Problems In Chapter 1 we saw networks consist of links
interconnecting nodes. How to connect two nodes together? We also
introduced the concept of cloud abstractions to represent a network
without revealing its internal complexities. How to connect a host
to a cloud?
Chapter Goal Exploring different communication medium over
which we can send data Understanding the issue of encoding bits
onto transmission medium so that they can be understood by the
receiving end Discussing the matter of delineating the sequence of
bits transmitted over the link into complete messages that can be
delivered to the end node Discussing different technique to detect
transmission errors and take the appropriate action
Slide 5
Discussing the issue of making the links reliable in spite of
transmission problems Introducing Media Access Control Problem
Introducing Carrier Sense Multiple Access (CSMA) networks
Introducing Wireless Networks with different available technologies
and protocol
Slide 6
Perspectives on Connecting An end-users view of the
Internet
Slide 7
Simplest network two hosts are directly connected by some
physical medium Medium a length of wire, a piece of optical fiber,
or air through which electromagnetic radiation (e.g., radio waves)
can be transmitted 7
Slide 8
8 Five issues encoding encode bits onto transmission medium so
that they can be understood by a receiving host framing delineate
the sequence of bits transmitted over the link into complete
messages (frames) that can be delivered to the end node error
detection detect transmitted frames errors and take appropriate
action
Slide 9
9 reliable delivery make a link appear reliable in spite of the
fact that it corrupts frames from time to time media access control
mediate multiple hosts access to a link (opposed to point-to-point
link)
Slide 10
Note Frames, Buffers and Messages From network's perspective,
the adaptor transmits frames from the host and receives frames into
the host From host architectures perspective, each frame is
received into or transmitted from a buffer, which is simply a
region of main memory of some length and starting at some address
From operating system's perspective, a message is an abstract
object that holds network frames Messages are implemented by a data
structure that includes pointers to different memory locations
(buffers) 10
Slide 11
11 2.1 Perspectives on Connecting Nodes Networks are
constructed from two classes of hardware building blocks nodes
links
Slide 12
Node Nodes general-purpose computers like a desktop
workstation, a multiprocessor, or a PC special-purpose hardware
considering performance and cost of a network node most commonly a
switch or router inside the network, rather than a host, is
implemented by special-purpose hardware 12
Slide 13
Assume a node is a workstation-class machine serve as a host
that users run application programs on serve as a switch that
forwards messages from one link to another (within a network) serve
as a router that forwards internet packets from one network to
another 13
Slide 14
14 Example workstation architecture
Slide 15
A workstation-class machine memory the memory on any given
machine is finite, it maybe 64MB, 1 GB or more, but it is not
infinite memory is a scarce resource because, on any node that
forwards packets, those packets must be buffered in memory while
waiting their turn to be transmitted over an outgoing link 15
Slide 16
while CPUs are becoming faster at an unbelievable pace, the
same is not true of memory recent performance trends show processor
speeds doubling every 18 months, but memory latency improving at a
rate of only 7% each year the network software needs to be careful
about how it uses memory and, in particular, about how many times
it accesses memory as it processes each message 16
Slide 17
network adaptor the component connects the rest of the
workstation to the link an active intermediary between node and
link, with its own internal processor role transmit data from the
workstation onto the link receive data from the link, storing it
for the workstation 17
Slide 18
the adaptor implements nearly all the networking functionality,
e.g. break data into frames that the link can transport detect
errors introduced as a frame travels over the link follow fairness
rules that allow a link to be shared by multiple workstations
18
Slide 19
two main components a bus interface that understands how to
communicate with the host a link interface that understands how to
use the link there is a communication path between these two
components, over which incoming and outgoing data is passed 19
Slide 20
20 Block diagram of a typical network adaptor
Slide 21
the adaptor exports a control status register (CSR) that is
readable and writeable from the CPU CSR is typically located at
some address in the memory, thereby making it possible for the CPU
to read and write just like any other memory location software on
the host - device driver - writes to the CSR to instruct it to
transmit and/or receive data and reads from the CSR to learn the
current state of the adaptor to notify the host of an asynchronous
event such as the reception of a frame, the adaptor interrupts the
host 21
Slide 22
how bytes of data are transferred between the adaptor and the
host memory direct memory access (DMA) the adaptor directly reads
and writes the hosts memory without any CPU involvement the host
simply gives the adaptor a memory address and the adaptor reads
from (or writes to) it 22
Slide 23
programmed I/O (PIO) the CPU is directly responsible for moving
data between the adaptor and the host memory to send a frame, the
CPU executes a tight loop that first reads a word from host memory
and then writes it to the adaptor (memory adaptor) to receive a
frame, the CPU reads words from the adaptor and writes them to
memory (adaptor memory) 23
Slide 24
the effective throughput of the memory system is defined by the
same two formulas Throughput = TransferSize/TransferTime
TransferTime = RTT + 1/Bandwidth TransferSize in the case of the
memory system, the transfer size corresponds to a unit of data we
can move across the bus in one transfer RTT corresponds to the
memory latency, that is, whether the memory is on-chip cache,
off-chip cache, or main memory the larger the transfer size and the
smaller the latency (RTT), the better the effective throughput
24
Slide 25
Link Network links are implemented on a variety of different
physical media twisted pair (home phone, Digital Subscriber Line
(DSL)) coaxial cable (TV) optical fiber (high-bandwidth, commercial
fiber-to-the home services, long-distance links in the Internets
backbone) air/free space (the stuff that radio waves, microwaves,
infrared beams propagate through) The physical media is used to
propagate signals 25
Slide 26
The signals are electromagnetic waves traveling at the speed of
light the speed of light is medium dependent - electromagnetic
waves traveling through copper and fiber do so at about two-thirds
the speed of light in a vacuum One characteristic of an
electromagnetic wave is frequency, measured in hertz (number of
cycles per second), with which the wave oscillates The distance
between a pair of adjacent maxima or minima of a wave, typically
measured in meters, is called the waves wavelength 26
Slide 27
Electromagnetic waves travel at the speed of light, that speed
divided by the wave's frequency is equal to its wavelength example
a 300-Hz wave traveling through copper would have a wavelength of
SpeedOfLightInCopper Frequency = (2/3 3 10 8 ) 300 = 667 10 3
meters 27
Slide 28
Electromagnetic waves span a much wider range of frequencies,
ranging from radio waves, to infrared light, to visible light, to
X-rays and gamma rays 28
Slide 29
29 Electromagnetic Spectrum
Slide 30
30
Slide 31
31
Slide 32
Links a physical medium carrying signals in the form of
electromagnetic waves at the speed of light transmits all sorts of
information, including binary data (1s and 0s), we say that the
binary data is encoded in the signal encoding & modulation
encoding: placing binary data on a signal modulation: modify
signals in terms of their frequency, amplitude, and phase 32
Slide 33
Frequency, Amplitude, Period, Phase, Wavelength Periodic signal
analog or digital signal pattern that repeats over time s(t +T ) =
s(t ) - < t < + where T is the period of the signal
Slide 34
Periodic Analog Signal (Sine Wave)
Slide 35
Aperiodic signal analog or digital signal pattern that doesn't
repeat over time Peak amplitude (A) maximum value or strength of
the signal over time typically measured in volts Frequency (f )
rate, in cycles per second, or Hertz (Hz) at which the signal
repeats
Slide 36
Period (T ) amount of time it takes for one repetition of the
signal T = 1/f Phase ( ) measure of the relative position in time
within a single period of a signal
Slide 37
Wavelength ( ) distance occupied by a single cycle of the
signal, or the distance between two points of corresponding phase
of two consecutive cycles
Slide 38
Encoding & Modulation The problem of encoding binary data
onto electromagnetic signals can be divided into two layers lower
layer concerned with modulation - varying the frequency, amplitude,
or phase of the signal to effect the transmission of information
example: vary the power (amplitude) of a single wavelength upper
layer encoding binary data onto high signals and low signals
38
Slide 39
How many bitstreams can be encoded on a link at a given time?
for share access to the link, the answer is only one
(multiple-access links) for point-to-point links it is often that
two bitstreams can be simultaneously transmitted over the link at
the same time, one going in each direction (full-duplex) if data
flowing in only one direction at a time - such a link is called
half-duplex - requires that the two nodes connected to the link
alternate using it 39
Slide 40
Different link types to build a computer network cables if the
nodes you want to connect are in the same room, in the same
building, or even on the same site (e.g., a campus), then you can
buy a piece of cable and physically string between the nodes leased
lines if the two nodes you want to connect are on opposite sides of
the country, or even across, you are recommended to lease a
dedicated link from the telephone company 40
Slide 41
last-mile links if you can't afford a dedicated leased line
then go for "last-mile" links, a less expensive option, they often
span the last mile from the home to a network service provider
41
Slide 42
42 Common Types of Cables and Fibers available for Local Links
CableTypical Bandwidths Distances Category 5 twisted pair 10-100
Mbps100m Thin-net coax10-100 Mbps200m Thick-net coax10-100 Mbps500m
Multimode fiber100 Mbps2km Single-mode fiber0.1-10 Gbps40km
Slide 43
43 Leased Lines DS (Digital Signals) Digital signal 1 (DS1,
also known as T1, sometimes "DS-1") is a T-carrier
(T:Telecommunication) signaling scheme devised by Bell Labs DS1 is
a widely used standard in telecommunications in North America and
Japan to transmit voice and data between devices E1 (E:European) is
used in place of T1 outside of North America and Japan DS: digital
signal STS: electrical signal OC: optical signal
Slide 44
44 DS1 is the transmission protocol used over a physical T1
line; however, the terms "DS1" and "T1" are often used
interchangeably DS1 is equal to the aggregation of 24 digital voice
circuits of 64 Kbps each (= 24 64 Kbps) DS3 (also known as T3) is
equal to 28 DS1 links DS1 and DS3 are relatively old technologies
that were originally defined for copper-based transmission media (=
28 1.544 Mbps)
Slide 45
45 Synchronous Transport Signal (STS) standard for electrical
signals a standard for data transmissions over SONET (Synchronous
Optical Networking) STS-1 is the base link speed STS-N has N times
the bandwidth of STS-1 STS-N links are for optical fiber STS-N
links are sometimes called OC-N (optical carrier) links DS: digital
signal STS: electrical signal OC: optical signal
Slide 46
46 Common Bandwidths available from the Carriers
ServiceBandwidth DS1 (T1)1.544 Mbps DS3 (T3)44.736 Mbps STS-151.840
Mbps STS-3155.250 Mbps STS-12622.080 Mbps STS-241.244160 Mbps
STS-482.488320 Mbps
Slide 47
Difference between STS and OC STS refers to the electrical
transmission on the devices connected to the link OC refers to the
actual optical signal that is propagated over the fiber Synchronous
optical networking a method for communicating digital information
using lasers or light-emitting diodes (LEDs) over optical fiber
47
Slide 48
48 There are multiple, very closely related standards that
describe synchronous optical networking SDH (Synchronous Digital
Hierarchy) standard developed by ITU, documented in standard G.707
and its extension G.708 SONET (Synchronous Optical Networking)
standard defined by GR-253-CORE from Telcordia both SDH and SONET
are widely used today: SONET in the U.S. and Canada, and SDH in the
rest of the world
Slide 49
49 OC (Optical Carrier) levels standard for optical signals
describe a range of digital signals that can be carried on SONET
fiber optic network the number in the Optical Carrier level is
directly proportional to the data rate of the bitstream carried by
the digital signal (the speed of OC-n is equal to n 51.8 Mbit/s)
DS: digital signal STS: electrical signal OC: optical signal
Slide 50
Slide 51
51 ServiceBandwidth POTS (Plain Old Telephone Service) 28.8-56
Kbps ISDN64-128 Kbps xDSL128 Kbps-100 Mbps CATV (CAble TV) 1-40
Mbps FTTH (Fiber To The Home) 50 Mbps-1 Gbps Common Services to
Connect Home (Last-Mile Links)
Slide 52
52 Plain Old Telephone Service (POTS) A term which describes
the voice-grade telephone service (e.g. through 56Kbps modem) that
remains the basic form of residential and small business service
connection to the telephone network in most parts of the world
Slide 53
Shannons Theorem How signals degrade over distance? How much
data a given signal can effectively carry? Shannons theorem gives
an upper bound to the capacity of a link, in terms of bits per
second (bps), as a function of the signal-to-noise ratio (S/N) of
the link, measured in decibels (dB) a high S/N means a high-quality
signal, low number of required intermediate repeaters Shannons
theorem can be used to determine the data rate at which a modem can
be expected to transmit binary data over a voice-grade phone line
without suffering from too high an error rate 53 C = B log 2 (1 +
S/N)
Slide 54
Note: SNR Signal-to-noise ratio (SNR or S/N) the ratio of the
power in a signal to the power contained in the noise that is
present at a particular point in the transmission typically
measured at a receiver represented in decibels
Slide 55
Example assume that a voice-grade phone connection supports a
frequency range of 300 to 3,300 Hz Shannons theorem is typically
given by the following formula: C = B log 2 (1 + S/N) where C is
the achievable channel capacity measured in hertz B is the
bandwidth of the line (3,300 Hz - 300 Hz = 3,000 Hz) S is the
average signal power N is the average noise power 55
Slide 56
the signal-to-noise ratio (S/N) is usually expressed in
decibels, related as follows: dB = 10 log 10 (S/N) assuming a
typical decibel ratio of 30 dB, this means that S/N = 1,000 thus,
we have C = 3,000 log 2 (l001) which equals approximately 30 Kbps,
roughly the limit of a 28.8 Kbps modem 56 C = B log 2 (1 +
S/N)
Slide 57
Given this fundamental limit, why is it possible for a 56 Kbps
modems? reason-1 such rates depend on improved line quality, i.e.,
a higher signal-to-noise ratio than 30 dB reason-2 the phone system
is changed to have largely eliminated analog lines that are
bandwidth limited to 3,300 Hz 57
Slide 58
58 Integrated Services Digital Network (ISDN) A
circuit-switched telephone network system, designed to allow
digital transmission of voice and data over ordinary telephone
copper wires, resulting in better quality and higher data speeds
than are available with analog An ISDN connection includes two 64
Kbps channels one can be used to transmit data another can be used
for digitized voice a device that encodes analog voice into a
digital ISDN link is called a CODEC (coder / decoder) When the
voice channel is not in use, it can be combined with the data
channel to support up to 128 Kbps of data bandwidth
Slide 59
59 Digital Subscriber Loop or Digital Subscriber Line (DSL or
xDSL) A family of technologies that provide digital data
transmission over the wires of a local telephone network High Data
Rate Digital Subscriber Line (HDSL) Symmetric Digital Subscriber
Line (SDSL) Asymmetric Digital Subscriber Line (ADSL) ISDN Digital
Subscriber Line (IDSL) Rate-Adaptive Digital Subscriber Line
(RADSL) Very High Speed Digital Subscriber Line (VDSL) Very High
Speed Digital Subscriber Line 2 (VDSL2) Symmetric High-speed
Digital Subscriber Line (G.SHDSL) Powerline Digital Subscriber Line
(PDSL) Uni-DSL (UDSL)
ITU G.992.3/4 ADSL2 12Mbps 1.0Mbps ITU G.992.3/4 Annex J ADSL2
12Mbps 3.5Mbps ITU G.992.5 ADSL2+ 24Mbps 1.0Mbps ITU G.992.5 Annex
M ADSL2+ 24Mbps 3.5Mbps 1.3km VDSL( 55.2Mbps 19.2Mbps) 61
Slide 62
A gateway is commonly used to make an ADSL connection. The
model pictured is also a wireless access point, hence the antenna
62
Slide 63
63 ADSL Wiring Standard twisted pair Connecting residence ADSL
modem COs ADSL modem Digital connection To local network To analog
phone To telephone switch Digital connection To provider Telephone
central office Local loop Central office Subscriber premises
Slide 64
64 Local loop Central office Subscriber premises ADSL Connects
the Subscriber to the Central Office via the Local Loop
Slide 65
65 VDSL Symmetric 12.96 to 55.2 Mbps (1,000 to 4,500 feet
(1.37km)) Fiber to the neighborhood put VDSL transmission hardware
in neighborhoods, with some other technology (e.g., STS-N running
over fiber) connecting the neighborhood to the central office
Slide 66
66 STS-N over fiber VDSL at 12.96 55.2 Mbps over 1000 4500 feet
of copper Central office Subscriber premises Neighborhood optical
network unit
Slide 67
67 Cable TV (CATV) Asymmetric Downstream: 40 Mbps / channel
Upstream: 20 Mbps / channel Unlike DSL, the bandwidth is shared
among all subscribers in a neighborhood
Slide 68
Wireless Links Wireless links transmit electromagnetic signals
- radio, microwave, infrared, or even visible light - through
space, even through vacuum Because wireless links all share the
same wire, the challenge is to share it efficiently, without unduly
interfering with each other It is feasible to limit the area
covered by an electromagnetic signal because such signals weaken,
or attenuate, with the distance from their origin To reduce the
area covered by your signal, reduce the power of your transmitter
68
Slide 69
Frequencies allocations are determined by government agencies
such as the Federal Communication Commission (FCC) in the United
States [NCC (National Communications Commission) in Taiwan]
Specific bands (frequency ranges) are allocated to certain uses
some bands are reserved for government use other bands are reserved
for uses such as AM radio, FM radio, television, satellite
communication, and cell phones specific frequencies within these
bands are then licensed to individual organizations for use within
certain geographical areas 69
Slide 70
Several frequency bands set aside for "license-exempt" usage -
bands in which a license is not needed devices that use
license-exempt frequencies are still subject to certain
restrictions 1 st restriction: a limit on transmission power this
limits the range of a signal, making it less likely to interfere
with another signal example, a cordless phone might have a range of
about 100 feet (30.48m) 2 nd restriction: requires use of spread
spectrum techniques 70
Slide 71
Spread spectrum spread the signal over a wider frequency band
than normal in such a way as to minimize the impact of interference
from other devices originally designed for military use, so these
other devices were often attempting to jam ( ) the signal 71
Slide 72
vs.
Slide 73
( Narrowband ) FM guard band (noise level) (jam)
Slide 74
Slide 75
( Spread band ) 1MHz 10W 20MHz (20 ) 10mW (1/1000 ) jam
jamming
Slide 76
1950 1960
Slide 77
Slide 78
Spread spectrum techniques Frequency Hopping Spread Spectrum
(FHSS) Direct Sequence Spread Spectrum (DSSS) 78
Slide 79
(FHSS) Frequency hopping transmit the signal over a random
sequence of frequencies, i.e., first transmit at one frequency;
then a second, then a third, and so on the sequence of frequencies
is not truly random, but is instead computed algorithmically by a
pseudorandom number generator the receiver uses the same algorithm
as the sender - and initializes it with the same seed - and hence
is able to hop frequencies in sync with the transmitter to
correctly receive the frame 79
Slide 80
FHSS = Frequency Hopping Spread Spectrum f 1, f 2, f 3, f
4,..., f k (PNS)
Slide 81
Slide 82
(narrowband) FHSS FCC ( Federal Communications Commission) 75
1W 4W
Slide 83
(channel) (hops) (dwell time) 400ms 2.5
Slide 84
(DSSS) Direct sequence add redundancy for greater tolerance of
interference each bit of data is represented by multiple bits in
the transmitted signal if some of the transmitted bits are damaged
by interference, there is usually enough redundancy to recover the
original bit for each bit the sender wants to transmit, it actually
sends the exclusive-OR of that bit and n random bits as with
frequency hopping, the sequence of random bits is generated by a
pseudorandom number generator known to both the sender and the
receiver 84
Slide 85
The transmitted values, known as an n-bit chipping code, spread
the signal across a frequency band that is n times wider than the
frame would have otherwise required 85 Example 4-bit chipping
sequence
Slide 86
DSSS = Direct Sequence Spread Spectrum 1 10 Spreading Ratio
(SR) SR SR ( )
Slide 87
spread spectrum
Slide 88
Slide 89
DSSS operation flow data signal DSSS (interference) pseudo code
de-spread data signal pseudo code de-spread filter desired
frequency signal
PN Code (Pseudorandom Code) PN code (orthogonal code) 0 example
PN code A 00011011 B 00101110 C 01011100 D 01000010
Slide 92
PN code Bipolar ( ) chip sequences A =(-1,-1,-1,1,1,-1,1,1) B
=(-1,-1,1,-1,1,1,1,-1) C =(-1,1,-1,1,1,1,-1,-1) D
=(-1,1,-1,-1,-1,-1,1,-1) (orthogonality) A*A=1+1+1+1+1+1+1+1=8
A*B=1+1-1-1+1-1+1-1=0 A*C=1-1+1+1+1-1-1-1=0
A*D=1-1+1-1-1+1+1-1=0
Slide 93
Local Oscillator Amplifier RbRb PN Code Generator Modulator
Data Input Amplifier Local Oscillator RbRb PN Code Generator
Demodulator Data Output Note: an electronic oscillator is an
electronic circuit that produces a repetitive electronic signal,
often a sine wave or a square wave.
Slide 94
DSSS cdmaOne (IS-95) cdma2000 W-CDMA WLAN
Slide 95
License & License-Exempt Frequencies It is these
license-exempt frequencies, with their spread spectrum techniques
and limited range, that are used by 802.11 (Wi-Fi), 802.15.1
(Bluetooth), and some flavors of 802.16 (WiMAX) Governments
regulate only the prime communication portion: the radio and
microwave ranges 95
Slide 96
Among the unregulated spectra are the infrared and visual light
ranges, which cannot penetrate walls Infrared is widely used in
remote controls for television and the like increasingly, it is
also used in a variety of short-range data exchange applications,
based on standards established by the Infrared Data Association
(IrDA) 96
Slide 97
For example, IrDA has defined a standard for "Point & Pay,
using infrared to conduct a financial transaction between a
handheld device, such as a mobile phone or PDA a stationary
financial terminal such as a cash register 97
Slide 98
Visual light or infrared can also be focused by a laser to
provide a high-bandwidth link between two stationary points - even
though no mobility is involved - in situations where a wired link
is less practical for some reason example, two buildings belonging
to the same organization but separated by a busy highway could
communicate with lasers 98
Slide 99
99 2.2 Encoding Link a physical medium carrying signals
(including 1s and 0s) in the form of electromagnetic waves Network
adaptor contain a signaling component that encode bits (binary
data) into signals at the sending node and decode signals into bits
at the receiving node
Slide 100
100 Signals travel between signaling components Bits flow
between adaptors
Slide 101
101
Slide 102
102 Encoding the process of transforming information from one
format into another Electronic encoding transforms a signal into a
form optimized for transmission or storage, generally done with a
codec (coder / decoder)
Slide 103
103 Encoder a device used to change a signal (such as a
bitstream) or data into a code the code may serve any of a number
of purposes such as compressing information for transmission or
storage encrypting or adding redundancies to the input code
translating from one code to another
Slide 104
104 Example of an Encoder OR NOT AND
Slide 105
105 Encoding Schemes Non-Return to Zero (NRZ) encode binary
data onto electromagnetic signals e.g., 0 as low signal and 1 as
high signal
Slide 106
Non-Return to Zero Inverted (NRZI) 1: make a transition from
current signal 0: stay at current signal Manchester transmit XOR of
the NRZ encoded data and the clock 106
Slide 107
107 Encodings
Slide 108
108 4B/5B idea insert extra bits into the bitstream so as to
break up long sequences of 0s or 1s every 4 bits of data encoded in
a 5-bit code that is then transmitted to the receiver
Slide 109
5-bit codes selected in the following way no more than one
leading 0 no more than two trailing 0s when sent back-to-back, no
pair of 5-bit codes results in more than three consecutive 0s being
transmitted the resulting 5-bit codes are transmitted using NRZI
109
Slide 110
Example of 4B/5B Encoding
Slide 111
The table gives the 5-bit codes that correspond to each of the
16 possible 4-bit data symbols 5 bits are enough to encode 32
different codes, and we are using only 16 of these for data 16
codes (=1+1+1+7+6) left for other purposes code 11111 used when the
line is idle code 00000 used when the line is dead code 00100
interpreted to mean halt 7 codes are not valid because they violate
the "one leading 0, two trailing 0s," rule 6 codes represent
various control symbols, e.g. used in framing protocols of FDDI
111
Slide 112
112 2.3 Framing Break sequence of bits into a frame Typically
implemented by network adaptor to enable the nodes to exchange
frames
Slide 113
Framing When node A wishes to transmit a frame to node B, it
tells its adaptor to transmit a frame from the nodes memory. This
results in a sequence of bits being sent over the link The adaptor
on node B then collects together the sequence of bits arriving on
the link and deposits the corresponding frame in Bs memory
Recognizing exactly what set of bits constitute a framethat is,
determining where the frame begins and endsis the central challenge
faced by the adaptor
Slide 114
114 Bits flow between adaptors and frames flow between
hosts
Slide 115
115 Approaches Byte-Oriented Protocols one of the oldest
approaches to framing root in connecting terminals to mainframes
view each frame as a collection of bytes (characters) rather than a
collection of bits
Slide 116
examples sentinel-based approaches Binary Synchronous
Communication (BISYNC) Point-to-Point Protocol (PPP) byte-counting
approach Digital Data Communications Message Protocol (DDCMP)
116
Slide 117
117 Bit-Oriented Protocols views the frame as a collection of
bits these bits might come from some character set, such as ASCII
pixel values in an image instructions and operands from an
executable file example High-Level Data Link Control (HDLC)
120 Byte-Oriented Protocol (BISYNC) Binary Synchronous
Communication (BISYNC) Developed by IBM in the late 1960s Use
sentinel characters to indicate start/end Format SYN
(synchronization) : beginning Data portion: between STX (start of
text) and ETX (end of text) SOH (Start Of Header) serves much the
same purpose as the STX field
Slide 121
Problem the ETX character might appear in the data portion of
the frame BISYNC overcomes this problem by escaping the ETX
character by preceding it with a data-link-escape (DLE) character
this approach is often called character stuffing because extra
characters are inserted in the data portion of the frame CRC
(Cyclic Redundancy Check): detect transmission errors 121
Slide 122
122 Byte-Oriented Protocol (PPP) Point-to-Point Protocol (PPP)
Commonly used to carry Internet Protocol packets over various sorts
of point-to-point links Also uses sentinels and character stuffing
Format Flag (start-of-text): 01111110 Address: default value
Control: default value
Slide 123
Protocol: used for demultiplexing, it identifies the high-level
protocol such as IP or IPX (Novell) Payload: size can be negotiated
(default is 1,500 bytes) Checksum: either 2 (by default) or 4 bytes
long 123
Slide 124
Several of the field sizes of the PPP frame format are
negotiated rather than fixed this negotiation is conducted by a
protocol called Link Control Protocol (LCP) PPP and LCP work in
tandem LCP sends control messages encapsulated in PPP frames and
then turns around and changes PPPs frame format based on the
information contained in those control messages 124
Slide 125
125 Byte-Oriented Protocol (DDCMP) Digital Data Communication
Message Protocol (DDCMP) Used in Digital Equipment Corporations
DECNET Count: specifies how many bytes are contained in the frames
body
Slide 126
One danger with this approach a transmission error could
corrupt the count field, in which case the end of the frame would
not be correctly detected should this happen, the receiver will
accumulate as many bytes as the bad COUNT field indicates and then
use the error detection field to determine that the frame is bad
(framing error) the receiver will then wait until it sees the next
SYN character to start collecting the bytes that make up the next
frame 126
Slide 127
127 Bit-Oriented Protocol (HDLC) Synchronous Data Link Control
(SDLC) protocol developed by IBM is an example of a bit-oriented
protocol SDLC was later standardized by the ISO as the High-Level
Data Link Control (HDLC) protocol Beginning and end of a frame:
01111110
Slide 128
Because this sequence (01111110) might appear anywhere in the
body of the frame in fact, the bits 01111110 might cross byte
boundaries bit-oriented protocols use the analog of the DLE (Data-
Link-Escape) character, a technique known as bit stuffing Operation
flow of bit stuffing on the sending side any time five consecutive
1s have been transmitted from the body of the message (i.e.,
excluding when the sender is trying to transmit the distinguished
01111110 sequence), the sender inserts a 0 before transmitting the
next bit 128
Slide 129
on the receiving side should five consecutive 1s arrive, the
receiver makes its decision based on the next bit it sees if the
next bit is a 0, it must have been stuffed, and so the receiver
removes it if the next bit is a 1, then one of two things is true
either this is the end-of-frame marker or an error has been
introduced into the bitstream 129
Slide 130
by looking at the next bit if it sees a 0 (i.e., 01111110),
then it is the end-of-frame marker if it sees a 1 (i.e., 01111111),
then there must have been an error and the whole frame is
discarded, in this case, the receiver has to wait for the next
01111110 before it can start receiving again 130
Slide 131
131 Clock-Based Framing (SONET) Synchronous Optical Network
(SONET) first proposed by Bell Communications Research (Bellcore)
then developed under the American National Standards Institute
(ANSI) for digital transmission over optical fiber, it has since
been adopted by the ITU-T The dominant standard for long-distance
transmission of data over optical networks
Slide 132
STS-1 frame runs at 51.84 Mbps arranged as 9 rows of 90 bytes
each the first 3 bytes of each row are overhead the rest is
available for data the first 2 bytes of the frame contain a special
bit pattern used for enabling the receiver to determine where the
frame starts 132
Slide 133
Frame overhead bits that are added at regular intervals to a
digital signal at the sending end of a digital link and used to
provide network functions such as framing, operations,
administration, and maintenance the overhead bytes of a SONET frame
are encoded using NRZ 133
Slide 134
134 STS-1 Frame
Slide 135
135 SONET supports multiplexing of multiple low-speed links A
given SONET link runs at one of a finite set of possible rates,
ranging from 51.48 Mbps (STS-1) to 2,448.32 (STS-48), and beyond A
single SONET frame can contain subframes for multiple lower-rate
channels Clock-based each frame is 125 s long
Slide 136
136 A SONET frame STS-N frame can be thought of as consisting
of N STS- 1 frames 810 bytes long at STS-1 rate 2,430 bytes long at
STS-3 rate The bytes from these frames are interleaved a byte from
the first frame is transmitted, then a byte from the second frame
is transmitted, and so on ensure that the bytes in each STS-1 frame
are evenly paced
Slide 137
STS-Nc link (concatenate) the payload from these N STS-1 frames
to form a larger STS-N payload 137 Three STS-1 frames multiplexed
onto one STS-3c frame
Slide 138
138 2.4 Error Detection Bit errors are introduced into frames
because of electrical interference and thermal noises Two basic
approaches that can be taken when the recipient of a message
detects an error one is to notify the sender that the message was
corrupted so that the sender can retransmit a copy of the message
alternatively, there are some types of error detection algorithms
that allow the recipient to reconstruct the correct message even
after it has been corrupted; such algorithms rely on error
correcting codes
Slide 139
Error detection techniques two-dimensional parity check used by
the BISYNC protocol when it is transmitting ASCII characters (CRC
is used as the error code when BISYNC is used to transmit EBCDIC)
Cyclic Redundancy Check (CRC) used in nearly all the link-level
protocols example HDLC, DDCMP, CSMA and token ring protocols
checksum used by several Internet protocols 139 BISYNC
Slide 140
140 Error Types
Slide 141
141 Error Detection
Slide 142
Slide 143
143 Parity Check
Slide 144
144
Slide 145
145
Slide 146
Slide 147
Cyclic Redundancy Check The theoretical foundation of the
cyclic redundancy check is rooted in a branch of mathematics called
finite fields In general, we can provide quite strong error
detection capability while sending n redundant bits for an m-bit
message, where n LAF then discarded (outside the receivers window)
RWS LFRLAF < LAF Largest Frame Acceptable LFR Last Frame
Received RWS Receive Window Size
Slide 184
Let SeqNumToAck denote the largest sequence number not yet
acknowledged, such that all frames with sequence numbers less than
or equal to SeqNumToAck have been received the receiver
acknowledges the receipt of SeqNumToAck this acknowledgement is
said to be cumulative sets LFR = SeqNumToAck & adjusts LAF =
LFR + RWS 184 LAF Largest Frame Acceptable LFR Last Frame Received
RWS Receive Window Size
Slide 185
Example suppose LFR = 5 (i.e., the last ACK the receiver sent
was for sequence number 5), and RWS = 4 this implies that LAF = 9
should frames 7 and 8 arrive, they will be buffered because they
are within the receivers window no ACK needs to be sent since frame
6 is yet to arrive frames7 and 8 are said to have arrived out of
order 185 LAF Largest Frame Acceptable LFR Last Frame Received RWS
Receive Window Size
Slide 186
should frame 6 then arrive perhaps it was lost and had to be
retransmitted, or perhaps it was simply delayed the receiver
acknowledges frame 8, bumps LFR to 8, and sets LAF to 12 if frame 6
was in fact lost then a timeout will have occurred at the sender,
causing it to retransmit frame 6 186 LAF Largest Frame Acceptable
LFR Last Frame Received RWS Receive Window Size
Slide 187
When a timeout occurs, the amount of data in transit decreases
since the sender is unable to advance its window until frame 6 is
acknowledged this means that when packet losses occur, this scheme
is no longer keeping the pipe full Early detection of packet losses
giving more information to the sender makes it potentially easier
for the sender to keep the pipe full, but adds complexity to the
implementation 187
Slide 188
sol-1 the receiver could send a negative acknowledgment (NAK)
for frame 6 as soon as frame 7 arrived (adds additional complexity
to the receiver) sol-2 send additional acknowledgments of frame 5
when frames 7 and 8 arrived, e.g., a sender can use duplicate ACKs
as a clue that a frame was lost sol-3 selective acknowledgment the
receiver acknowledges exactly those frames it has received, rather
than just the highest- numbered frame received in order, e.g., the
receiver could acknowledge the receipt of frames 7 and 8 188
Slide 189
Send window size selected according to how many frames we want
to have outstanding on the link at a given time SWS (Send Window
Size) is easy to compute for a given delay bandwidth product
189
Slide 190
Receive window size the receiver can set RWS to whatever it
wants RWS = 1 the receiver will not buffer any frames that arrive
out of order RWS = SWS the receiver can buffer any of the frames
the sender transmits RWS > SWS makes no sense, its impossible
for more than SWS frames to arrive out of order 190
Slide 191
191 Sequence Number Space SeqNum field is finite; sequence
numbers wrap around Sequence number space must be larger then
number of outstanding frames, e.g. stop-and-wait (1-bit): allows 1
outstanding frame at a time and has two distinct sequence numbers
(0 &1) a 3-bit field: allows 7 outstanding frame at a time and
has 8 distinct sequence numbers (0..7)
Slide 192
192 Is SWS MaxSeqNum-1 ( MaxSeqNum is the number of available
sequence numbers) correct? if RWS = 1, then MaxSeqNum SWS+1 is
correct [the receiver will not buffer any frames that arrive out of
order] if RWS=SWS, then MaxSeqNum SWS+1 is incorrect example (
MaxSeqNum =8 & SWS =7) assumption 3-bit SeqNum field (sequence
numbers 0..7) SWS=RWS=7 scenario (we want to avoid this) the sender
transmits frames 0..6, successfully arrive, but ACK lost the
receiver is now expecting frames the 7, 0..5 (sequence numbers wrap
around) the sender times out and sends 0..6 SWS Send Window Size
RWS Receive Window Size
Slide 193
when RWS=SWS sending window size ( SWS ) can be no more than
half as big as the number of available sequence numbers SWS <
(MaxSeqNum+1)/2 general rule for arbitrary values of RWS and SWS
MaxSeqNum SWS+RWS (exercise) Implementation of Sliding Window (pp.
109-114 (4 th ed.), pp.111-117 (5 th ed.)) 193
Slide 194
Frame Order and Flow Control Sliding window protocol can be
used to serve three different roles reliably deliver frames across
an unreliable link (the core function of the algorithm) preserve
the order in which frames are transmitted this is easy to do at the
receiver - since each frame has a sequence number, the receiver
just makes sure that it does not pass a frame until it has already
passed up all frames with a smaller sequence number i.e., the
receiver buffers out-of-order frames 194
Slide 195
support flow control a feedback mechanism by which the receiver
is able to throttle the sender keep the sender from overrunning the
receiver, that is, from transmitting more data than the receiver is
able to process this is usually accomplished by augmenting the
sliding window protocol so that the receiver not only acknowledges
frames it has received, but also informs the sender of how many
frames it has room (free buffer space) to receive 195
Slide 196
196 2.6 Ethernet (802.3) History developed by Xerox Palo Alto
Research Center (PARC) in mid-1970s roots in Aloha packet-radio
network standardized by Xerox, DEC, and Intel in 1978 Ethernet is
standardized as IEEE 802.3
Slide 197
197 Ethernet a family of frame-based computer networking
technologies for local area networks (LANs) originally used a
shared coaxial cable winding around a building or campus to every
attached machine a working example of the carrier sense, multiple
access with collision detection (CSMA/CD) governed the way the
computers shared the channel
Slide 198
carrier sense means that all the nodes can distinguish between
an idle and a busy link collision detect means that a node listens
as it transmits and can therefore detect when a frame it is
transmitting has interfered (collide) with a frame transmitted by
another node 198
Slide 199
a multiple-access network meaning that a set of nodes send and
receive frames over a shared link Ethernet can be thought of as
being like a bus that has multiple stations plugged into it data
rates Ethernet 10 Mbps Fast Ethernet 100 Mbps Gigabit Ethernet
1,000 Mbps 199
Slide 200
CSMA/CD Shared Medium Ethernet Main procedure frame ready for
transmission is medium idle? If not, wait until it becomes ready
and wait the interframe gap period (9.6 s in 10 Mbit/s Ethernet)
start transmitting did a collision occur? If so, go to collision
detected procedure reset retransmission counters and end frame
transmission 200
Slide 201
Collision detected procedure continue transmission until
minimum packet time is reached (jam signal) to ensure that all
receivers detect the collision increment retransmission counter was
the maximum number of transmission attempts reached? If so, abort
transmission calculate and wait random backoff period based on
number of collision re-enter main procedure at stage 1 201
Slide 202
202 Physical Properties Ethernet segment implemented on a
coaxial cable of up to 500m (impedance of 50-ohm) this cable is
similar to the type used for cable TV, except cable TVs 75 ohms
Host connects to the Ethernet segment by tapping ( ) into it; taps
must be 2.5m apart
Slide 203
203 Transceiver a small device directly attached to the tap ( )
detects when the line is idle and drives the signal when the host
is transmitting receives incoming signals connects to an Ethernet
adaptor, which is plugged into the host Adaptor all the logic that
makes up the Ethernet protocol is implemented in the adaptor
Slide 204
Ethernet transceiver and adaptor
Slide 205
205 Repeater multiple Ethernet segments can be joined together
by repeaters a device that forwards digital signals, much like an
amplifier forwards analog signals no more than four repeaters may
be positioned between any pair of hosts an Ethernet has a total
reach of only 2,500m an Ethernet is limited to support a maximum of
1,024 hosts
Slide 206
Ethernet repeater
Slide 207
Example of Ethernet repeater use just two repeaters between any
pair of hosts a segment running down the spine of a building with a
segment on each floor 207
Slide 208
208 Any signal placed on the Ethernet is broadcast over the
entire network signal propagated in both directions repeater
forward the signal on all ongoing segments
Slide 209
209 terminator attached to the end of each segment absorb the
signal and keep it from bouncing back and interfering with trailing
signals Encoding Ethernet uses the Manchester encoding scheme
Slide 210
210 Coax cable evolution 50-ohm coax cable thick-net 10Base5 10
10Mbps Base baseband system 5 a given segment can be no longer than
500m
Slide 211
thin-net 10Base2 10 10Mbps 2 a given segment can be no longer
than 200m 10BaseT 10 10Mbps T twisted pair (Category 5 twisted
pair) limited to under 100m in length 211
Slide 212
212 100BaseT 100 100Mbps Category 5 twisted pair limited to
under 100m in length 1000BaseT 1000 1000Mbps Category 5 twisted
pair limited to under 100m in length
Slide 213
213 10BaseT several point-to-point segments coming out of a
multiway repeater (hub) Ethernet hub
Slide 214
214 Note Ethernet hub (or concentrator) a device for connecting
multiple twisted pair or fiber optic Ethernet devices together,
making them act as a single segment a form of multiport repeater
responsible for forwarding a jam signal to all ports if it detects
a collision
Slide 215
215
Slide 216
Varieties of Ethernet 216
Slide 217
217
Slide 218
218
Slide 219
219 Access Protocol Media Access Control (MAC) control access
to the shared Ethernet link implemented in hardware on the network
adaptor Ethernet is a bit oriented framing protocol Frame format
Preamble (64 bits) allows the receiver to synchronize with the
signal a sequence of alternating 0s and 1s
Slide 220
Dest addr & Src addr (48 + 48 bits) both the source and
destination hosts are identified with a 48-b address Type (16 bits)
demultiplexing key it identifies to which of possibly many higher-
level protocols this frame should be delivered Body 46 (used to
detect a collision) to 1,500 bytes of data 220
Slide 221
CRC (32 bits) each frame includes a 32-bit CRC from the host
perspective, an Ethernet frame a 14- byte header two 6-byte
addresses [Dest addr + Src addr] a 2-byte type field [Type] the
sending adaptor attaches the preamble, CRC, and postamble before
transmitting, and the receiving adaptor removes them 221
Slide 222
222 Ethernet a bit-oriented framing protocol Ethernet frame
format
Slide 223
223 Addresses The address belongs to the adaptor, not the host,
it is actually burned into ROM Addresses a sequence of six numbers
(48 bits, hexadecimal digits) separated by colons, example:
8:0:2b:e4:b1:2 00001000 00000000 00101011 11100100 10110001
00000010 unique: 48-bit unicast address assigned to each adapter
broadcast: all 1 s multicast: first bit is 1
Slide 224
Each frame transmitted on an Ethernet is received by every
adaptor connected to that Ethernet each adaptor recognizes those
frames addressed to its address and passes only those frames on to
the host an adaptor can also be programmed to run in promiscuous
mode, in which case it delivers all received frames to the host,
but this is not the normal mode 224
Slide 225
Summary an Ethernet adaptor receives all frames and accepts
frames addressed to its own address frames addressed to the
broadcast address frames addressed to a multicast address, if it
has been instructed to listen to that address all frames, if it has
been placed in promiscuous mode an Ethernet adaptor passes to the
host only the frames that it accepts 225
Slide 226
226 Transmit Algorithm When the adaptor has a frame to send if
line is idle send immediately (no negotiation with the other
adaptors) upper bound message size of 1500 bytes (means that the
adaptor can occupy the line for only a fixed length of time) must
wait 9.6 s after the end of one frame before beginning to transmit
the next frame if line is busy wait until idle and transmit
immediately
Slide 227
227 Ethernet is said to be a 1-persistent protocol because an
adaptor with a frame to send transmits with probability 1 whenever
a busy line goes idle In general, a p-persistent algorithm
transmits with probability 0 p 1 after a line becomes idle, and
defers with probability q = 1- p (there might be multiple adaptors
waiting for the busy line to become idle, and we dont want all of
them to begin transmitting at the same time)
Slide 228
Because there is no centralized control it is possible for two
(or more) adaptors to begin transmitting at the same time either
because both found the line to be idle or because both had been
waiting for a busy line to become idle when this happens, the two
(or more) frames are said to collide on the network 228
Slide 229
229 If an adaptor detects that its frame is colliding with
another transmits a 32-bit jamming sequence and then stop
transmitting frame a transmitter will minimally send 64-bit
preamble plus 32-bit jamming sequence = 96 bits (runt frame) in the
case of collision The worst case scenario (the two hosts are at
opposite ends of the Ethernet) transmitter needs to send 512 bits
to make sure that the frame it just sent did not collide with
another frame 512 bits is the minimum frame size 64 bytes (14 bytes
header + 46 bytes of data + 4 bytes of CRC)
Slide 230
230 scenario (a) A sends a frame at time t (b) the first bit of
As frame arrives at B at time t+d (d is link latency) (c) suppose
an instant before host As frame arrives, B still sees an idle line
and begins transmitting at time t+d and collides with As frame
(this collision is detected by host B) (d) Bs runt (32-bit jamming
sequence) frame arrives at A at time t+2d A will not know that the
collision occurred until Bs runt reaches it A must transmit for 2 d
to be sure that it detects all possible collisions
Slide 231
231 Worst-case scenario: (a) A sends a frame at time t; (b) As
frame arrives at B at time t + d; (c) B begins transmitting at time
t + d and collides with As frame; (d) Bs runt (32-bit) frame
arrives at A at time t + 2d
Slide 232
232 round-trip delay and try again each time it tries to
transmit but fails, the adaptor doubles the amount of time it waits
before trying again (exponential backoff) 1 st collision delay : 0
or 51.2 s (round-trip delay, which on a 10-Mbps Ethernet
corresponds to 512 bits), selected at random 2 nd collision delay:
0, 51.2, 102.4, or 153.6 s, selected at random (k=0..2 2 1) n th
collision delay: k x 51.2 s, for randomly selected k=0..2 n 1
adaptor typically retry up to 16 times
Slide 233
2.7 Wireless Wireless technologies differ in a variety of
dimensions how much bandwidth they provide how far apart
communicating nodes can be which part of the electromagnetic
spectrum they use (including whether it requires a license) how
much power they consume (important for mobile nodes) 233
Slide 234
234 Overview of Leading Wireless Technologies Bluetooth
802.15.1 Wi-Fi 802.11WiMAX 802.16 3G Cellular Typical link length
10m100m10kmTens of km Typical bandwidth 2 Mbps (shared) 54 Mbps
(shared) 70 Mbps (shared) 384+ Kbps (per connection) Typical
useLink a peripheral to a notebook computer Link a notebook
computer to a wired base Link a building to a wired tower Link a
cell phone to a wired tower Wired technology analogy
USBEthernetCoaxial cable DSL
Slide 235
The most widely used wireless links today are usually
asymmetric, i.e., the two endpoints are usually different kinds of
nodes base station usually has no mobility, but has a wired (or at
lease high bandwidth) connection to the Internet or other networks
client node often mobile, and relies on its link to the base
station for all its communication with other nodes 235
Slide 236
A Wireless Network using a Base Station 236
Slide 237
This topology implies three qualitatively different levels of
mobility 1 st level no mobility such as when a receiver must be in
a fixed location to receive a directional transmission from the
base station example: the Initial version of WiMAX 237
Slide 238
WiMAX Consumer Last Mile 238
Slide 239
WiMAX Nomadic / Portable 239
Slide 240
2 nd level mobility within the range of a base station example:
Bluetooth 3 rd level mobility between base stations examples: cell
phones and Wi-Fi 240
Slide 241
A Wireless Ad Hoc or Mesh Network An alternative topology is
the mesh or ad hoc network nodes are peers (i.e., there is no
special base station node) messages may be forwarded via a chain of
peer nodes as long as each node is within range of the preceding
node 241
Slide 242
242
Slide 243
This allows the wireless portion of a network to extend beyond
the limited range of a single radio this allows a shorter-range
technology to extend its range and potentially compete with a
longer range technology Meshes also offer fault tolerance by
providing multiple routes for a message to get from point A to
point B 243
Slide 244
IEEE 802.11 / Wi-Fi [/ WA FA /] Wireless Fidelity ( ) A
wireless-technology brand owned by the Wi-Fi alliance Promotes
standards with the aim of improving the interoperability of
wireless local area network products based on the IEEE 802.11
standards
Slide 245
Common applications for Wi-Fi Internet and VoIP phone access,
gaming network connectivity for consumer electronics such as
televisions, DVD players, and digital cameras Wi-Fi Alliance a
consortium of separate and independent companies agrees on a set of
common interoperable products based on the family of IEEE 802.11
standards
Slide 246
IEEE 802.11 Standard and Amendments IEEE 802.11 - The WLAN
standard was original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and
infrared [IR] standard (1997), all the others listed below are
Amendments to this standard, except for Recommended Practices
802.11F and 802.11T. IEEE 802.11a - 54 Mbit/s, 5 GHz standard
(1999, shipping products in 2001) IEEE 802.11b - Enhancements to
802.11 to support 5.5 and 11 Mbit/s (1999) IEEE 802.11c Bridge
operation procedures; included in the IEEE 802.1D standard (2001)
IEEE 802.11d - International (country-to-country) roaming
extensions (2001) IEEE 802.11e - Enhancements: QoS, including
packet bursting (2005) IEEE 802.11f - Inter-Access Point Protocol
(2003) Withdrawn February 2006 IEEE 802.11g - 54 Mbit/s, 2.4 GHz
standard (backwards compatible with b) (2003) IEEE 802.11h -
Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
IEEE 802.11i - Enhanced security (2004) IEEE 802.11j - Extensions
for Japan (2004) IEEE 802.11-2007 - A new release of the standard
that includes amendments a, b, d, e, g, h, i & j. (July
2007)
Slide 247
IEEE 802.11k - Radio resource measurement enhancements (2008)
IEEE 802.11n - Higher throughput improvements using MIMO (multiple
input, multiple output antennas) (September 2009) IEEE 802.11p -
WAVE Wireless Access for the Vehicular Environment (such as
ambulances and passenger cars) (working June 2010) IEEE 802.11r -
Fast roaming Working "Task Group r" - (2008) IEEE 802.11s - Mesh
Networking, Extended Service Set (ESS) (working September 2010)
IEEE 802.11T Wireless Performance Prediction (WPP) - test methods
and metrics Recommendation cancelled IEEE 802.11u - Interworking
with non-802 networks (for example, cellular) (working September
2010)
Slide 248
IEEE 802.11v - Wireless network management (working June 2010)
IEEE 802.11w - Protected Management Frames (September 2009) IEEE
802.11y - 3650-3700 MHz Operation in the U.S. (2008) IEEE 802.11z -
Extensions to Direct Link Setup (DLS) (August 2007 - December 2011)
IEEE 802.11aa - Robust streaming of Audio Video Transport Streams
(March 2008 - June 2011) IEEE 802.11mb Maintenance of the standard.
Expected to become 802.11-2011. (ongoing) IEEE 802.11ac - Very High
Throughput
