Computer Networks

1. Computer Networks
Chapter 1:Introduction
Computer Networking:
A Top Down Approach ,
4th edition.
Jim Kurose, Keith Ross

2. Today’s Lecture
 Layered Architecture
 Brief description of Five Layers

3. Internet Protocol Stack
 To provide structure to design of
network
protocols,
network
designers organize protocols in
layers
 Service – says what a layer does
 Protocol – says how the service is
implemented
 Advantages
 Drawbacks
 When
taken
together
the
protocols of various layers are
called the Protocol Stack.
 Internet Protocol Stack consists
of Five layers
 Physical,
Link,
Network,
Transport and Application layers .
 Organization of Book

4. Internet Protocol Stack
 To provide structure to design of
network
protocols,
network
designers organize protocols in
layers
 Service – says what a layer does
 Protocol – says how the service is
implemented
 Advantages
 Drawbacks
 When
taken
together
the
protocols of various layers are
called the Protocol Stack.
 Internet Protocol Stack consists
of Five layers
 Physical,
Link,
Network,
Transport and Application layers .
 Organization of Book

5. Internet Protocol Stack
• Application Layer:
 Network applications and their application layer protocols
reside.
 Provides user interfaces and support for services such as email, file transfer etc.
Hyper Text Transfer Protocol (HTTP)
File Transfer Protocol (FTP)
Session Initiation Protocol (SIP)
 An application layer protocol is distributed over multiple end
systems
 The packets of information at the application layer is called
as a message.

6. Internet Protocol Stack
• Transport Layer:
 Transports application-layer messages between application
end points.
 Transport layer packet is called as a segment
 Breaks long messages into shorter segments
 There are two Transport Layer Protocols
 Transmission Control Protocol (TCP)
 Connection Oriented service
 Guaranteed delivery of application layer messages
 Flow control
 Congestion Control
 User Datagram Protocol (UDP)
 Connectionless service
 No reliability, flow control and congestion control

7. Internet Protocol Stack
• Network Layer:
Responsible for moving network layer packets
known as datagrams from one host to another.
Transport layer passes a transport layer
segment and a destination address to the
network layer.
Network layer includes IP Protocol
Defines the fields in the datagram as well as
how end systems and routers act on these
fields
Commonly referred as IP layer.
Different routing protocols.
Determine the route that datagrams take
between source and destination

8. Internet Protocol Stack
• Link Layer:
Moves a packet from one node (host or router)
to the next node in the route.
Divide the stream of bits received from the
network layer into manageable data units called
frames.
Transforms a raw transmission facility to a
reliable link.
Mechanism to detect and retransmit damaged
or lost frames
Example of link layer protocols include WiFi,
Ethernet etc.

9. Internet Protocol Stack
• Physical Layer:
The job of this layer is to move the individual
bits with in frames from one node to next.
Representation of bits
Physical Layer data consists of a stream of
bits (0 or 1)
To be transmitted bits must be encoded into
signals. The physical layer defines the type of
encoding.
The protocol in this layer depend on the
actual transmission medium of the link.

10. Internet Protocol Stack
 Application: Provides user interfaces and
support for services such as e-mail, file
transfer etc.

FTP, HTTP
 Transport: Transports application-layer
messages between application end points.


Segmentation and reassembly
TCP, UDP
 Network: Routing of Datagrams from
source to destination

IP, routing protocols
 Link: Move a packet from one node (host or
router) to the next node in the route.
 Ethernet, WiFi
 Physical: Move the individual bits with in
frames from one node to next
Application
Transport
Network
Link
Physical

11. OSI Reference Model
 In 1970 International Organization for
Standardization proposed a seven layered
model called Open Systems
Interconnection (OSI) model.
 Presentation Layer: Provide services such
as data encryption, compression.
 Session Layer: Synchronization points
(checkpointing) and recovery of data
exchange.
 Internet stack “missing” these layers!
 these services, if needed, must be
implemented in the application by the
application developer.

12. Encapsulation
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn H t
M
router

13. Throughput
 The rate (bits/sec) at which bits are transferred
between sender/receiver
 Difference between Bandwidth and Throughput?
 ISPs sell bandwidth
 In computer networks, the throughput is less
than the bandwidth for several reasons
• The channel may be shared by other users
• Packet loss due to congestion
• Packet loss due to bit errors
• Noise in the channel
• Transmission rates of the link over which the
data flows.

14. Throughput
 Rs
< Rc What is average end-end throughput?
Rs bits/sec
 Rs
Rc bits/sec
> Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
Throughput is min {Rs,Rc}
Transmission Rate of the bottleneck link

15. Throughput
 10 clients/servers pairs,
 Common link R traversed by
all 10.
 Rate of the link R is very
large then the throughput
is min {Rs , Rc}
 Rs=2Mbps, Rc=1Mbps,
R=5Mbps
 Common link divides
transmission rate equally
among the 10 downloads
 500kbps to each download
 Shared Link R is now the
bottleneck.
Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections share bottleneck
link R