PowerPoint Presentation
Computer Systems
Introduction to Networks (Part II)
Dr. Mian M. Hamayun
m.m. .uk
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Lecture Objective
The objective of this lecture is to develop a basic
understanding of different types of network
delays, protocol layering and OSI model.
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Lecture Outline
Packet Loss and Delays
What is a Protocol?
Protocol Layers and Encapsulation
Open Systems Interconnection Model
Large Messages and Message Consistency
Ports and Addressing
Summary
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How do loss and delay occur?
Packets queue in router buffers
packet arrival rate to link (temporarily) exceeds output link
capacity
packets queue, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
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How do loss and delay occur?
d
proc
: nodal processing
check bit errors
determine output link
typically < msec A B propagation transmission nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop d queue : queueing delay time waiting at output link for transmission depends on congestion level of router Slide #6 of 36 How do loss and delay occur? d trans : transmission delay L: packet length (bits) R: link bandwidth (bps) d trans = L/R d prop : propagation delay d: length of physical link s: propagation speed in medium (~2x108 m/sec) d prop = d/sdtrans and dprop very different A B propagation transmission nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop Slide #7 of 36 Caravan Analogy cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (bit transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? toll booth toll booth ten-car caravan 100 km 100 km time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes Slide #8 of 36 Caravan Analogy (more) suppose cars now “propagate” at 1000 km/hr and suppose toll booth now takes one min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at first booth? A: Yes! after 7 min, 1st car arrives at second booth; three cars still at 1st booth. toll booth toll booth ten-car caravan 100 km 100 km Slide #9 of 36 Packet Loss queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area) Slide #10 of 36 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server sends bits (fluid) into pipe pipe that can carry fluid at rate Rs bits/sec) pipe that can carry fluid at rate Rc bits/sec) server, with file of F bits to send to client link capacity Rs bits/sec link capacity Rc bits/sec Slide #11 of 36 Throughput (more) R s < R c What is average end-end throughput? Rs bits/sec Rc bits/sec R s > R
c
What is average end-end throughput?
Rs bits/sec Rc bits/sec
link on end-end path that constrains end-end throughput
bottleneck link
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What is a Protocol?
To effectively communicate, network devices use
protocols
Set of guidelines
Must specify:
The sequence of messages to be exchanged
The format of the data in the messages
Implemented by pair of software modules in sending
and receiving computer
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Protocol – Example 1
Transport protocol: transmits message of a length
from a sending process to a receiving process.
A process wishing to send message passes it to
transport protocol module
Transport software divides message into packets
These are then transmitted using network protocol
Inverse operations are performed at other end
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Protocol – Example 2
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Message: Frame
10101010 1010101 etc…
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Message: Frame
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Inside a Frame: Packet
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Protocol Layers
Network software arranged in hierarchy of layers
Each layer presents interface to layer above
Every computer in network should have these layers
Layers communicate with those above and below
through procedure calls
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Conceptual Layering of Protocol Software
Layer n
Layer 2
Layer 1
Message sent Message received
Communication
medium
Sender Recipient
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair,
Distributed Systems: Concepts and Design Edn. 5
© 2012
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Protocol Layers
Each layer accepts items of data in a specified format
from layer above
Transformations applied to encapsulate data
Then data passed to layer below
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Protocol Layers – Example
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Encapsulation – in Layered Protocols
Complete set of protocol layers is a protocol suite or
protocol stack
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Open Systems Interconnection (OSI) Model
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OSI Protocol Summary
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Pros and Cons of Protocol Layering
Simplifies and generalizes software interfaces
accessing communication services
Downside: performance costs.
N layers = N control transfers
N copies of the data due to encapsulation
Hence, data transfer rate much lower than available
network bandwidth
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What does the Internet Use?
Differs slightly from OSI model
Application, presentation and session layers not clearly
distinguished in Internet protocol stack
► Applications decide how to treat these
Session layer integrated with transport layer
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Large Messages
Problem: entire message may need more than one
frame
An Ethernet frame can only hold 1500 bytes of data
Any large messages will have to be broken down
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Message Consistency
Reliable data transfer
Sender & Receiver communicate using TCP
If there are errors in the packets, the receiver
notifies sender and the sender resends those
packets again.
If it was one large packet, there could be issues
due to poor connection
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How could we define what the
packet order should be?
Each packet contains a sequence number!
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Ports
Transport layer’s task is to provide a network-
independent message transfer service between pairs of
network ports
Software defined destination points at a host computer
Port 25 for email…
Attached to processes
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Addressing
Transport layer must deliver messages to destinations
with transport addresses
Composed of network address and a port number
Numeric identifier
In the Internet, every host computer is assigned an IP number
Identifies it and the subnet to which it is connected
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Datagram Packet Delivery
Datagram – like a letter/telegram
Delivery of each packet is a one-shot process
Network retains no info once delivered
Sequence of packets may take different routes from host to
destination, so may arrive out of sequence
Datagrams contain:
Full network address of source and destination
Internet’s network layer based on datagram delivery
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Hosts, Routers, and Link Layer Switches
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Summary
In this lecture we have been introduced to:
The different sources of delays in networks
The notion of a protocol and protocol layering
The OSI Model
Packet Assembly, Ports & Addressing
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References / Links
Chapter #1: Computer Networks and the Internet,
Computer Networking: A Top-Down Approach (7th
edition)
by Kurose & 1
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