Communication and Networks Assignment

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Communications and Networks

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Diploma in Information Technology

Lesson 17: Ethernet

Lesson 17 Learning Outcomes

Describe the different channelized, controlled access and random access protocols

Identify the frame format used in Ethernet

Describe the evolution of Ethernet

Describe the hardware used on the Ethernet

Lesson 17 Outline

Multi-access Protocols

Ethernet Standards

Ethernet Wiring

Multi-Access Protocols

Channelized access: like multiplexing

Controlled access: need to grant access

Random access: computer’s own judgement

Source: Douglas, C (2016) Computer Networks and Internets

Channelized Access

Channelization: mapping between a given communication and channel in underlying system

1-to-1 or static channel allocation: works well when communicating entities does not change

Dynamic channel allocation: needed where communicating entities varies

E.g. cellular telephony

Source: Douglas, C (2016) Computer Networks and Internets

Controlled Access

Controlled access is a distributed version of statistical multiplexing

Source: Douglas, C (2016) Computer Networks and Internets

Controlled Access: Polling

Polling: centralised controller that cycles through stations on the network and gives each an opportunity to transmit a packet

Round robin order: transmit on rotation basis

Each station equal opportunity to transmit

Priority order: based on highest priority

Priority stations more opportunity to send

Example: IP phone higher priority than PC

Controlled Access: Reservation

Reservation: employs a two-step process which each round of transmissions is planned in advanced

Step 1: each sender specifies whether they have packets to send and controller broadcast a transmission schedule

Step 2: stations use the schedule to transmit

Variation: an alternate channel to gather reservations for next round and main channel for current round of transmission

Controlled Access: Token Passing

Token passing: a special control message (token) circulate the network continuously

Computers who want to transmit must capture the token

Ring topology: order of circulation is defined, clockwise or anti-clockwise

Bus topology: each station is assigned a position in a logical sequence and the token is passed according to the assigned sequence

Random Access Protocols

Computers attempt to access the shared medium without coordination

Random: access only given when station has packet to send

Prevent all computers from using a medium at the same time

Source: Douglas, C (2016) Computer Networks and Internets

ALOHAnet

ALOHAnet: an early network in Hawaii that pioneered the concept of random access

no longer used, but ideas have been extended

ALOHAnet consist of a powerful transmitter in a central geographic location

Surrounded by a set of stations/computer

Stations can reach the central transmitter but not able to reach all the other stations

ALOHAnet Illustration

ALOHAnet used two carrier frequencies for broadcasting:

Outbound: central transmitter to all stations

Inbound: stations to the central transmitter

Source: Douglas, C (2016) Computer Networks and Internets

ALOHA Protocol

When a station has a packet to send, it transmits the packet on inbound frequency

Central transmitter repeats the transmission on the outbound frequency

To ensure transmission is successful, sender listens to outbound channel

Copy arrives: sender moves to next packet

No copy arrives: sender waits and try again

Transmission Collision

Interference can occur if two stations transmit simultaneously

Collision occurs when two signals interfere and becomes will be garbled

OR two transmitted packets collide

The protocol handles a collision by requiring sender to retransmit each lost packet

Handling Collisions in Ethernet

In 1978, Ethernet was created by Digital Equipment Corporation, Intel, and Xerox

Uses shared wired medium (cable) instead of broadcasting through the atmosphere

Ethernet uses 3 mechanisms to handle collisions:

Carrier sense

Collision detection

Binary exponential backoff

Carrier Sense

Carrier sense: each station monitor cable to detect if another transmission is in progress

Prevents most collision problems and improves network utilisation

Limitation: collision occurs if two stations finds cable idle, both transmit

Some time is required for signal to travel down the cable, a station may not know instantly when another station begins to transmit

Collision Detection

Collision detection: if signal on cable differs from signal that is transmitted, collision has occurred

If collision occurs, sender aborts transmission

Many details complicate Ethernet transmission

Following a transmission, stations must wait for an interpacket gap (9.6s for 10 Mbps Ethernet) to ensure that all stations sense an idle network and have a chance to transmit

Backoff Mechanism

Backoff: after a collision, sender must wait for cable to be idle before transmitting

Specifies max delay d, and a random delay less than d after collision

Randomisation is used to avoid multiple stations transmit simultaneously when cable is idle

If two or more sender choose nearly same amount of delay, there’s risk of second collision

Source: Bing, licensed under CC BY-NC-ND

Binary Exponential Concept

Binary exponential: doubling the range of the random delay

To avoid a sequence of collisions each computer will double range of delay after each collision

Random delay 0 - d, 0 - 2d, 0 - 4d etc.

After few collisions, range becomes large, lowers risks of choosing similar delay values

CSMA/CD

Binary exponential backoff doubles range of the random delay for transmission after each collision

Allows Ethernet to recover quickly after collision

Guarantees reduced contention

Carrier Sense Multi-Access with Collision Detection (CSMA/CD) combines all techniques

Carrier Sense, Collision Detection and Binary Exponential backoff

CSMA/CD Limitations

CSMA/CD does not work as well in wireless LANs (WLAN) as transmitter in WLAN has limited range

A receiver > δ distance away from transmitter will not receive signal and cannot detect carrier

Source: Douglas, C (2016) Computer Networks and Internets

Hidden Station Problem

Hidden station problem: some stations are not visible to others in a WLAN (> δ distance away)

WLAN use a modified access protocol CSMA with Collision Avoidance (CSMA/CA)

CSMA/CA triggers a brief transmission from the intended receiver before transmitting a packet

CSMA/CA

When both sender and receiver transmit a message, all computers within range of either will know a packet transmission is starting

Source: Douglas, C (2016) Computer Networks and Internets

CSMA/CA Steps

Comp3 sends a short message to announce it is ready to transmit a packet to Comp2

Computers in range of Comp3 receive this

Comp2 responds by sending a short message announcing it is ready to receive the packet

Computers in range of Comp2 receive this

Comp1 knows a packet transmission is taking place, even though it cannot receive the signal or sense a carrier

CSMA/CA Collision

Collisions of control messages can occur in CSMA/CA, but can be handled easily

If Comp1 and Comp3 each transmit a packet to Comp2 at the same time, their control messages will collide

When collision occurs, both senders apply random backoff before retransmission

Control messages are smaller than a packet, probability of second collision is low

Automotive Ethernet

Source: https://www.youtube.com/watch?v=BW57JpmZEcc

Practice 17.1

Explain why, for CSMA approaches, slower computers might not get an opportunity to transmit.

What is an alternative multi-access protocol for such cases?

Lesson 17 Outline

Multi-access Protocols

Ethernet Standards

Ethernet Wiring

Ethernet Compatibility

Ethernet versions are backward compatible

New version can automatically adapt to accommodate the older technology

Ethernet are also compatible with newer versions

Ethernet frame format has remained constant since it was created in 1970

Ethernet Frame Format

Frame format: the way a packet is organised

including details like size and meaning of individual fields

Ethernet frame consists of a fixed-length header, a variable-length payload, and a fixed-length CRC

Source: Douglas, C (2016) Computer Networks and Internets

Ethernet Multiplexing

Type field in Ethernet frame allows a computer run multiple protocols operating simultaneously e.g.:

hexadecimal 0800 (IP datagrams) & 0806 (ARP messages)

Receiver will use the type field to determine which software module should process the frame

Source: Douglas, C (2016) Computer Networks and Internets

Ethernet 802.3

IEEE developed standard for Ethernet 802.3 in 1983 which interprets the original type field as a packet length

But also adds 8-byte header that is known as a Logical Link Control / Sub-Network Attachment Point (LLC/SNAP)

Source: Douglas, C (2016) Computer Networks and Internets

Ethernet 802.3 Frame Size

Overall frame size in 802.3 Ethernet remains same at 1514 bytes

Payload is reduced from 1500 to 1492 bytes with 8 bytes SNAP header

To keep the two versions of Ethernet compatible, a convention is used:

If bytes 13-14 of a frame contain a numeric value less than 1500, the field is interpreted as packet length and 802.3 standard applies

Practice 17.2

How does Ethernet achieve multiplexing?

What field does it examine to determine which software program should process the frame?

Lesson 17 Outline

Multi-access Protocols

Ethernet Standards

Ethernet Wiring

Original Ethernet wiring scheme was informally called thick wire Ethernet or Thicknet (10Base5)

Consisted of heavy coaxial cable

Hardware used with Thicknet is divided into two:

Network Interface Card (NIC) to handle digital aspects of communication

Transceiver to handles carrier detection and conversion between digital and analog data

Thicknet Wiring

A physical cable, Attachment Unit Interface (AUI) is connects a transceiver to a NIC

Transceiver is usually separated from a computer

In an office building, transceivers might attach to an Ethernet in a hallway ceiling

Source: Douglas, C (2016) Computer Networks and Internets

2nd Generation Ethernet Wiring

Thicknet evolves to Thinwire Ethernet or Thinnet (10Base2) that use a thinner coaxial cable

Integrates transceiver directly on NIC and runs a coaxial cable from one computer to another

Source: Douglas, C (2016) Computer Networks and Internets

Thinnet Benefits & Issue

Benefits:

Lower overall cost and ease of installation

No external transceivers were needed

Can be installed in a convenient path

Issue:

Entire network is vulnerable if user unplugged a segment of the network, the entire network would stop working

3rd Generation Ethernet Wiring

Instead of coax, it use a central electronic device (Hub) separate from the computers attached to the network and uses twisted pair

Informally known as twisted pair Ethernet

Hubs are available in a variety of sizes with cost proportional to size

Source: Douglas, C (2016) Computer Networks and Internets

Ethernet Wiring Emulation

A hub emulates a physical cable that makes entire system works like a conventional Ethernet

Uses CSMA/CD

Twisted pair Ethernet retains same frame format as the previous versions

Software on computers do not distinguish between thicknet, thinnet or twisted pair Ethernet

NIC handles details and hides any differences

Ethernet Topology

Hub can be thought of as a bus in a box

Logical and physical topologies:

Logically: twisted pair Ethernet employs a bus topology

Physically: twisted pair Ethernet forms a star-shaped topology

Wiring in Office Buildings

Styles of wiring used for LANs make little difference in a machine room or laboratory

Type of wiring makes a difference in terms of:

type

number of wires needed

distance spanned

cost

Wiring Illustration

Twisted pair Ethernet may require many individual cables to go between offices and a central point, wiring closet

Requires careful labeling of cables

Source: Douglas, C (2016) Computer Networks and Internets

Variants of Ethernet

Higher-speed Ethernet use an electronic device known as a switch

To remain backward compatible:

interfaces automatically sense (autosense) the speed at which a connection can operate; slows down to accommodate older devices

Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Connectors

Twisted pair Ethernet uses RJ45 connectors

RJ45 are larger versions of the RJ11 connectors used to connect telephones

Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Cables

Straight cable connects a computer and a switch

connects each pin of the RJ45 attached to one end of the cable directly to the corresponding pin on the RJ45 at the other end

Crossed cable connects two switches

connects a pin on one end to a different pin on the other end

To ensure correct connections, Cat 5 or Cat 6 cable are coated with colored plastic

Practice 17.3

Why is the 2nd generation of Ethernet known as Thinnet?

Reading

Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 14, 15

End of Lesson