Overview/Questions
CS101 Lecture 18:
Networking:
Topology, Packet Switching, Protocols
± What is a communications network?
± Why connect computers together?
± How do computers connect to each other?
± What are the typical models for creating
computer networks?
± How is data transferred across a network?
Some diagrams copyright Jones and Bartlett
1
The Telephone Network
2
POTS Circuit Switching
Telephone wires leave your house, and connect to the central office.
office.
POTS (the plain old telephone system), a.k.a PSTN
To connect a phone call, the caller¶s phone must be physically
connected to connect to the receiver¶s phone.
Connecting these circuits (called switching) takes place at dedicated
facilities called central offices.
offices.
Image from
www.exegesis.uklinux.
net.
At the central office, connections are made to other telephone lines«
lines«
4
1
POTS Circuit Switching
Computer Networking
Telephone operators used to actually switch wires to connect the calls.
Computer Network
A collection of computing devices connected in
order to communicate and share resources.
Connections between computing devices can be
physical using wires or cables, or wireless using
radio waves or infrared signals.
Why connect computers together?
In the PSTN, this connection (called switching) is done electronically.
electronically.
5
Networking
Networking Terms
Computer networks have opened up an
entire frontier in the world of computing
called the client/server model.
Figure 15.1 Client/Server interaction
6
Client
A computer which uses (consumes) resources
from the network.
Server
A computer that stores and manages files or
applications for multiple users on a network.
Example: Web Server
A computer dedicated to responding to requests
(from the browser client) for web pages.
7
8
2
Networking Terms
Network Nodes
Bandwidth (data transfer rate)
The speed at which data is transferred over
a telecommunications link on a network.
Bandwidth is measured in bits per second
(bps/Kbps/Mbps).
A network node is any device on a network:
± Computer (desktop, laptop, PDA, etc.)
± Server (web server, mail server, etc. )
± Router (device which directs traffic)
± Firewall (access control device)
Why is bandwidth such a big deal?
9
10
Network Interface Controller
Network Topologies
Each node has a network interface controller (NIC)
connected to its circuit board.
LocalLocal-area Network (LAN)
A network that connects a relatively small number
of machines in a relatively close geographical area.
The CPU treats the NIC
as an input/output
device.
Network Topology
Describes the physical wiring plan for connections
between nodes on a network. Also describes how
messages are sent between nodes.
It communicates by
reading or writing
bytes of data to the NIC.
Each NIC has a unique Media Access Control (MAC)
address, which distinguishes it from all other NICs.
NICs.
11
12
3
Ring Network Topology
A ring network
connects all nodes in a
closed loop on which
messages travel in one
direction.
A star network centers
around one node to
which all others are
connected and through
which all messages are
sent.
How many hops?
Complexity?
How many hops?
Complexity?
Bus Network Topology
On a bus network
nodes are connected to
a single communication
line that carries
messages in both
directions.
How many hops?
Complexity?
Star Network Topology
Ethernet The industry standard
bus technology for locallocal-area
networks.
Types of Networks
WideWide-area network (WAN)
A network that connects locallocal-area networks over a
potentially large geographic distance.
MetropolitanMetropolitan-area network (MAN)
The communication infrastructures that have been
developed in and around large cities or campuses.
Gateway/Router
One particular computer on a LAN which
coordinates/directs all communication going between
that LAN and other networks.
16
4
Circuit Switched Network
Creating a WAN
Computers can be connected over a circuit
switched network (e.g. phone lines), creating a
circuit between the source and destination.
LANs separated by a great distance are connected by
High speed communication links to create a WAN.
A Switched Circuit connects devices A and B.
Image from
www.tcpipguide.com.
17
Data Transmissions
Bursty Transmissions
Computers send data in irregular bursts ± a bunch
of data at one shot, and then nothing for a long
time in between messages.
Using a switched network connection is wasteful ±
the wires must be connected even if no data is
being sent!
Packet Switching
Packet
A unit of data sent across a network.
Packet switching
Messages are divided into fixedfixed-sized, numbered
packets. Packets are individually routed to their
destination, then reassembled into messages.
Router
A network device that directs a packet between
networks toward its final destination.
5
Packet Switching Network
Packet Switching
Nodes send packets of data along routes to a
destination, without a dedicated circuit.
Image from
www.tcpipguide.com.
Figure 15.4
Messages sent by packet switching
21
Benefits of Packet Switching
Communication links between nodes are only
allocated while transmitting each packet.
- Longer messages require multiple packets, but the
link is not dedicated when there is no data to send.
Packet switching enables data to travel from its
source to its destination over any number of
possible routes.
- Increases reliability of message delivery.
Packets (even in the same transmission)
can take different routes.
22
Network Protocols
Protocol
A set of rules that defines how data is formatted
and processed on a network.
Open System
A system which is based on published standards
for how protocols should work. Open systems
enable interoperability of network software and
hardware components from different vendors.
6
The OSI SevenSeven-Layer Model
for Networking Applications
Open Systems Interconnection
Open Systems Interconnection (OSI) Reference Model
A sevenseven-layer logical break down of network interaction to facilitate
communication standards.
Each layer deals with a
particular aspect of
network communication.
Network protocols are
usually specific to one
layer of this model.
Figure 15.5 The layers of the OSI Reference Model
25
Network Protocols
Layer Name
Layer Functions
Application
Specifies the syntax to be used between sending and
receiving processes.
Presentation
Converts between native and network data representations,
allowing machines of different architectures to communicate
in a common "network"network-neutral" format.
Session
Handles login, restart, check pointing, and other session
state maintenance issues.
Transport
Provides reliability of packet delivery where appropriate.
Network
Handles pointpoint-toto-point routing across the network,
fragments data into chunks appropriate to datalink technology
to be used, and reassembles on receipt.
Datalink
Takes networknetwork-layer datagrams and frames them for
transmission.
Physical
Converts datalink frames into bit streams for transmission
over some physical medium.
26
TCP/IP Suite
Internet Protocol (IP)
Software that deals with the routing of
packets through the maze of interconnected
networks to their final destination.
Protocol Stack
Network protocols are layered such that
each protocol relies on the protocols that
underlie it.
± tracks each packet¶s source, destination,
packet size, and maximum number of hops.
IP does not guarantee delivery of packets.
Rather, IP is a bestbest-effort protocol.
Figure 15.6 Layering of key network protocols (diagram showing layers 3-4-5)
27
28
7
TCP/IP Suite
HighHigh-Level Protocols
Transmission Control Protocol (TCP)
Software that breaks messages into packets,
hands them off to the IP software for delivery, and
then orders and reassembles the packets at their
destination.
Port
A numeric
designation
that
corresponds to
a particular
high-level
protocol
± TCP guarantees delivery of packets.
User Datagram Protocol (UDP)
An alternative to TCP that is faster but less
reliable.
Figure 15.7
Some high-level protocols which rely upon
TCP/IP, and the ports they use.
± UDP does not guarantee delivery of packets.
29
30
Summary
± Networks enable the clientclient-server model for sharing
data and applications between users.
± Topology describes the physical layout of a network
as well as the number of hops for message
transmission.
± Packet switching makes most efficient use of network
transmission lines, and provides multiple routes from
source to destination.
± Protocols describe how data is exchanged over a
network.
31
8