FIT 1005 Networks & Data Communications
Lecture 9 – High Speed LANs
Reference: Chapter 16
Data and Computer Communications
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Modified Lecture Slides::
http://users.monash.edu.au/~amkhan/fit1005/
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Introduction
• range of technologies
– Fast and Gigabit Ethernet
– Fibre Channel
– High Speed Wireless LANs
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Why High Speed LANs?
• speed and power of PCs has risen
– graphics-intensive applications and GUIs
• LANs are seen as essential to organizations
– for client/server computing
• now have requirements for
– centralized server farms
– power workgroups
– high-speed local backbone
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Ethernet (CSMA/CD)
• most widely used LAN standard
• developed by
– Xerox - original Ethernet
– IEEE 802.3
• Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
– random / contention access to media
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ALOHA
•
•
•
•
•
•
•
developed for packet radio nets
when station has frame, it sends
then listens for a bit over max round trip time
– if it receive ACK then fine
– if not, it retransmit
– if no ACK after repeated transmissions, give up
uses a frame check sequence (as in HDLC)
frame may be damaged by noise or by another station transmitting at
the same time (collision)
any overlap of frames causes collision
Very simple scheme and hence pay’s penalty of max utilization of 18%
Aloha - (From the Hawaiian greeting) A system of contention resolution devised
at The University of Hawaii. Packets are broadcast when ready, the sender
listens to see if they collide and if so re-transmits after a random time.
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Slotted ALOHA
• To improve efficiency on ALOHA (pure) we have slotted
ALOHA
• time on channel is based on uniform slots equal to
frame transmission time
– need central clock (or other sync mechanism)
• transmission begins at slot boundary
– frames will either miss or overlap totally
– this increases max utilization to 37%
• Both (pure aloha and slotted aloha) have poor utilization
• fail to use the fact that propagation time is much less
than frame transmission time
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CSMA
•
•
•
•
•
replacement to ALOHA
stations soon know transmission has started
so first listen for clear medium (carrier sense)
if medium is idle, then transmit, if not wait
if two stations start at the same instant, we
have collision then;
– Station waits reasonable time after tx ACK
– if no ACK then retransmit
– collisions occur at leading edge of frame
• max utilization depends on propagation
time (i.e. medium length) and frame length
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Nonpersistent CSMA
With CSMA, an algorithm is needed to specify what a station should do if
the medium is found busy. One algorithm is nonpersistent CSMA. A station
wishing to transmit listens to the medium and obeys the following rules:
• Nonpersistent CSMA rules:
Sensing Media
Wait back off
– Step-1: if medium is idle, transmit
timer (Probability
– Step-2: if medium is busy, wait for an
distributed)
Is
amount of time drawn from
Media
probability distribution
Busy
Yes
(retransmission delay) & retry
No
• random delays reduces probability of
Transmit frame
collisions
• capacity is wasted because medium will remain idle following
end of transmission
• nonpersistent stations are deferential
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1-persistent CSMA
• 1-persistent CSMA avoids idle channel time
• 1-persistent CSMA rules:
– Step 1: If the medium is idle, transmit
immediately
– Step 2: If the medium is busy, continue to
listen until medium becomes idle, and then
transmit immediately
Sensing Media
Is
Media
Busy
Yes
No
Transmit frame
• In 1-persistent CSMA stations are selfish
• if two or more stations waiting to transmit, a collision is
guaranteed
• Relatively high collision rate
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P-persistent CSMA
• a compromise to try and reduce
collisions and idle time
• p-persistent CSMA rules:
– If the medium is idle it may or may not send
the frame
– Sends with probability p
– Refrains with probability 1-p
– A random number generator is used to
determine whether the station sends
– If the station refrains from sending, it waits
one time slot before sensing again
– Fewer collisions than 1-Persistent
Sensing Media
Is
Media
Busy
Wait a
time slot
Yes
No
Select a random
probability # n from a
set
Yes
• issue of choosing effective value of p to
avoid instability under heavy load
#n,m
No
Transmit frame
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Value of p?
• have n stations waiting to send
• at end of tx, expected no of stations is np
– if np > 1 on average there will be a collision
• repeated tx attempts mean collisions is likely
• eventually when all stations trying to send have
continuous collisions hence zero throughput
• thus we want np<1 for expected peaks of n
– if heavy load expected, then p is small
– but smaller p means stations wait longer
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CSMA/CD Description
•
•
•
with CSMA alone, collision occupies medium for
duration of transmission
better if stations listen whilst transmitting; CMSMA/CD
CSMA/CD rules:
1.
2.
3.
4.
if medium idle, transmit
if busy, listen for idle, then transmit
if collision detected, jam and then cease transmission
after jam, wait random time then retry
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CSMA/CD
Operation
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Which Persistence Algorithm?
• IEEE 802.3 uses 1-persistent
• both nonpersistent and p-persistent have
performance problems
• 1-persistent seems more unstable than p-persistent
– because of greed of the stations
– Time wasted due to collisions
– with random back-off time the stations are unlikely to collide
on next attempt to send
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Binary Exponential Backoff
• for backoff stability, IEEE 802.3 and Ethernet both
use binary exponential backoff
• stations repeatedly resend when collide
– For first 10 retransmission attempts, mean-random delay is
doubled
– The mean value then remains same for 6 further attempts
– after 16 unsuccessful attempts, station gives up and reports
error
• 1-persistent algorithm with binary exponential
backoff is efficient over wide range of loads
• but backoff algorithm has last-in, first-out effect
(stations with no or few collisions will have a chance to transmit before stations that have waited longer)
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Collision Detection
• on baseband bus
– collision produces higher signal voltage
– collision detected if cable signal greater than single
station signal
– signal is attenuated over distance
– limit to 500m (10Base5) Thick Ethernet
– Limit to 200m (10Base2) Thin Ethernet
• on twisted pair (star-topology) Shared Ethernet
– Activity (collision signal) on more than one port is collision
(HUB)
– Detect by use of special collision presence signal
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IEEE 802.3 Frame Format
Preamble: 7-octet pattern of alternating 0s and 1s used for bit synchronization.
Start Frame Delimiter (SFD)
is a sequence 10101011,
locates the start of the frame
Destination and Source MAC
/ Physical 48 bit address
Frame Check Sequence
i.e. CRC of data from DA to Pad
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10Mbps Specification (Ethernet)
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100Mbps Fast Ethernet
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100BASE-X
• uses a unidirectional data rate 100 Mbps over
single twisted pair or optical fiber link
• two physical medium specifications
– 100BASE-TX
> uses two pairs of twisted-pair cable for tx & rx
> STP and Category 5 UTP allowed
> MTL-3 signaling scheme is used
– 100BASE-FX
> uses two optical fiber cables for tx & rx
> convert 4B/5B-NRZI code group into optical signals
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100BASE-T4
• 100-Mbps over lower-quality Cat 3 UTP
– takes advantage of large installed base of cat 3 cabling
– does not transmit continuous signal between packets
– useful in battery-powered applications
• cannot get 100 Mbps on single twisted pair
–
–
–
–
so data stream split into three separate streams
four twisted pairs used
data transmitted and received using three pairs
two pairs configured for bidirectional transmission
• use ternary signaling scheme (8B6T)
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100BASE-T Options
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Full Duplex Operation
• traditional Ethernet was only half duplex
• using full-duplex, station can transmit and
receive simultaneously
• 100-Mbps Ethernet in full-duplex mode, giving a
theoretical transfer rate of 200 Mbps
• stations must have full-duplex adapter cards
• and must use switching (switch)
– each station constitutes separate collision domain
– CSMA/CD algorithm no longer needed
– 802.3 MAC frame format used
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Mixed Configurations
•
•
Fast Ethernet LANs supports mixture of existing 10-Mbps LANs and
newer 100-Mbps LANs
supporting older and newer technologies
– e.g. 100-Mbps backbone LAN supports 10-Mbps hubs
> stations attach to 10-Mbps hubs using 10BASE-T
> hubs connected to switching hubs using 100BASE-T
> high-capacity workstations and servers attach directly to 10/100
switches
> switches connected to 100-Mbps hubs use 100-Mbps backbone
links
> 100-Mbps hubs provide building backbone
> connected to router providing connection to WAN
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Mixed Configurations (2)
•
•
Gigabit Ethernet supports mixture of existing 100 Mbps and 10
Mbps
supporting older and newer technologies
– e.g. 1000-Mbps backbone LAN supports 100-Mbps switches
> stations attach to 10/100-Mbps switch using 100BASE-T
> Standard workstations and servers attach directly to 10/100
switches
> high-capacity workstations and servers attach directly to 1000Mbps switches
> switches connected to 10/100-Mbps switch use 1000-Mbps
backbone links
> 1000-Mbps switches provide building blocks for backbone
> connected to router providing connection to WAN
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Gigabit Ethernet Configuration
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Gigabit Ethernet – Physical
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10Gbps Ethernet
• growing interest in 10Gbps Ethernet
– for high-speed backbone use
– with future wider deployment
• alternative to ATM and other WAN technologies
• uniform technology for LAN, MAN, or WAN
• advantages of 10Gbps Ethernet
– no expensive, bandwidth-consuming conversion between
Ethernet packets and ATM cells
– IP and Ethernet together offers QoS and traffic policing
approach ATM
– have a variety of standard optical interfaces
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10Gbps Ethernet Configurations
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10Gbps Ethernet Options
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Summary
• High speed LANs emergence
• Ethernet technologies
–
–
–
–
–
CSMA & CSMA/CD media access
10Mbps ethernet
100Mbps ethernet
1Gbps ethernet
10Gbps ethernet
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Ethernet Designations
Designation
10Base-2
10Base-5
10Base-36
10Base-F
10Base-FB
10Base-FL
10Base-FP
10Base-T
10Broad-36
Description
10 Mbps baseband Ethernet over coaxial cable with a maximum distance of
185 meters. Also referred to as Thin Ethernet or Thinnet or Thinwire.
10 Mbps baseband Ethernet over coaxial cable with a maximum distance of
500 meters. Also referred to as Thick Ethernet or Thicknet or Thickwire.
10 Mbps baseband Ethernet over multi-channel coaxial cable with a
maximum distance of 3,600 meters.
10 Mbps baseband Ethernet over optical fiber.
10 Mbps baseband Ethernet over two multi-mode optical fibers using a
synchronous active hub.
10 Mbps baseband Ethernet over two optical fibers and can include an
optional asynchronous hub.
10 Mbps baseband Ethernet over two optical fibers using a passive hub to
connect communication devices.
10 Mbps baseband Ethernet over twisted pair cables with a maximum
length of 100 meters.
10 Mbps baseband Ethernet over three channels of a cable television
system with a maximum cable length of 3,600 meters.
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Fast Ethernet Designations
Designation
Description
100Base-FX
100 Mbps baseband Ethernet over two multimode optical fibers.
100Base-T
100 Mbps baseband Ethernet over twisted pair cable.
100Base-T2
100 Mbps baseband Ethernet over two pairs of Category 3 or
higher unshielded twisted pair cable.
100Base-T4
100 Mbps baseband Ethernet over four pairs of Category 3 or
higher unshielded twisted pair cable.
100Base-TX
100 Mbps baseband Ethernet over two pairs of shielded twisted
pair or Category 4 twisted pair cable.
100Base-X
A generic name for 100 Mbps Ethernet systems.
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Gigabit Ethernet Designations
Designation
Description
1000Base-CX
1000 Mbps baseband Ethernet over two pairs of 150 shielded
twisted pair cable.
1000Base-LX
1000 Mbps baseband Ethernet over two multimode or single-mode
optical fibers using longwave laser optics.
1000Base-SX
1000 Mbps baseband Ethernet over two multimode optical fibers
using shortwave laser optics.
1000Base-T
1000 Mbps baseband Ethernet over four pairs of Category 5
unshielded twisted pair cable.
1000Base-X
A generic name for 1000 Mbps Ethernet systems.
Designation
Description
Ethernet at 10 billion bits per second over optical fiber. Multimode
fiber supports distances up to 300 meters; single mode fiber
supports distances up to 40 kilometers.
10Gigabit
Ethernet
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