introduction to computer networks
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Introduction To Computer Networks
Presented ByMd. Asadul Islam
Lecturer,Dept. Of CSE,KUET
1
Referances
1. Computer Networking By F.Kuross & W.Ross
2. Computer Networks By Tanenbaum
3. Data Communication and Networking By A.Forouzan
4. And WWW.
2
Network A network can be defined as a number of
autonomus device connected together in such a way that they can share resources.
The purpose of a network is to share resources A resource may be:
– A file– A folder– A printer– A disk drive– Or just about anything else that exists on a computer.
3
Network(More defination) A network is simply a collection of computers or
other hardware devices that are connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate.
Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise working with networks and network technologies.
4
Applications of Networks Resource Sharing
Hardware (computing resources, disks, printers) Software (application software)
Information Sharing Easy accessibility from anywhere (files, databases) Search Capability (WWW)
Connectivity and Communication Email Message broadcast
Remote computing Distributed processing (GRID Computing) or Performance
Enhancement and Balancing Internet Access Data Security and Management Entertainment
5
Fundamental Network Classifications Local Area Networks (LANs):
A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings
Metropolitan Area Network (MAN): Network in a City is call MAN (Metropolitan Area Network) Covered by even a large local area network (LAN) but smaller than
the area covered by a wide area network (WAN). It is also used to mean the interconnection of several local area
networks by bridging them with backbone lines. Wide Area Networks (WANs):
Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)
A network that uses routers and public communications links The largest and most well-known example of a WAN is the
Internet. WANs are used to connect LANs and other types of networks
together. 6
7
Packet Transmission Modes
• Unicast– Transmission to single specific receiver
• Broadcast– Transmission to all network nodes
• Multicast– Transmission to specific subset of nodes
• Anycast– Transmission to one of a specific subset of nodes
Network Topology Defines the way in which
computers, printers, and other devices are connected.
Describes the layout of the wire and devices as well as the paths used by data transmissions.
8
Bus Topology A bus is the simplest physical topology. It consists of
a single cable that runs to every workstation This topology uses the least amount of cabling, but
also covers the shortest amount of distance. Each computer shares the same data and address
path. With a logical bus topology, messages pass through the trunk, and each workstation checks to see if the message is addressed to itself. If the address of the message matches the workstation’s address, the network adapter copies the message.
9
Star & Tree Topology The star topology is the most
commonly used architecture in Ethernet LANs.
When installed, the star topology resembles spokes in a bicycle wheel.
Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.
Star Topology
Tree Topology10
Ring Topology A frame travels around the ring,
stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame.
The frame then continues around the ring until it finds the destination node, which takes the data out of the frame.
Single ring – All the devices on the network share a single cable
Dual ring – The dual ring topology allows data to be sent in both directions.
Ring Topology
Dual Ring Topology11
Mesh Topology
The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance.
It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions.
Implementing the mesh topology is expensive and difficult. Mesh Topology
12
Topology(cont.)
Topology Advantages Disadvantages
Bus Cheap. Easy to install. Difficult to reconfigure.
Break in bus disables
entire network.
Star Cheap. Easy to install.
Easy to reconfigure.
Fault tolerant.
More expensive than bus.
Ring Efficient. Easy to install. Reconfiguration difficult.
Very expensive.
Mesh Simplest. Most fault tolerant. Reconfiguration extremely difficult.
Extremely expensive.
Very complex.
Advantages and Disadvantages of Network Topologies
13
Intranet, Internet & Extranet Internet:
Is a worldwide system of computer networks The Internet is an open, public space.
Intranet: An intranet is a private network that is contained within an enterprise. It may consist of many interlinked local area networks and
also use leased lines in the wide area network. An intranet may be accessible from the Internet, but as a
rule it's protected by a password and accessible only to employees or other authorized users.
Extranet: Is a portion of an organization's Intranet accessible to authorized outside users without full access to
an entire organization's intranet. 14
1-15
What’s the Internet: “nuts and bolts” view
millions of connected computing devices: hosts = end systems – running network apps
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptop
cellular handheld
wiredlinks
access points
communication links fiber, copper, radio,
satellite transmission rate =
bandwidth
routers: forward packets (chunks of data)
Introduction 1-16
What’s the Internet: “nuts and bolts” view
protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype,
Ethernet Internet: “network of
networks” loosely hierarchical public Internet versus private
intranet Internet standards
RFC: Request for comments IETF: Internet Engineering Task
Force
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Introduction 1-17
What’s the Internet: a service view communication infrastructure
enables distributed applications: Web, VoIP, email, games, e-
commerce, file sharing communication services
provided to apps: reliable data delivery from
source to destination “best effort” (unreliable) data
delivery
Introduction 1-18
What’s a protocol?Protocols define format, order of msgs sent and received among network entities, and actions taken on msg
transmission, receipt a human protocol and a computer network protocol:
Q: Other human protocols?
Hi
Hi
Got thetime?2:00
TCP connection request
TCP connectionresponseGet http://www.awl.com/kurose-ross
<file>
time
Introduction 1-19
A closer look at network structure
• network edge: applications and hosts
access networks, physical media: wired, wireless communication links network core:
interconnected routers network of networks
Introduction 1-20
The network edge:• end systems (hosts):
– run application programs– e.g. Web, email– at “edge of network”
client/server
peer-peer
client/server model client host requests,
receives service from always-on server
e.g. Web browser/server; email client/server peer-peer model:
minimal (or no) use of dedicated servers
e.g. Skype, BitTorrent
Introduction 1-21
Physical Media
• Bit: propagates betweentransmitter/rcvr pairs
• physical link: what lies between transmitter & receiver
• guided media: – signals propagate in solid media:
copper, fiber, coax
• unguided media: – signals propagate freely, e.g.,
radio
Twisted Pair (TP)• two insulated copper
wires– Category 3: traditional
phone wires, 10 Mbps Ethernet
– Category 5: 100Mbps Ethernet
Introduction 1-22
Physical Media: coax, fiber
Coaxial cable:• two concentric copper
conductors• bidirectional• baseband:
– single channel on cable– legacy Ethernet
• broadband:– multiple channels on cable– HFC
Fiber optic cable: glass fiber carrying light pulses,
each pulse a bit high-speed operation:
high-speed point-to-point transmission (e.g., 10’s-100’s Gps)
low error rate: repeaters spaced far apart ; immune to electromagnetic noise
Introduction 1-23
Physical media: radio
• signal carried in electromagnetic spectrum
• no physical “wire”• bidirectional• propagation environment
effects:– reflection – obstruction by objects– interference
Radio link types: terrestrial microwave
e.g. up to 45 Mbps channels
LAN (e.g., Wifi) 11Mbps, 54 Mbps
wide-area (e.g., cellular) 3G cellular: ~ 1 Mbps
satellite Kbps to 45Mbps channel
(or multiple smaller channels)
270 msec end-end delay geosynchronous versus low
altitude
Introduction 1-24
The Network Core
• mesh of interconnected routers
• the fundamental question: how is data transferred through net?– circuit switching:
dedicated circuit per call: telephone net
– packet-switching: data sent thru net in discrete “chunks”
Introduction 1-25
Network Core: Circuit Switching
End-end resources reserved for “call”
• link bandwidth, switch capacity
• dedicated resources: no sharing
• circuit-like (guaranteed) performance
• call setup required
Introduction 1-26
Network Core: Circuit Switchingnetwork resources (e.g., bandwidth) divided into “pieces”• pieces allocated to calls• resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces”
frequency division: total frequency bands are divided into several users eg : television broad casting
time division: total available time is divided into several user eg: telephone system
wdm: Total wave lengnth is divided in to number of users eg: optical networking
Introduction 1-27
Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
Introduction 1-28
Numerical example
• How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?– All links are 1.536 Mbps– Each link uses TDM with 24 slots/sec– 500 msec to establish end-to-end circuit
Let’s work it out!
Introduction 1-29
Network Core: Packet Switching
each end-end data stream divided into packets
• user A, B packets share network resources
• each packet uses full link bandwidth
• resources used as needed
resource contention: aggregate resource demand
can exceed amount available
congestion: packets queue, wait for link use
store and forward: packets move one hop at a time Node receives complete
packet before forwarding
Bandwidth division into “pieces”
Dedicated allocationResource reservation
Introduction 1-30
Packet Switching: Statistical Multiplexing
Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
A
B
C100 Mb/sEthernet
1.5 Mb/s
D E
statistical multiplexing
queue of packetswaiting for output
link
Introduction 1-31
Packet-switching: store-and-forward
• takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
• store and forward: entire packet must arrive at router before it can be transmitted on next link
• delay = 3L/R (assuming zero propagation delay)
Example:• L = 7.5 Mbits• R = 1.5 Mbps• transmission delay = 15
sec
R R RL
more on delay shortly …
Introduction 1-32
Packet switching versus circuit switching
• 1 Mb/s link• each user:
– 100 kb/s when “active”– active 10% of time
• circuit-switching: – 10 users
• packet switching: – with 35 users, probability >
10 active at same time is less than .0004
Packet switching allows more users to use network!
N users
1 Mbps link
Q: how did we get value 0.0004?
33
Network Core: Packet Switching
Packet-switching: Pipelining Discard error packet Carry packet header
34
Differences Between Circuit & Packet Switching
Circuit-switching Packet-Switching
Guaranteed capacity No guarantees (best effort)
Capacity is wasted if data is bursty
More efficient
Before sending data establishes a path
Send data immediately
All data in a single flow follow one path
Different packets might follow different paths
No reordering; constant delay; no pkt drops
Packets may be reordered, delayed, or dropped
35
Types of ISPs
• Tier-1 ISPs: Backbone networks
• Tier-2 ISPs: National coverage
• Tier-3 ISPs: Directly attached to customers
Tier3
Tier2
Tier1
PP
PP
P
P
PP
P
P
P
ISPInterconnection of ISP
36
Delay in packet-switched networks
packets experience delay on end-to-end path
• four sources of delay at each hop
• nodal processing: – check bit errors– determine output link
• queueing– time waiting at output link for
transmission – depends on congestion level of
router
A
B
propagation
transmission
nodalprocessing queueing
37
Delay in packet-switched networks
Transmission delay:• R=link bandwidth (bps)• L=packet length (bits)• time to send bits into link = L/R
Propagation delay:• d = length of physical link• s = propagation speed in
medium (~2x108 m/sec)• propagation delay = d/s
A
B
propagation
transmission
nodalprocessing queueing
Note: s and R are very different quantitites!
38
Queueing delay (revisited)
• R=link bandwidth (bps)• L=packet length (bits)• a=average packet arrival
rate
traffic intensity = La/R
La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay
infinite!
Introduction 1-39
Why layering?Dealing with complex systems:• explicit structure allows identification, relationship of
complex system’s pieces– layered reference model for discussion
• modularization eases maintenance, updating of system– change of implementation of layer’s service
transparent to rest of system– e.g., change in gate procedure doesn’t affect
rest of system• layering considered harmful?
Introduction 1-40
Internet protocol stack• application: supporting network
applications– FTP, SMTP, HTTP
• transport: process-process data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
Introduction 1-41
ISO/OSI reference model• presentation: allow applications to
interpret meaning of data, e.g., encryption, compression, machine-specific conventions
• session: synchronization, checkpointing, recovery of data exchange
• Internet stack “missing” these layers!– these services, if needed, must be
implemented in application– needed?
Application
Presentation
session
Transport
Network
link
physical
TCP/IP Referances Model
42
Introduction 1-43
source
application
transportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
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