical Link Control (LLC) sublayer

• Medium Access Control (MAC) sublayer

• Any of the LAN protocols (802 series) may be utilised as the MAC protocol.


Fiber Distributed Data Interface (FDDI)FDDI employs

• Ring topology

• Fiber-optic cabling as its physical medium

• 100Mbps


FDDI--architecture

• FDDI’s counter rotating ring architecture.

• The primary ring is active in normal operation.

• The secondary ring provides redundancy.

• All devices on the ring are dualattachment stations (Class A nodes) or dual-attachment hubs.


FDDI– self healFDDI has the ability to “self-heal” if the ring topology is cut in a single spot. This is called autowrapping.

In the event of a fiber cut or an inoperative node,

• FDDI automatically wrap the ring at the point of failure.

• interconnecting the primary and secondary rings into a single functional ring.


FDDI– data

In FDDI, the data around the ring is not phase locked. Each station receives the bit stream and extracts the clock to decodethe data then transmits the data using its own clock.

Clocking information is included in the bit stream with a 4B/5B code and a NRZI line code.


FDDI– dataIn FDDI, the frames and tokens are similar to those used in token ring.

A 10 bit buffer (latency buffer is used to store 2 nibbles to identify the start delimiter.

The local clock runs at 125 MHz. This gives a bit rate of 100 Mbps (4 bits code into 5 bits)


FDDI– receiving/transmission


FDDIFDDI uses early token release. The latency of an FDDI ring is large as each station has a 10 bit buffer and there could be 1000 stations.

Priorities are handled similarly to token bus. The Token Rotation Time (TRT) increases as the ring becomes more heavily loaded. Each station measures the TRT and will only transmit a frame when theTRT becomes lower than the TRT target (TRTT) for that frame. High priority frames have high TRTT’s.

The system has the ability to send synchronous data, i.e., real time data like digitized speech. These frames have maximum priority.


Bridges, Repeaters and Switches--RepeaterWe often wish to join LAN’s together, a repeater will simply relay data from one segment to another. It is simply a dumb physical layer device.


Bridges, Repeaters and Switches--BridgeA bridge however normally has up to link layer intelligence. So it receives, check and retransmits the data.

Advantages• Removes LAN physical restrains, i.e., number of stations may be

increased• Isolation between LAN’s of different MAC protocol.• Bridges only look at MAC sub addresses so may isolate LAN’s using

different high level protocols.• Performance monitoring, security and management of the LAN may be

improved.• Small LAN segments are more reliable and serviceable.


Bridges, Repeaters and Switches--Bridge

Disadvantages• Bridges introduce store and forward delay• The MAC layer has no provision for flow control so the

bridge may overflow its buffers during heavy traffic.• When placed between LAN’s of different protocol, the

bridge must generate a new FCS. Errors introduced in the bridge will go undetected.

Often networks of different types need to be connected. Because of the difficulties involved between differing network types often a layer 3 relay device is used.

These are called Routers. Their function is similar to a bridge.


Bridges, Repeaters and Switches—Switches

Switches are now more commonly used for lower level rout controland network segmentation. Ethernet switches are a layer 2 device. They are different to bridges in that they can implement multiple switched connections between ports on the switch.

Switch may be :Store and forward switches work like a bridge where they store the packet before retransmission. They therefore have high latency (1 ms or more)Cut through switches divert the packet out the appropriate port as soon as they read the MAC address. Latency for these switches isvery low (20µs)Hybrid switches can do both and will change from one method to the other as traffic demands.

Documents

Module 2 Computer Communications and … · CCN: Lecture Notes Computer Comm. & Networking 1 Module 2 Computer Networking. A specific and very important application of data communications