Data Communications and
Networking
Chapter 9
High Speed LANs and Wireless LANs
References:
Book Chapters 16 and 17
Data and Computer Communications, 8th edition
By William Stallings
Outline
• Fundamentals of Ethernet
—ALOHA, slotted ALOHA, CSMA
—CSMA/CD
• Ethernet Examples
—10-Mbps Ethernet
—Fast Ethernet
—Gigabit Ethernet
—10-Gbps Ethernet
• 802.11 Wireless LANs
2
Ethernet
• Most widely used high-speed LANs
— Ethernet (10Mbps, 100Mbps, 1Gbps, 10Gbps)
— Fibre channel
— High-speed wireless LANs
• Ethernet protocol is developed by IEEE 802.3 standards
committee, consisting of
— Medium Access Control (MAC) Layer (CSMA/CD protocol)
• the key part of this chapter
— Physical Layer
• Earlier MAC schemes:
— ALOHA
— Slotted ALOHA
— CSMA
3
ALOHA
•
ALOHA protocol is developed for packet radio networks, but applicable to
any shared transmission medium.
— A number of stations share the transmission medium. Two or more
simultaneous transmissions will cause a collision.
•
Sender
•
Receiver
•
Frame may be damaged by noise or by another station transmitting at the
same time (collision). Overlap of frames also causes collision.
ALOHA is simple, but very inefficient
•
—
—
—
—
—
When station has frame, it sends
Station listens for an amount of time
If its hears an ACK, fine. If not, it retransmits the frame after a random time
If no ACK after several transmissions, it gives up
Frame check sequence can be used for error detection
— If frame is OK and address matches receiver, sends ACK
— Otherwise, ignores this frame and does nothing
— Assuming random traffic, the maximum channel utilization is only about 18%
4
ALOHA
5
Slotted ALOHA
• To improve efficiency, a modification of ALOHA,
known as slotted ALOHA, was developed.
• Time is organized into uniform slots whose size
equals the frame transmission time
—Need a central clock (or other sync mechanism)
• Transmission begins only at a slot boundary
—Consequence: frames either miss or overlap totally
• Maximum channel utilization can be improved to
about 37%
6
Slotted ALOHA
7
CSMA
• Why ALOHA and slotted ALOHA are so inefficient?
— Stations don’t check the channel status. They just send out
frames without considering whether the channel is free or not,
which creates too many collisions.
• In fact, it is not difficult for a station to “sense” the
channel status (free or not).
• CSMA: Carrier Sense Multiple Access
— Stations listen to the channel (carrier sense)
— Stations “know” whether the channel is free or not
— Stations transmit only if the channel is free
— Collisions become rare
• Only if two or more stations attempt to transmit at about the same
time, collisions could happen.
8
CSMA (Cont.)
• In traditional LANs, propagation time is much less than
frame transmission time
— Remark: this may not be true for 1Gbps and 10Gbps Ethernet
• All stations know that a transmission has started almost
immediately by “carrier sense”
• Details of CSMA
— Stations first listen for clear medium (carrier sense)
— If medium is idle, transmit the frame
— If two or more stations start at about the same instant, there
will be a collision.
• To account for this, a station waits for an ACK
• If no ACK after a reasonable time, then retransmit
• What should a station do if the medium is found busy?
— Three different approaches: nonpersistent CSMA, 1-persistent
CSMA, and p-persistent CSMA
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Nonpersistent CSMA
•
•
A station wishing to transmit listens to the medium and
obeys the following rules:
1. If medium is idle, transmit; otherwise, go to step 2
2. If medium is busy, wait an amount of time drawn from a
probability distribution and repeat step 1
The use of random delays reduces probability of
collisions
— Consider two stations become ready to transmit at about the
same time while another transmission is in progress
— If both stations delay the same amount of time before retrying,
both will attempt to transmit at same time  collision
•
Drawback:
— Capacity is wasted because medium will generally remain idle
following the end of a transmission, even if there are one or
more stations waiting to transmit.
10
1-persistent CSMA
•
To avoid idle channel time, 1-persistent protocol can be
used
• A station wishing to transmit listens to the medium and
obeys the following rules:
1. If medium is idle, transmit; otherwise, go to step 2
2. If medium is busy, continue to listen until the channel
is sensed idle; then transmit immediately.
• 1-persistent stations are selfish
• Drawback:
— If two or more stations are waiting to transmit, a collision is
guaranteed.
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p-persistent CSMA
•
•
p-persistent CSMA is a compromise that attempts to
reduce collisions, like nonpersistent, and reduce idle
time, like 1-persistent
Rules:
1. If the medium is idle, transmit with probability p, and delay one
time unit with probability (1 – p)
•
The time unit is typically equal to the maximum propagation delay
2. If the medium is busy, continue to listen until the channel is
idle and repeat step 1
3. If transmission is delayed one time unit, repeat step 1
•
Question:
— What is an effective value of p?
12
Value of p?
•
•
•
Assume n stations are waiting to transmit while a transmission is
taking place
At the end of transmission, the expected number of stations
attempting to transmit is the number of stations ready (i.e., n)
times the probability of transmitting (i.e., p): np
If np > 1, on average there will be a collision
— Repeated attempts to transmit almost guarantee more collisions
— Retries compete with new transmissions from other stations
— Eventually, all stations try to send
•
•
•
Continuous collisions; zero throughput
So np should be less than 1 for expected peaks of n
Drawback of p-persistent:
— If heavy load is expected, p should be small such that np < 1.
However, as p is made smaller, stations must wait longer to attempt
transmission, which results in very long delays.
13
CSMA/CD:
CSMA with Collision Detection
•
•
•
1.
2.
3.
4.
The problem of CSMA:
—
A collision occupies medium for the duration of a frame transmission,
which is not good for long frames.
Collision Detection:
—
Stations listen whilst transmitting. If collision is detected, stop
transmission immediately.
Rules of CSMA/CD:
If the medium is idle, transmit; otherwise, go to step 2
If the medium is busy, continue to listen until the channel is idle,
then transmit immediately
If a collision is detected during transmission, transmit a brief
jamming signal to assure that all stations know that there has
been a collision and then cease transmission
After transmitting the jamming signal, wait a random amount of
time, referred to as backoff, then attempt to transmit again
(repeat from step 1)
14
CSMA/CD
Operation
C detects a collision!
A detects a collision!
15
Collision Detection
The amount of time that it takes to detect a collision is no greater than twice the end-toend propagation delay.
Frames should be long enough to allow collision detection prior to the end of
transmission. If shorter frames are used, then collision detection does not occur.
For 10 and 100Mbps Ethernet, the frame length is at least 512 bits.
For 1Gbps Ethernet, the frame length is at least 4096 bits, using carrier extension or
frame bursting.
16
Which Persistence Algorithm?
• IEEE 802.3 uses 1-persistent
• Both nonpersistent and p-persistent have
performance problems
• 1-persistent seems to be more unstable than ppersistent, due to the greed of the stations
• But wasted time due to collisions is short (if
frames are long relative to propagation delay)
• With random backoff, stations involved in a
collision are unlikely to collide on next tries
—To ensure backoff maintains stability, IEEE 802.3 and
Ethernet use binary exponential backoff
17
Binary Exponential Backoff
• Rules of binary exponential backoff:
— A station attempts to transmit repeatedly in the face of repeated
collisions
— For the first 10 attempts, the mean value of the random delay is
doubled
— The mean value then remains the same for 6 additional attempts
— After 16 unsuccessful attempts, the station gives up and reports an
error
• As congestion increases, stations back off by larger and larger
amounts to reduce the probability of collision.
• 1-persistent algorithm with binary exponential backoff is efficient
over a wide range of loads
— At low loads, 1-persistence guarantees that a station can seize channel
as soon as the channel goes idle
— At high loads, it is at least as stable as the other techniques
• Problem: Backoff algorithm gives last-in, first-out effect
— Stations with no or few collisions will have a chance to transmit before
stations that have waited longer
18
IEEE 802.3 Frame Format
≥
≥
Preamble: 7 octets of 10101010
SFD: 10101011
Length: the maximum frame size is 1518 octets, excluding the preamble and SFD.
Pad: octets added to ensure that the frame is long enough for collision detection
FCS: 32-bit CRC, based on all fields except preamble, SFD, and FCS
19
10Mbps Specification
(Ethernet)
• IEEE 802.3 defined a number of physical configurations.
• The nation follows the rule of
— <data rate in Mbps><Signaling method><Max segment length
in hundreds of meters>
20
100Mbps Fast Ethernet
•
•
Fast Ethernet refers to a set of specifications developed by IEEE 802.3 committee to
provide a low-cost, Ethernet-compatible LAN operating at 100 Mbps.
— Use IEEE 802.3 MAC protocol and frame format
100BASE-X refers to a set of options that use two physical links between nodes: one
for ttransmission and one for reception
— 100BASE-TX for twisted pair
— 100BASE-FX for optical fiber
•
100BASE-T4 can use Category 3 UTP cable
•
Star-wire topology
•
•
Uses four twisted-pair lines between nodes
Data transmission uses three pairs in one direction at a time
— Similar to 10BASE-T
21
Gigabit Ethernet
• The Gigabit Ethernet uses the same CSMA/CD frame
format and MAC protocol as used in the 10Mbps and
100Mbps version of IEEE 802.3.
• For shared-medium hub operation, there are two
enhancements to the basic CSMA/CD
— Carrier extension
• Appends a set of special symbols to the end of short MAC frames
so that the resulting block is at least 4096 bit-times in duration
(512 bit-times for 10/100Mbps)
— Frame bursting
• Allows for multiple short frames to be transmitted consecutively (up
to a limit) without giving up control for CSMA/CD between frames.
It avoids the overhead of carrier extension when a single station
has a number of small frames ready to send.
• Not necessary for Ethernet switches because there is no
contention for a shared medium.
22
Gigabit Ethernet Configuration
23
10-Gbps Ethernet
• 10-Gbps Ethernet can be used to provide high-speed, local
backbone interconnection between large-capacity switches.
— Our departmental Ethernet uses 10-Gbps Ethernet at the backbone.
24
10-Gbps Ethernet Configuration
10-Gbps Ethernet also
targets at Metropolitan
Area Networks (MANs)
and WANs.
25
Wireless LANs
• Wireless LAN makes use of a wireless
transmission medium.
• Wireless LAN applications
—LAN Extension
—Cross-building Interconnection
—Nomadic Access
—Ad Hoc Networking
• An ad hoc network is a peer-to-peer network (without
centralized server) set up temporarily to meet some
immediate need.
26
WLAN Configurations (option1 of 2):
Infrastructure Wireless LAN
27
WLAN Configurations (option2 of 2):
Ad Hoc LAN
28
Wireless LAN Technology
• Infrared (IR) LANs
—An individual cell of an IR LAN is limited to a single
room, because infrared light does not penetrate
opaque walls
• Spread spectrum LANs
—The most popular type
—In United States, three microwave bands for
unlicensed use
• 915-MHz band (902-928 MHz) ---- 26 MHz of bandwidth
• 2.4-GHz band (2.4-2.4835 GHz) ---- 83.5 MHz of bandwidth
• 5.8-GHz band (5.725-5.825 GHz) ---- 100 MHz of bandwidth
29
IEEE 802.11
• IEEE 802.11 is devoted to wireless LANs.
— Consists of MAC and physical layer protocols for wireless LANs
• The Wi-Fi Alliance (Wi-Fi: Wireless Fidelity)
— An industry consortium
— To certify interoperability for 802.11 products
• IEEE 802.11 Architecture
— The smallest building block is Basic Service Set (BSS)
• A number of stations executing the same MAC protocol
• Shared wireless medium
• BSS corresponds to a cell
— A BSS may be isolated, or may connect to a Backbone Distribution
System (DS) through an Access Point (AP)
•
•
•
•
AP functions as a bridge and a relay point
AP could be a station which has the logic to provide DS services
AP corresponds to a Control Module (CM)
DS can be a switch, wired network, or wireless network
— An Extended Service Set (ESS) consists of two or more BSSs
interconnected by a DS.
30
IEEE 802.11 Architecture
31
IEEE 802.11 Protocol
Architecture
Physical
Layer
32
802.11 Physical Layer
• Issued in four stages
• First part in 1997
—
—
—
—
IEEE 802.11
Includes MAC layer and three physical layer specifications
Two in 2.4-GHz band and one infrared
All operating at 1 and 2 Mbps
• Two additional parts in 1999
— IEEE 802.11a
• 5-GHz band, data rate up to 54 Mbps
— IEEE 802.11b
• 2.4-GHz band, data rate at 5.5 and 11 Mbps
• Most recent in 2002
— IEEE 802.11g extends IEEE 802.11b to higher data rates, up to 54
Mbps
• At present
— IEEE 802.11n: data rate up to hundreds of Mbps
33
Medium Access Control
• MAC layer has three functions
—Reliable data delivery
• Different from Ethernet, wireless LANs suffer from
considerable unreliability.
—Access control
• Distributed access
• Centralized access
—Security
34
Reliable Data Delivery
•
802.11 physical and MAC layers subject to unreliability
•
802.11 includes frame exchange protocol
— Noise, interference, and other propagation effects result in loss of frames
— Even with error-correction codes, frames may not successfully be received
— Station receiving frame returns acknowledgment (ACK) frame
— The exchange of these two frames is treated as atomic unit
• Not interrupted by any other station
— If no ACK frame received within short period of time, retransmit
•
To further enhance reliability, four-frame exchange may be used
•
The purpose of RTS/CTS
—
—
—
—
Source issues a Request to Send (RTS) frame to destination
Destination responds with Clear to Send (CTS)
After receiving CTS, source transmits data
RTS and CTS notify other stations that a transmission is under way, so they
refrain from transmission in order to avoid collision
— Destination responds with ACK
— The collision time is shortened, because collision only occurs on the RTS frame,
which is very short compared with data frames
35
Hidden Terminal Problem
A and B can hear each other. B and C
can hear each other. But A and C cannot
hear each other.
collision
A
B
C
When A is sending data to B, C cannot
sense this activity and hence C is
allowed to send data to B at the same
time. This will cause a collision at B.
Consider the effect of RTS/CTS:
RTS alerts all stations within range of source (i.e., A) that
exchange is under way; CTS alerts all stations within
range of destination (i.e., B).
36
Medium Access Control
• Two sublayers
• Lower sublayer is distributed coordination function
(DCF)
— Uses a contention algorithm to provide access to all traffic
• Higher sublayer is point coordination function (PCF)
— Uses a centralized algorithm
— Contention free
— Implemented on top of DCF
• Remark: PCF has not been popularly implemented in
today’s 802.11 products. DCF is widely used.
37
Distributed Coordination
Function: CSMA/CA
•
DCF sublayer uses CSMA/CA protocol, where CA refers to as Collision
Avoidance
1.
2.
3.
4.
•
•
A station with a frame to transmit senses the medium. If the medium is idle, it
waits to see if the medium remains idle for a time equals to a delay called
Interframe Space (IFS). If so, the station may transmit immediately.
If the medium is busy, the station defers transmission and continues to
monitor the medium until the current transmission is over.
Once the transmission is over, the station delays another IFS. If the medium
remains idle for this period, then the station backs off a random amount of
time and again senses the medium. If the medium is still idle, the station may
transmit. During the backoff time, if the medium becomes busy, the backoff
timer is halted and resumes when the medium becomes idle.
If the transmission is unsuccessful, which is determined by the absence of an
ACK, then it is assumed that a collision has occurred.
To ensure that backoff maintains stability, binary exponential backoff
is used.
Why not collision detection?
—
—
—
Collision detection is not practical on wireless networks
The dynamic range of wireless signals is very large
The transmitting station cannot distinguish incoming weak signals from noise
and/or effects of own transmission
38
IEEE 802.11 MAC Frame Format
• Frame Control: Indicates the type of frame
• control, management, or data
• Control Frames:
• RTS, CTS, ACK (Acknowledgement), etc.
• Management Frames:
• to manage communications between stations and APs.
39
MAC Frame Fields
• Duration/Connection ID:
— If used as duration field, indicates the time (in s) the channel will be
allocated for successful transmission of a MAC frame
— In some control frames, contains association or connection identifier
• Addresses:
— The number and meaning of the 48-bit address fields depend on
context
— source address, destination address, transmitter address, receiver
address
• Sequence Control:
— 4-bit fragment number subfield used for fragmentation and reassembly
— 12-bit sequence number used to number frames between given
transmitter and receiver
• Frame Body:
— MSDU or a fragment of an MSDU
• Frame Check Sequence:
— 32-bit cyclic redundancy check
40