Download Ethernet Overview

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Rivier College
CS575: Advanced LANs
Ethernet
10BASE-T
Hub
Collision Domain
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Overview
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A Brief History of Ethernet
IEEE 802.3 10BASE5 Standard
IEEE 802.3 10BASE2 Standard
IEEE 802.3 10BASE-T Standard
Media Access Control
Ethernet Frame Format
Collision Domain Concept
Bridge Concept
Ethernet Switches
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A Brief History of Ethernet
0 Ethernet was invented by Bob Metcalfe and David Boggs in 1973 at
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the Xerox Palo Alto Research Center (PARC) as a local area
network (LAN) for resource sharing among the researchers at the
Center
The original PARC Ethernet was developed to run at 3 Mbps data
rate
In 1979, Digital, Intel, and Xerox (DIX) formed a consortium to
develop Ethernet specification
In 1980, the DIX Ethernet specification (Ethernet blue Book),
Ethernet Version 1.0, was published
The data rate for the DIX Ethernet was specified as 10 Mbps
Over the next two years, DIX refined the Ethernet and issued the
Ethernet Version 2.0 specification in 1982
In the same year, the Ethernet Version 2.0 specification was
submitted it to IEEE for standardization
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A Brief History of Ethernet (concluded)
0 On December 19, 1982, IEEE made some minor changes to the
Ethernet specification Version 2.0 and released the first version of
the IEEE 802.3 standard
0 The IEEE 802.3 standard has been adopted by numerous national
and international standards bodies, including the National Institute
of Standards and Technology (NIST), the European Computer
manufacturers Association (ECMA), and the American National
Standards Institute (ANSI)
0 In 1990, the IEEE 802.3 standard became part of the International
Standardization Organization (ISO) standards, known as ISO/IEC
8802-3 standard
0 More than 85% of all installed network connections were Ethernet
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IEEE 802.3 10BASE5 Standard
0 In 1983, the IEEE 802.3 standard was finalized as the 10BASE5
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standard
802.3 is technically different from DIX Ethernet 2.0, but the
differences are minor
Today, Ethernet and 802.3 are considered as synonymous
The DIX Ethernet 2.0 specification and the original IEEE 802.3
standard specified the use of a 50-ohm, 0.4-inch (10-mm) diameter,
baseband coaxial cable as the transmission medium
This is sometimes referred to as the Thick Ethernet
A bus topology was specified to support 10 Mbps Manchester digital
signaling
With these parameters, the maximum length of the cable is set at
500 meters
Stations attach to the cable by means of taps (transceivers)
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IEEE 802.3 10BASE5 Standard (concluded)
0 A maximum of 100 taps is allowed on each 500-meter cable segment
0 This Ethernet configuration is referred as the 10BASE5 Ethernet
0 To extend the length of the network, repeaters may be used to joint
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cable segments
The standard allows a maximum of 4 repeaters in the path between
any two stations extending the effective length of the medium to
2500 meters (5 cable segments maximum)
This is known as the 4 repeaters and 5 cable segments rule
Since all stations are sharing the same cable, there will be a collision
when two or more stations transmit at the same time
A medium access control protocol must be used to arbitrate the
access of the network
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A Single Segment 10BASE5 Ethernet Example
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A Multiple Segment 10BASE5 Ethernet Example
Repeater
Repeater
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10BASE5 Medium Attachment Unit
0 10BASE5 specifies the use of an external Medium Attachment Unit
(MAU) to attach a station to a cable segment
0 The MAU is most often implemented in a device called a transceiver
0 A transceiver is clamped directly onto the coaxial cable
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10BASE5 Medium Attachment Unit (concluded)
0 An Attachment Unit Interface (AUI) cable is used to connect a
station to its MAU
0 The AUI defines the cable and the connectors to be used
0 The maximum length for the AUI cable (also called the transceiver
cable) is limited to 50 meters
0 Transceiver (MAU) functions
- Transmit
- Receive
- Collision detection
- Jabber Interrupt
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IEEE 802.3 10BASE2 Standard
0 To provide a lower cost system for personal computer LANs, IEEE
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802.3 added a 10BASE2 standard in 1984
The main difference is to implement a less expensive, thinner, 0.25inch (5-mm) diameter 50-ohm coaxial cable
As with 10BASE5, 10BASE2 specified the same bus topology and
Manchester digital signaling at a data rate of 10 Mbps
Since the thinner cable is more flexible, it is easier to bend around
corners and bring to the workstation
The thinner cable also suffers greater attenuation and lower noise
resistance than the thick cable
The maximum cable segment length is limited to 185 meters and up
to 30 stations are supported per cable segment
To extend the length of the network, repeaters may be used to joint
cable segments
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IEEE 802.3 10BASE2 Standard (concluded)
0 This Ethernet configuration is referred as the 10BASE2 Ethernet or
sometimes dubbed as the CheaperNet
0 Cost savings for 10BASE2
- Cheaper, thinner cable
- No need for external transceiver (MAU) and drop cable
- The MAU is integrated with the station (NIC)
- A BNC “T” connector is used to connect a station to the cable
0 A workstation with 10BASE5 NIC can attach to 10BASE2 cable
with an AUI to 10BASE2 adapter
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10BASE2 Ethernet Example
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10BASE2 Ethernet with A Repeater
Source: LANTRONIX Ethernet Product Guide
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IEEE 802.3 10BASE-T Standard
0 By sacrificing some distance, it is possible to develop a 10 Mbps
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LAN using the unshielded twisted-pair (UTP) medium
In 1990, IEEE 802.3 added the 10BASE-T standard to the 802.3
family
The 10BASE-T specification defines a star-shaped topology
A simple system consists of a number of stations connected to a
central point, referred as a hub, or multiport repeater, via two
unshielded twisted pair cable
The type of connector most often used with 10BASE-T cabling is the
RJ-45 modular connector
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IEEE 802.3 10BASE-T Standard (concluded)
0 Adapters are also available for stations having AUI interfaces or
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10BASE2 connectors to be connected to a 10BASE-T hub
The data rate is 10 Mbps using Manchester encoding
Because of the high data rate and the poor transmission quality of
unshielded twisted pair, the length of the link is limited to 100
meters
UTP cable offers many advantages over thick and thin coaxial cable
UPT is less expensive, easier to handle, and similar to the telephone
cable that may already be installed in the building
UTP cables come in a variety of grades
Category 3 and category 5 UTP cables have received most attention
for LAN applications
Cat. 3 UTP is less expensive and more popular for 10BASE-T
configurations, but Cat. 5 UTP is becoming increasingly common
for pre-installation in new office buildings
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10BASE-T Ethernet Example
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10BASE-T Ethernet Expended Configuration
Source: LANTRONIX Ethernet Product Guide
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OSI Reference Model and IEEE 802 Protocol
Layers
Logical Link Control
Medium Access Control
Physical
Medium
Medium
IEEE
OSI
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Media Access Control
0 Since all devices on an Ethernet share the same medium, to avoid
collision, only one station is allowed to transmit at any time
0 Ethernet and 802.3 specified a Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) protocol to control the access of the
medium
0 The basic rules for the CSMA/CD protocol
- A station must listen to the medium first before transmit
- If the medium is idle, the station is allowed to transmit
- If the medium is busy, the station must continue to listen until
the channel is idle, then transmit immediately
- While transmitting, the station must continue to listen to the
channel
- If a collision is detected during transmission, the station must
transmit a brief jamming signal to assure that all stations know
that there has been a collision and then cease transmission
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Media Access Control (continued)
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- After transmitting the jamming signal, the station must wait a
random amount of time and then attempt to transmit again
With random delay, the two stations involved in a collision are
unlikely to collide on their next try
To ensure network stability, IEEE 802 and Ethernet specified a
technique known as binary exponential backoff
A station will attempt to transmit repeatedly in the face of repeated
collisions, but after each collision, the mean value of the random
delay is doubled
After 16 unsuccessful attempts, the station gives up and reports an
error
The key advantage of the CSMA/CD protocol with binary
exponential backoff is that access to the medium is typically very fast
as long as traffic is not heavy, since a station can transmit at any
time the medium is idle
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Media Access Control (continued)
0 Since signal is Manchester encoded, any transmission on the
medium will be detected
Source: Stallings: Data and Computer Communications
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Media Access Control (concluded)
0 Performance issues
- Under heavier traffic loads, the number of collisions increases
and the time spent responding to collisions and retransmission
may cause performance to degrade
- Reasons for 4-repeater/5-segment rule
- Reasons why the number of stations per segment is limited
0 Collision detection for 10BASE-T via the repeater (hub)
- A hub receives a signal on any input link is repeated on all
output links
- If two inputs occur, causing a collision, a collision enforcement
signal is transmitted on all links
- If a collision enforcement signal is detected on any input, it is
repeated on all other links
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Ethernet Frame Format
7 bytes
1 byte
Preamble
SFD
6 bytes
Destination
Address
6 bytes
Source
Address
2 bytes
Length
0 -1500 bytes
Data
0 -46 bytes 4 bytes
Pad
FCS
IEEE 802.3 Frame Format
8 bytes
Preamble
6 bytes
Destination
Address
6 bytes
Source
Address
2 bytes
Type
46 -1500 bytes
4 bytes
Data
FCS
DIX Ethernet Frame Format
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Ethernet Frame Format (concluded)
Preamble
Start Frame Delimiter
MAC Addresses
Type/Length
Maximum Frame Size: 1518 bytes (14 bytes header + 1500 bytes
data + 4 bytes FCS)
0 Minimum Frame Size : 64 bytes (14 bytes header + 46 bytes data +
4 bytes FCS)
0 Interframe Gap: 9.6 microseconds
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Summary Table
Parameter
Transmission Medium
Signaling Technique
Data Rate (Mbps)
Maximum Segment Length (m)
Network Span (m)
Node per Segment
Node Space (m)
Cable Diameter (mm)
Slot Time (bit times)
Inter-Frame Gap (microsecond)
Attempt Limit
Backoff Limit
Jam Size (bits)
Maximum Frame Size (bytes)
Minimum Frame Size (bytes)
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10BASE5
Thick coaxial
cable (50 ohm)
Baseband
(Manchester)
10
500
2500
100
2.5
10
512
9.6
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10
32
1518
64
Ethernet
10BASE2
Thin coaxial
cable (50 ohm)
Baseband
(Manchester)
10
185
925
30
0.5
5
512
9.6
16
10
32
1518
64
10BASE-T
Unshielded
twist pair
Baseband
(Manchester)
10
100
500
NA
NA
0.4 – 0.6
512
9.6
16
10
32
1518
64
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Collision Domain Concept
10BASE-T
Hub
Collision Domain
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Collision Domain Concept (concluded)
10BASE-T
Hub
10BASE-T
Hub
Collision Domain
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Bridge Concept
Bridge
Fast Ethernet
Repeater
Fast Ethernet
Repeater
Collision Domain
Collision Domain
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Bridge Concept (continued)
0 A bridge is a network interconnection device used to join together
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two or more separate LANs to create a larger or an extended LAN
Sometimes it may be necessary to use a bridge to split what is
logically a single LAN into separate LANs to accommodate the
traffic load
A bridge performs its functions in the OSI model Data Link layer
The device is designed for use between LANs that use identical
protocols for the physical and medium access layer (e.g., all
conforming to IEEE 802.3 or all conforming to IEEE 802.5)
Because the devices all use the same protocols, the amount of
processing required at the bridge is minimal
Low cost and ease of use are the advantages of the bridge
Some more intelligent bridges are designed to for use between LANs
with dissimilar protocols
Two types of bridges
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Bridge Concept (continued)
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- Transparent bridge
- Source routing bridge
In a building with multiple LANs, bridges may need to determine
how to forward frames in a very complex network configuration to
reach the destination
It is difficult and time consuming to manually configure all the
bridges
A transparent bridge is a self learning, plug-and-play bridge
Once power up, it observes transmissions that take place on each
connected LAN segment and learns which stations can be reached
over which LAN segment
A transparent bridge is sometimes called a spanning tree bridge
The operation of transparent bridge is specified in the IEEE 802.1d
standard
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Bridge Concept (continued)
Multiple LANs
Interconnected
by Bridges
Source: Stallings: Local & metropolitan area Networks
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Bridge Concept (continued)
Source: Stallings: Local & metropolitan area Networks
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Bridge Concept (concluded)
0 Source routing bridges use routing information included in the
message header to determine the path the frame should take
through the complex network configuration
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Ethernet Switches
0 10BASE-T Ethernet can be implemented in either hubs (repeaters)
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or switches
In 1990, Kalpana developed the first Ethernet switch, EtherSwitch
EPS-700
The technology of Ethernet switch is similar to the intelligent
multiport bridge
The hub is a shared medium technology (medium access control
such as CSMA/CD is needed) and the switch is a dedicated medium
technology (no medium access controls are needed)
Support multiple, independent conversations, i.e., messages are
simultaneously switched between any idle ports (like a telephone
switch or ATM switch)
Network capacity scales with number of ports
Improve network privacy due to dedicated bandwidth
Provide wire-speed performance
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Ethernet Switches (continued)
0 Support full-duplex operation, which increases the effective
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throughput to 20 Mbps per port
Use existing 10BaseT wiring infrastructure
CSMA/CD protocol is no longer needed; no collisions can happen
Multiple switches can be used to extend Ethernet network diameter
as well as supporting more nodes
Most switches are “plug-and-play” self learning devices for packet
forwarding
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Ethernet Switch Switching Schemes
0Three switching schemes
- Store-and-forward
- Cut-through
- Fragment Free (Modified cut-through)
7 bytes
1 byte
Preamble
SFD
6 bytes
6 bytes
2 bytes
1500 bytes Max
Destination
Address
Source
Address
Length
Data
4 bytes
FCS
After 64 bytes
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Cut-through
Fragment Free
Lowest latency
Low latency, collision detection
Ethernet
Store-and-forward
High latency, error detection
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Cost & References
Source: Data Comm Warehouse (1999)
COST:
0 Asante 8-port 10BASE-T Hub: $59.95
0 Asante 24-port 10BASE-T Hub: $269.99
0 Allied Telesyn 8-port 10BASE-T Switch: $419.99
0 Allied Telesyn 16-port 10BASE-T Switch: $1999.99
REFERENCES:
0 W. Stalling, Local and Metropolitan Area Networks, 6th edition,
Prentice Hall, 2000, Chapters 7
0 W. Stalling, Data and Computer Communications, 6th edition,
Prentice Hall, 2002, Chapters 13-14
0 A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier
College, 2001.
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