Transcript Computer networks

Unit – I
Introduction to Computer Networks
By
N.Gopinath
AP/CSE
Introduction to Computer Networks
Computer Networks
Computer network connects
two or more autonomous
computers.
The computers
geographically
anywhere.
can be
located
Introduction to Computer Networks
LAN, MAN & WAN
Network in small geographical Area (Room, Building or a
Campus) is called LAN (Local Area Network)
Network in a City is call MAN (Metropolitan Area Network)
Network spread geographically (Country or across Globe) is
called WAN (Wide Area Network)
Introduction to Computer Networks
Applications of Networks
Resource Sharing
Hardware (computing resources, disks, printers)
Software (application software)
Information Sharing
Easy accessibility from anywhere (files, databases)
Search Capability (WWW)
Communication
Email
Message broadcast
Remote computing
Distributed processing (GRID Computing)
Introduction to Computer Networks
Network Topology
The network topology
defines the way in which
computers, printers, and
other
devices
are
connected. A network
topology describes the
layout of the wire and
devices as well as the
paths used by data
transmissions.
Introduction to Computer Networks
Bus Topology
Commonly referred to as a
linear bus, all the devices
on a bus topology are
connected by one single
cable.
Introduction to Computer Networks
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.
Introduction to Computer Networks
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.
Introduction to Computer Networks
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.
Introduction to Computer Networks
Network Components
Physical Media
Interconnecting Devices
Computers
Networking Software
Applications
Introduction to Computer Networks
Networking Media
Networking media can be
defined simply as the
means by which signals
(data) are sent from one
computer
to
another
(either by cable or wireless
means).
Introduction to Computer Networks
Networking Devices
HUB,
Switches,
Wireless
Access
Modems etc.
Routers,
Points,
Introduction to Computer Networks
Computers: Clients and Servers
In a client/server network
arrangement,
network
services are located in a
dedicated computer whose
only function is to respond
to the requests of clients.
The server contains the
file, print, application,
security, and other services
in a central computer that
is continuously available to
respond to client requests.
Introduction to Computer Networks
Networking Protocol: TCP/IP
Introduction to Computer Networks
Applications
E-mail
Searchable Data (Web Sites)
E-Commerce
News Groups
Internet Telephony (VoIP)
Video Conferencing
Chat Groups
Instant Messengers
Internet Radio
Network Architecture
• Provides a general, effective, fair, and robust connectivity of
computers
• Provides a blueprint
– Types
• OSI Architecture
• Internet Architecture
OSI ARCHITECTURE
• Open Systems Interconnection (OSI) model is a reference model developed
by ISO (International Organization for Standardization) in 1984
OSI model defines the communications process into Layers
Provides a standards for communication in the
network
Primary architectural model for inter-computing and Inter networking
communications.
network communication protocols have a structure based on OSI Model
OSI Architecture
Direct Links: Outline
• Physical Layer
– Link technologies
– Encoding
• Link Layer
–
–
–
–
Framing
Error Detection
Reliable Transmission (ARQ protocols)
Medium Access Control:
• Existing protocols: Ethernet, Token Rings, Wireless
Link Technologies
• Cables:
–
–
–
–
Cat 5 twisted pair, 10-100Mbps, 100m
Thin-net coax, 10-100Mbps, 200m
Thick-net coax, 10-100Mbps, 500m
Fiber, 100Mbps-2.4Gbps, 2-40km
• Leased Lines:
– Copper based: T1 (1.544Mbps), T3 (44.736Mbps)
– Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)
Link Technologies
• Last-Mile Links:
– POTS (56Kbps), ISDN (2*64Kbps)
– xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL (12.96Mbps55.2Mbps)
– CATV: 40Mbps downstream, 20Mbps upstream
• Wireless Links: Cellular, Satellite, Wireless Local Loop
FRAMING
• An efficient data transmission technique
• It is a message forwarding system in which data packets, called
frames, are passed from one or many start-points to one
Approaches
• Byte oriented Protocol(PPP)
BISYNC
Binary Synchronous Communication
DDCMP
Digital Data Communication Message Protocol
• Bit oriented Protocol(HDLC)
• Clock based Framing(SONET)
Byte oriented Protocol(PPP)
BISYNC FRAME FORMAT
SYH
SYH
SOH
Header
STX
Body
ETX
PPP Frame Format
Flag
Address
Control
Protocol
Payload
Flag
CRC
DDCMP Frame Format
SYN
SYN
Class
Count
Header
Body
CRC
Bit Oriented Protocol(HDLC)
• Collection of Bits
1.HDLC
High-Level Data Link Control
2.Closed Based Framing(SONET)
Synchronous Optical Network
HDLC Frame Format
Beginning
sequence
Header
Body
CRC
Bit Stufffing
After 5 consecutive 1s insert 0
Next bit is 0 – stuffed removed
Next bit is 1 –end of frame or erorr
Ending
sequence
Closed Based Framing(SONET)
• STS-1 Frame
9 rows of 90 byte each
First 3 byte for overhead rest contains data
Payload bytes scrambled- exclusive OR
Supports Multiplexing
Payloads
9 rows
90 columuns
ERROR DETECTION
• Detecting Errors In Transmission
Electrical Interference, thermal noise
Approaches
Two Dimensional Parity
Internet Checksum Algorithm
Cyclic Redundancy Check
Two Dimensional Parity
7 bits of data
8 bits including parity
Number of 1s
even
odd
0000000 (0)
00000000
100000000
1010001 (3)
11010001
01010001
1101001 (4)
01101001
11101001
1111111 (7)
11111111
01111111
Transmission sent using even parity:
• A wants to transmit: 1001
• A computes parity bit value: 1^0^0^1 = 0
•
A adds parity bit and sends: 10010
• B receives: 10010 B computes parity: 1^0^0^1^0 = 0
• B reports correct transmission after observing expected even result.
Transmission sent using odd parity:
•
•
•
•
•
•
A wants to transmit: 1001
A computes parity bit value: ~(1^0^0^1) = 1
A adds parity bit and sends: 10011
B receives: 10011
B computes overall parity: 1^0^0^1^1 = 1
B reports correct transmission after observing expected odd result.
Reliable Transmission
Deliver Frames Reliably
Accomplished by Acknowledgements and Timeouts
ARQ-Automatic Repeat Request
Mechanism:
Stop and Wait
Sliding Window
Concurrent Logical Channels
Stop And Wait ARQ
• The source station transmits a single frame and then waits for an
acknowledgement (ACK).
• Data frames cannot be sent until the destination station’s reply
arrives at the source station.
• It discards the frame and sends a negative acknowledgement (NAK)
back to the sender
• causes the source to retransmit the damaged frame in case of error
Acknowledgements & Timeouts
Sender
Receiver
Sender
Timeout
ACK
Timeout
Timeout
Fram
e
(a)
Timeout
Fram
e
Fram
e
ACK
Sender
Timeout
Receiver
Fram
e
ACK
(c)
Timeout
Sender
Timeout
Time
Fram
e
Receiver
Receiver
Fram
e
ACK
Fram
e
ACK
ACK
(b)
(d)
Stop & wait sequence numbers
Receiver
Sender
Receiver
Sender
Receiver
Timeout
Timeout
Timeout
Timeout
Sender
(c)
(d)
(e)
• Simple sequence numbers enable the client to discard
duplicate copies of the same frame
• Stop & wait allows one outstanding frame, requires two
distinct sequence numbers
Stop And Wait
Sliding Window
•
bi-directional data transmission protocol used in the data link layer
(OSI model) as well as in TCP
• It is used to keep a record of the frame sequences sent
•
respective acknowledgements received by both the users.
Sliding Window: Sender
•
•
Assign sequence number to each frame (SeqNum)
Maintain three state variables:
– send window size (SWS)
– last acknowledgment received (LAR)
– last frame sent (LFS)
•
•
•
Maintain invariant: LFS - LAR <= SWS
Advance LAR when ACK arrives
 SWS
Buffer up to SWS frames
…
…
LAR
LFS
Sequence Number Space
•
•
•
SeqNum field is finite; sequence numbers wrap around
Sequence number space must be larger then number of outstanding frames
SWS <= MaxSeqNum-1 is not sufficient
–
–
–
–
–
–
•
•
suppose 3-bit SeqNum field (0..7)
SWS=RWS=7
sender transmit frames 0..6
arrive successfully, but ACKs lost
sender retransmits 0..6
receiver expecting 7, 0..5, but receives the original incarnation of 0..5
SWS < (MaxSeqNum+1)/2 is correct rule
Intuitively, SeqNum “slides” between two halves of sequence number space
Sliding Window: Receiver
• Maintain three state variables
– receive window size (RWS)
– largest frame acceptable (LFA)
– last frame received (LFR)
• Maintain invariant: LFA - LFR
<= RWS
 RWS
…
…
LFR
• Frame SeqNum arrives:
LFA
– if LFR < SeqNum < = LFA
accept
– if SeqNum < = LFR or SeqNum > LFA
discarded
• Send cumulative ACKs – send ACK for largest frame such that all
frames less than this have been received
Ehernet
• local-area network (LAN) covered by the IEEE
802.3.
• two modes of operation:
– half-duplex
– full-duplex modes.
.
Three basic elements :
1. the physical medium used to carry Ethernet signals between
computers,
2. a set of medium access control rules embedded in each
Ethernet interface that allow multiple computers to fairly
arbitrate access to the shared Ethernet channel,
3. an Ethernet frame that consists of a standardized set of bits
used to carry data over the system
IEEE 802.5 Format
Frame Format IEEE 802.5
IEEE 802.3 MAC Data Frame Format
Wireless
• The process by which the radio waves are propagated through air
and transmits data
• Wireless technologies are differentiated by :
• Protocol
• Connection type—Point-to-Point (P2P)
• Spectrum—Licensed or unlicensed
Types
• Infrared Wireless Transmission
– Tranmission of data signals using infrared-light
waves
• Microwave Radio
– sends data over long distances (regions, states,
countries) at up to 2 megabits per second (AM/FM
Radio)
• Communications Satellites
– microwave relay stations in orbit around the earth.
UNIT III Packet Switching
• Is a network communications method
• Groups all transmitted data, irrespective of content, type, or structure
into suitably-sized blocks, called packets.
• Optimize utilization of available link capacity
• Increase the robustness of communication.
• When traversing network adapters, switches and other network nodes
• packets are buffered and queued, resulting in variable delay and
throughput, depending on the traffic
Types
• Connectionless
• each packet is labeled with a connection ID rather than
an address.
• Example:Datagram packet switching
• connection-oriented
– each packet is labeled with a destination address
– Example:X.25 vs. Frame Relay
Star Topology
Source Routing
0 Sw itch 1
3
0
1
3
2 Sw itch 2
2
3 0 1
3
1
1
2
1 3 0
0
Host A
0 1 3
1
0 Sw itch 3
3
2
Host B
Virtual Circuit Switching
• Explicit connection setup (and tear-down) phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented model
0 Switch 1
1
3
2
5


Analogy: phone
call
Each switch
maintains a VC
table
3
11
2 Switch 2
1
0
Host A
7
1
0 Switch 3
3
4
2
Host B
Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless
model
Host D


Analogy: postal
system
Each switch
maintains a
forwarding
(routing) table
Host E
0 Switch 1
3
Host C
Host F
1
2 Switch 2
2
3
1
0
Host A
Host G
1
0 Switch 3 Host B
3
2
Host H
Virtual Circuit Model
• Typically wait full RTT for connection setup before sending first
data packet.
• While the connection request contains the full address for
destination
• each data packet contains only a small identifier, making the
per-packet header overhead small.
• If a switch or a link in a connection fails, the connection is
broken and a new one needs to be established.
• Connection setup provides an opportunity to reserve resources.
Datagram Model
• There is no round trip delay waiting for connection setup; a
host can send data as soon as it is ready.
• Source host has no way of knowing if the network is capable of
delivering a packet or if the destination host is even up.
• Since packets are treated independently, it is possible to route
around link and node failures.
• Since every packet must carry the full address of the
destination, the overhead per packet is higher than for the
connection-oriented model.
Bridges and Extended LANs
• LANs have physical limitations (e.g., 2500m)
• Connect two or more LANs with a bridge
– accept and forward strategy
– level 2 connection (does not add packet header)
A
B
C
Port 1
Bridge
Port 2
• Ethernet Switch = Bridge on Steroids
X
Y
Z
Spanning Tree Algorithm
• Problem: loops
A
B
B3
C
B5
D
B2
B7
E
K
F
B1
G
H
B6
B4
I
J
• Bridges run a distributed spanning tree algorithm
– select which bridges actively forward
– developed by Radia Perlman
– now IEEE 802.1 specification
Algorithm Details
• Bridges exchange configuration messages
– id for bridge sending the message
– id for what the sending bridge believes to be root bridge
– distance (hops) from sending bridge to root bridge
• Each bridge records current best configuration message for
each port
• Initially, each bridge believes it is the root
Algorithm Details
• Bridges exchange configuration messages
– id for bridge sending the message
– id for what the sending bridge believes to be root bridge
– distance (hops) from sending bridge to root bridge
• Each bridge records current best configuration message for
each port
• Initially, each bridge believes it is the root
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