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Chapter 12
Networking, and
Distributed
Computing
Introduction
• Networking of computers has profoundly
changed the way that we interact and conduct
business:
• Internet
• World Wide Web
• Business
• Software and application designs
• Gaming
• Social Media & interactions
Introduction
– the central role of networking in today’s computer
systems.
– various networking types and topologies.
– the TCP/IP protocol stack.
– the capabilities of TCP/IP’s application, transport,
network and link layers.
– protocols such as HTTP, FTP, TCP, UDP, IP and IPv6.
– network hardware and hardware protocols such as
Ethernet and Wireless 802.11.
– the client/server networking model.
Distributed systems
– Remote computers cooperate via a network to
appear as a local machine
– Users are given the impression that they are
interacting with just one machine
– Spread computation and storage throughout a
network of computers
– Applications are able to execute code on local
machines and remote machines and to share data,
files and other resources among these machines
Ethernet
• Networking technologies for local area (LAN) and
larger networks.
• It was commercially introduced in 1980 while it
was first standardized in 1983 as IEEE 802.3, and
has since been refined to support higher bit rates
and longer link distances.
• Networking: Allowing computers to communicate
and exchange information
– Media (cables, wires, frequency, etc.)
– Protocol is set of rules:
• rules for sharing access to the media
• Format for packaging data with control information
Network Types
• Local area network (LAN)
– Limited geographic dispersion and designed
to optimize data transfer rates between its
hosts
– Interconnect resources using high-speed
communication paths with optimized network
protocols for local area environments
– Error rates lower than those of larger
networks
– Greater management flexibility
– Independence from constraints of public
networking system
• Wide area networks (WANs)
– Broader than LAN, connecting two or more
LANs
– Example: the Internet
– Generally employ mesh topology
– Operate at slower speeds than LANs and have
Network Topologies
Ethernet Basics
• Ethernet is contention-based
• Shared media (originally a cable)
• Stations may compete for access to shared
media
• Roots in WWII radio network
• Carrier-Sense (listen before talk)
• Multiple-Access (multiple stations)
• Collision-Detection (listen while transmitting
to detect collisions)
• CSMA-CD (Carrier Sense Multiple Access with Collision
detection)
CSMA-CD Protocol
• Carrier Sense Multiple Access/Collision Detect
(CSMA/CD) is the protocol for carrier
transmission access in Ethernet networks.
• How are collisions possible?
– Listening is not mutually exclusive
– Multiple stations listen at same time, detect idle,
transmit
• What happens when two or more stations
transmit at the same time:
– Collision – signals on the media are
distorted/garbled
Collision
• When PC-D tries to
initiate/send a data to PC-A
via hub (a half-duplex device)
through it’s connected
channel to hub, if some other
data traffic of some other PC
in the network is already
coming down in the same
channel through the hub
down to PC-A, then the data
hit each other and form a
collision.
• 
How does a CSMA/CD detects and
avoids data collision ?
Since, CSMA/CD is a Media Access Control mechanism,
whenever any PC wants to send a data, it first sends out a
dummy electrical signal into the channel to check whether any
incoming data traffic is coming down or the line is free.
If the traffic is already coming down, then the dummy
signal collides with the incoming traffic and sends
back a jam signal to the PC so that, the PC is
notified to hold its traffic for a random amount of time
before attempting to send the data again.
This process will continue until the line/channel becomes free.
The moment line is free, the PC immediately initiates/send it's
data to the intended destination. This is how the data collision is
avoided in a half-duplex channels.
Propagation delay
• Propagation delay: is a technical term that can
have a different meaning depending on the
context. It can relate to networking, electronics
or physics. In general it is the length of time
taken for the quantity of interest to reach its
destination.
• Is a time required for signal to go the distance.
• How long depends on how many cable segments
and the delay of the repeaters that connect
segments.
Propagation delay
• Detecting collisions. Worst case is
Transmit
Collision
Detect
• Twice the propagation delay
• Of the longest circuit =21.6ns
• Must add in delay of repeaters
• 8 repeaters @ 4.5ns each = 36ns
Propagation delay
• 57.6 seconds worst case for detection
Transmit
Collision
Detect
Collision
• Sender only listens while transmitting, so
• Transmission must last long enough to
catch the worst-case for a collision
• =57.6ns
Propagation delay
• 57.6 ns seconds worst case for detection
Transmit
Collision
Detect
•
•
•
•
Transmitting at 10million bits/second
Speed of original Ethernet
57.6ns X 10Mbps = 576 bits
Minimum frame length is 576 bits!
Propagation delay
• Propagation delay: time required for signal to
go the distance.
• How long depends on how many cable
segments and the delay of the repeaters that
connect segments.
• Original thick cable Ethernet: 5 segments of
500 meters each = 2500 meters
• At 77% of speed of light
– 0.77 X 3 X 108 meters per second
• 2500 / 0.77 X 3 X 108 = 10.825 nanoseconds
(billionths of a second)
Propagation delay
• Detecting collisions. Worst case is
T
C
D
• Twice the propagation delay
10Base5 – thick wire
•
•
•
•
•
10 million bits per second
2500 meter distance (plus repeaters)
576 bit minimum frame size
At constant fraction of speed-of-light
To run at 100 million bits per second, with
repeaters that are 10 times as fast: the 576
bits will be on the network 1/10th the time, so
the distance to detect collisions must be
1/10th the distance, or 250 meters
Other media
• 10Mbps, baseband signaling, media
– (broadband implies FDM)
• 10Base2 – thin wire Ethernet (like cable TV
size cable)
• 10BaseT –
– Ethernet on twisted-pair, rather than cable.
– Star topology – all wires run to an Ethernet
concentrator or hub, now use fast switches.
Other MAC
• MAC – Media Access Control
• Ethernet uses CSMA/CD: collisions and
retransmittion reduce the realized BW from
the theoretical (about half)
• Other strategies include
• Token-Passing
– Possession of the token grants the right to
transmit
– Token must be passed to the next machine after
some period
Token Passing
• An IBM protocol, and ARC-NET
• Possession of the token grants the right to
transmit
• Token must be passed to the next machine
after some period
• Token is passed in a known order
• Observations: fair, every machine gets a turn;
less overhead, no collisions or retransmission
• So, why did Ethernet become a widespread
“standard”?
Token Passing
• Token
– When machine owns the token, it generates data, places it
in the frame and sends the frame to its neighbor
– Each machine forwards the token until it reaches its
destination
• At the destination, the machine
– Copies the content of the message
– Marks the frame as having been delivered
– Passes frame to its neighbor
• When the original sender receives the frame, it
– Removes the message from the frame
– Passes the token to its neighbor
Token Passing
Internetworking & TCP/IP
• TCP/IP protocol stack
– Composed of four logical levels called layers
• Application layer
– Highest level
– Provides protocols for applications to communicate
• Transport layer
– End-to-end communication
– Relies on network layer to determine proper path from one end of
communication to the other
• Network layer
– Moving data between computers
• Link layer
– Provides an interface between the network layer and the
underlying physical medium of the connection
Internetworking
• How to interconnect Local Area Networks
• If same type (frame, MAC strategy), and the same media,
can use a repeater
• If the same type, but different media, can use a bridge
• If different type (different frame, MAC strategy – I.e.
token to Ethernet, need to convert the protocol –
requires more logic; router, switch, gateway
• Additional features filter down to lower devices, so
functions change – moving target.
OSI (Open Systems Interconnection)
• Data Link Layer
• Layer 2 in the OSI Reference Model
• Translates and passes data between Physical
Layer and Network Layer
• Ethernet operates at this level
• Has two sublayers, based on functionality
– Logical Link Control (LLC)
– Media Access Control (MAC)
•
Logical Link Control (LLC) Sublayer
•
•
•
•
IEEE 802.2
Interfaces with Network Layer protocols (e.g., IPv4, IPv6, ICMP)
Translates IP packet into a frame
Performs error checking and control—requests retransmission of frames that fail error
checking
– Error checking is performed by destination node
– Frame is discarded and retransmitted
Manages flow control between nodes
NOTE: Flow control and error checking is also performed by Transport Layer protocols (e.g.,
•
•
TCP/IP Model Vs. OSI Model
Transmission Control Protocol (TCP)
is used by Internet applications like
email, world wide web, FTP, etc.
TCP/IP was developed by the
Department of Defense (DOD) to
connect various devices to a
common network (Internet).
The main purpose behind
developing the protocol was to build
a robust and automatically
recovering phone line failure while
on the battlefield. On the other hand,
Open Systems Interconnection (OSI)
was developed by the International
Organization for Standardization
(ISO). This model was made up of
two components, namely, sevenlayer model and the subset of
protocols.
OSI Reference Model
TCP/IP
Application Layer
Presentation Layer
Application Layer
Session Layer
Transport Layer
Transport Layer
Network Layer
Internet Layer
Data Link Layer
Physical Layer
Link Layer
TCP/IP Protocol Stack
• TCP/IP protocol stack
– Composed of four logical levels called layers
• Application layer
– Highest level
– Provides protocols for applications to communicate
• Transport layer
– End-to-end communication
– Relies on network layer to determine proper path from one end of
communication to the other
• Network layer
– Moving data between computers
• Link layer
– Provides an interface between the network layer and the
underlying physical medium of the connection
TCP/IP Protocol
TCP/IP Protocol
TCP/IP
• TCP – Transmission Control Protocol
– OSI Transport Layer
– Connection-Oriented Strategy
• UDP – User Datagram protocol
– OSI Transport Layer
– Connectionless Strategy
• IP – Internet Protocol
– Network Layer
• Application layer software has been
standardized on top of TCP
• No real protocol at Session and Presentation
layer.
TCP/IP
• Evolved from defense department research –
funded,
• ARPANET, and experimental internetwork
World Wide Web
Uses the internet
Convenient way to share information, locate
information & web sites
Markup Language
• Markup Language: this is the language that specifies
codes that allow the developer to format the
appearance of web pages. The web browser must
understand and be able to interpret these codes,
which are standardized as HTML (Hyper-Text Markup Language). For instance, to designate that text is
to appear in bold, a web page might contain the
following line:
• The word <strong> BOLD</strong> is in bold text.
• The <strong> and </strong> are used to begin and
end the section of text that is to be displayed as in
bold typeface.
URL
• Uniform Resource Locator (URL): this is a label
for the address and location of page or resource
somewhere on the internet. This label is
translated to an actual IP address by servers on
the Internet. An example URL is;
• http://www.nsf.gov/
• which is the URL for the U.S. Government’s
National Science Foundation.
Hypertext
• Hypertext or Hyperlinks: These are embedded links
in a web page, that can be used link pages together.
• An example of a hyperlink that can be embedded in a
web page to link to the Linux operating system
organization’s home page:
• <A href=" http://www.linux.org/"> Linux </A>
• Hyperlink has a URL embedded within it between the
double-quotes (“).
• Note also the use of the markup-language < and > to
begin and end the html commands.
Client-Server
• Client-Server model of computing: This is the idea
that an application can be composed of parts that
may exist at different machines connected by the
Internet, that work together.
• In regards to the web, there are at least two
machines that work together:
– The user’s PC is the client
– A server with web pages or web applications.
• There are a number of other machines that work
behind the scenes to make the internet work, and to
handle functions like translating the URLs into IP
addresses.
DNS & DHCP
http://technet.microsoft.com/en-us/library/cc958921.aspx
Domain Name System (DNS) servers provide name
resolution for network clients.
DNS maintains (among other things) information that
links a computer's fully qualified domain name
(FQDN) to its assigned IP address(es).
Dynamic Host Configuration Protocol (DHCP)
Provides a powerful mechanism for automatically
configuring client IP addresses, until recently DHCP
did not notify the DNS service to update the DNS
records on the client; specifically, updating the
client name to an IP address, and IP address to
name mappings maintained by a DNS server.
DNS
• Because internet addresses are numbers, intelligible
names are assigned in place of internet addresses,
for our convenience in every day use.
• Domain Name System (DNS) does the translation.
• Example, the US President’s web site at
www.whitehouse.gov is translated to an IP address
of 92.123.72.136.
Parallel Computing
Enhances performance
Multiple “execution engines”
Multiple machines/computers
Multiple CPUs
Multiple core on a chip
Multiple pipelines
Multiple stages on a pipeline
Multiple Levels of Parallelism
Need for Parallel Processing
• Every generation of processor technology is
faster, with more transistors in a small space.
• Processor technologies often require less
voltage.
• Based on advances in manufacturing
transistors.
• Regardless of how fast our computers are, we
always seem to want more power and speed.
CPU Cycles
• Consider a personal computer (PC).
• When you are typing on a word processor, the CPU
doesn’t have much to do.
• CPU runs in millions or billions of cycles per second.
This is MUCH faster than human time can perceive.
• A PC CPU is typically wasting most of its processor
time.
• BUT, when we have lots of processing for the CPU to
do, we want it done ASAP.
• Processing cycles are so cheap, that we can buy fast
processors even when most of their power is wasted.
Multicore Technologies
• Each core is almost a separate processor.
• A 4-core processor is like having the power of 4
CPUs in one.
• More processors means more power and speed
(and more wasted CPU cycles, but they are so
cheap we don’t care).
• Multiple Core or multiple CPUs allow faster
completion of work by dividing the work
between more than one CPU.
Classic Model of Parallel Processing
• Multiple Processors
available (4)
• A Process can be divided
into serial and parallel
portions
• The parallel parts are
executed concurrently
• Serial Time: 10 time
units
• Parallel Time: 4 time
units