ECE 4371, Fall, 2009
Department of Electrical and Computer Engineering
Multiaccess vs. Point
Multiaccess means shared medium.
systems share the same physical
communication resources (
wire, frequency, ...)
There must be some arbitration mechanism.
only 2 systems involved
no doubt about where data came from !
Local Area Network
Connects computers that are physically close together ( < 1
10 Mbps, 100Mbps
Wide Area Network
Connects computers that are physically far apart. “long
Metropolitan Area Network
Larger than a LAN and smaller than a WAN
Connection of 2 or more distinct (possibly dissimilar) networks.
Requires some kind of network device to facilitate the
ISO/OSI Reference Model
To address the growing tangle of incompatible
proprietary network protocols, in 1984 the ISO formed a
committee to devise a unified protocol standard.
The result of this effort is the ISO
Interconnect Reference Model
The ISO’s work is called a reference model because
virtually no commercial system uses all of the features
precisely as specified in the model.
The ISO/OSI model does, however, lend itself to
understanding the concept of a unified communications
ISO/OSI Reference Model
The OSI RM contains seven protocol layers, starting with physical
media interconnections at Layer 1, through applications at Layer 7.
OSI model defines only the functions of each of the seven layers
and the interfaces between them.
details are not part
of the model.
ISO/OSI Reference Model: Physical Layer
The Physical layer receives a stream of
bits from the Data Link layer above it,
encodes them and places them on the
The Physical layer conveys
transmission frames, called
Protocol Data Units
Each physical PDU carries an address
and has delimiter signal patterns that
, or contents, of
mechanical and electrical interfaces
time per bit
Process of varying a carrier signal
in order to use that signal to
Carrier signal can transmit far
away, but information cannot
Modem: amplitude, phase, and
Analog: AM, amplitude, FM,
frequency, Vestigial sideband
Digital: mapping digital
information to different
ISO/OSI Reference Model: Data Link
The Data Link layer negotiates frame sizes
and the speed at which they are sent with
the Data Link layer at the other end.
The timing of frame transmission is
Data Link layers at both ends acknowledge
packets as they are exchanged. The sender
retransmits the packet if no
acknowledgement is received within a given
Medium Access Control
(dividing data into chunks)
header & trailer bits
Alice and Bob on their cell phones
Both Alice and Bob are talking
What if Alice couldn’t understand Bob?
Bob asks Alice to repeat what she said
What if Bob hasn’t heard Alice for a while?
Is Alice just being quiet?
Or, have Bob and Alice lost reception?
How long should Bob just keep on talking?
Maybe Alice should periodically say “uh huh”
… or Bob should ask “Can you hear me now?”
Block diagram of TDM system.
ISO/OSI Reference Model: Network
At the originating computers, the
Network layer adds addressing
information to the Transport layer
The Network layer establishes the
route and ensures that the PDU size
is compatible with all of the equipment
between the source and the
Its most important job is in moving
London Metro Map
ISO/OSI Reference Model: Transport
the OSI Transport layer provides end
end acknowledgement and error correction
through its handshaking with the Transport
layer at the other end of the conversation.
The Transport layer is the lowest layer
of the OSI model at which there is any
awareness of the network or its
Transport layer assures the Session layer
that there are no network
induced errors in
error detection: CRC
Add one bit so that xor of all bit is zero
Send, correction, miss
Add vertically or horizontally
Applications: ASCII, Serial port transmission
ISO/OSI Reference Model: Session
The Session layer arbitrates the dialogue
between two communicating nodes,
opening and closing that dialogue as
It controls the direction and mode (
It also supplies recovery
Checkpoints are issued each time a block
of data is acknowledged as being received
in good condition.
establishes, manages, and terminates sessions
service location lookup
ISO/OSI Reference Model: Presetation
The Presentation layer provides
level data interpretation
services for the Application
layer above it, such as
Presentation layer services are
also called into play if we use
encryption or certain types of
Shift Cipher (Caesar’s Cipher)
I CAME I SAW I CONQUERED
H BZLD H TZV H BNMPTDSDC
Julius Caesar to communicate with his army
Language, wind talker
ISO/OSI Reference Model
The Application layer supplies
meaningful information and services to
users at one end of the communication
and interfaces with system resources
(programs and data files) at the other
end of the communication.
All that applications need to do is to send
messages to the Presentation layer, and
the lower layers take care of the hard
application level protocols
appropriate selection of “type of service”
anything not provided by any of the other
TCP/IP is the de facto
It has a lean 3
protocol stack that can be
mapped to five of the
seven in the OSI model.
TCP/IP can be used with
any type of network, even
different types of networks
within a single session.
The IP Layer of the TCP/IP
protocol stack provides
essentially the same
services as the Network
and Data Link layers of the
OSI Reference Model.
It divides TCP packets into
protocol data units called
, and then
The concept of the datagram was fundamental to the
robustness of ARPAnet, and now, the Internet.
Datagrams can take any route available to them
without human intervention.
TCP/IP Current and Future
The current version of IP, IPv4, was never designed to
serve millions of network components scattered across
It limitations include 32
bit addresses, a packet length
limited to 65,635 bytes, and that all security measures
Furthermore, network addresses have been assigned
with little planning which has resulted in slow and
cumbersome routing hardware and software.
We will see later how these problems have been
addressed by IPv6.
Layering & Headers
Each layer needs to add some control information to the data to do it’s job.
This information is typically pre
pended to the data before being given to the
Once the lower layers deliver the data and control information
the peer layer
uses the control information.
Protocols and networks in the TCP/IP model
How a call is made?
Physical: Language between two machines
Link: communication between machines on the same
Network: communication between machines on possibly
Transport: communication between processes (running on
machines on possibly different networks).
data link layer
network layer and above.
IEEE 802 Standards
The 802 working groups. The important ones are marked with *. The ones
are hibernating. The one marked with † gave up.
Wireless Network Tutorial.
First Chapter of my book, in print by Cambridge University Press
Wireless Metropolitan Area Network (WMAN): WIMAX
Wireless Local Area Network (WLAN): WIFI
Wireless Personal Area Network (WPAN)
Ultra Wide Band (UWB)
Ad Hoc Networks
Cognitive Radio Networks
(Local Area Network)
(Wide Area Network)
(Metropolitan Area Network)
Bluetooth < 1 Mbps
b: 11 to g: 54 Mbps
Last Mile Access
Cellular Networks Road Map
: high speed circuit switched data: multiple time slots, realtime 57.6 kbps
: general packet radio service: non
realtime, 171.2 kbps
: Enhanced Data
Rates for GSM Evolution
Multiple modulation and
coding schemes (MCS)
New hardware 384kbps
: multiple code
Medium data rate (MDR)
Up to 8 codes, 64kbps
Easy to update
3G Standards Comparison
3GPP and 3GPP2
CDMA2000: easy to upgrade. WCDMA: compatible with GSM
Replace cable or low speed fiber in the last mile
Comparison of 802.11 Standards
g is back compatible with b. but b is supported by Intel
CDMA vs. OFDM
Free WIFI only in Boise airport.
Contention based multiple access
Personal Area Networks
mix of multiple
2.4GHz band with
Spread spectrum frequency
“always on” user
Combination of packet
switching & circuit
switching (good for
data & voice)
3 voice channels
Low power, low cost
Transparently connects “office” devices
Laptop, Desktop, PDA, Phone, printer
Bridging capability: network
: low power devices
Ultra Wide Band
High speed at short range:
480 Mb/s at ~3m. Does not penetrate walls
Very low power
CDMA vs. OFDM
cellular wireless technologies all competing for
802.11 WLANs offer “hotspots” at nominal cost (sometimes “free”)
Cellular services used worldwide
802.15 offers bridging options for WLAN and cellular services
Alliances, Partnerships, Coalitions,…
AT&T, Intel, IBM (and investors) form “Cometa”, a company to provide
wireless hot spots across the country
Motorola, Proxim and Avaya form partnership to provide seamless
roaming between WiFi and cellular networks
HP and Transat Technologies collaborating on project to link 2G/3G to
Speed and Range
Ad Hoc Network
Mobile Ad Hoc Networks (MANETs)
collection of mobile users that communicate over
relatively bandwidth constrained wireless links.
Since the nodes are mobile, the network
topology may change
and unpredictably over time.
The network is
, where all network activity including
discovering the topology and delivering messages must be executed by
the nodes themselves. MANETs need efficient distributed algorithms to
determine network organization, link scheduling, and routing.
The set of applications for MANETs is
, ranging from
small, static networks that are constrained by power sources, to large
scale, mobile, highly dynamic networks
In a military environment, preservation of
, latency, reliability,
intentional jamming, and recovery from failure are significant concerns
Ad hoc mode of WIFI
Wireless Sensor Network
Consists of a number of sensors spread across a geographical area.
sensor has wireless communication capability and some level of intelligence
for signal processing and networking of the data
Military sensor networks to detect and gain as much information as
possible about enemy movements, explosions, and other phenomena of
Sensor networks to detect and characterize Chemical, Biological,
Radiological, Nuclear, and Explosive (CBRNE) attacks and material.
Sensor networks to detect and monitor environmental changes in plains,
forests, oceans, etc.
Wireless traffic sensor networks to monitor vehicle traffic on highways
or in congested parts of a city.
Wireless surveillance sensor networks for providing security in
shopping malls, parking garages, and other facilities.
Wireless parking lot sensor networks to determine which spots are
occupied and which are free.
Wireless Sensor Networks
Large number of (mostly stationary) sensors
Aside from the deployment of sensors on the
ocean surface or the use of mobile, unmanned, robotic sensors in military operations, most
nodes in a smart sensor network are stationary.
Networks of 10,000 or even 100,000 nodes are
envisioned, so scalability is a major issue.
Low energy use
Since in many applications the sensor nodes will be placed in a remote area,
service of a node may not be possible.
In this case, the lifetime of a node may be determined
by the battery life, thereby requiring the minimization of energy expenditure.
Given the large number of nodes and their potential placement in
hostile locations, it is essential that the network be able to self
organize; manual configuration
is not feasible.
Moreover, nodes may fail (either from lack of energy or from physical
destruction), and new nodes may join the network.
Therefore, the network must be able to
periodically reconfigure itself so that it can continue to function.
Individual nodes may
become disconnected from the rest of the network, but a high degree of connectivity must be
Collaborative signal processing
Yet another factor that distinguishes these networks from
MANETs is that the end goal is detection/estimation of some events of interest, and not just
To improve the detection/estimation performance, it is often quite useful to
fuse data from multiple sensors.
This data fusion requires the transmission of data and control
messages, and so it may put constraints on the network architecture.
A user may want to query an individual node or a group of nodes for
information collected in the region.
Depending on the amount of data fusion performed, it
may not be feasible to transmit a large amount of the data across the network.
local sink nodes will collect the data from a given area and create summary messages.
query may be directed to the sink node nearest to the desired location.
Inefficient usage and over crowded of some spectrums
Can change modulation
carrier frequency to