Transcript Packet switched

Fundamentals of Mobile
Computing
CE00375-2
Network Technology
[email protected]
Ext 3292, Room C203, Beacon Building
Mobile Computing
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Content
• What is a network?
• Common Network Types
• Network Technologies
Mobile Computing
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Network
• Can be anything which needs to communicate
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Does not need to be electrical
• A group of friends who talk is a network
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Each person in the group is receiving and transferring
information (Voice)
In a group they become a network
• Telephone companies are a good example of a
wide scale network
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The phone being the device and the phone number being
the unique address
Computer Network
• This is the same with a number of devices trying
to communicate
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The word devices is used, due to more than just
computers need to communicate i.e. printers, routers etc
Mobile Computing
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Usage
• The increasing use of computers with network
communications has allowed sharing of all
electronic resources
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Increasingly all devices are usage digital signals
With these developments it was then straight forward to
move these digital signals on to wires as 1’s and 0’s
This allows sharing of an increasing number of devices
• Beyond the “normal” office communications of files and
printers
• Digital Cameras are a recent example where a picture can be
placed on the network
• Telecommunications with the introduction of Voice Over
Internet Protocol (VOIP)
• CCTV cameras streaming the pictures to a console for viewing
 Based on a digital stream that viewing console could be
anywhere on the planet
Mobile Computing
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To allow communication the devices need to
be connected via wires or wireless
technology
• This presentation will concentrate on wired as it is
initially easier to follow
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In the diagram below the two machines are
connected and can send electronic message
between them
• There are many ways of connecting devices to
allow them all to communicate
Types of Network
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Types of Wired Networks
• Three main types for wired network but there are more
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Local Area Network (LAN)
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High speed networks
Cheap to install and run
Short distances between nodes
Usual environment, Offices, Homes
Metropolitan Area Network (MAN)
• Medium speed but is increasing
• Covers a large area of a town
• This universities network is an example of a MAN
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Wide Area Network (WAN)
• Slow throughput
 Can be increased but at substantial cost
• Expensive to install and run
• Networks can cover large areas, like countries or the global!
• Usual users large scale corporations, governments or the Internet
Types of Network
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Types of Networks
• For wireless communications we also have
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Personal Area Network (PAN)
• These are device which transmit in a very small area around
the device
• Usually distance of 10 metres or less
 Due to the range of these devices
• Common example is Bluetooth used to communicate with a
mobile phone
• Transmissions are between 100 Kbps to 1 Mbps
 Usually slow to maximise the battery life
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Body Area Networks (BAN)
• These operate is close proximity to the user
 Very low Kbps but a long battery life
 Healthcare is given as an example of using this tech
 There is a standards group IEEE 802.15.6, working on a
standard
 http://ieee802.org/15/pub/TG6.html
Mobile Computing
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The most common communications in the
LAN networks is Ethernet
• Ethernet allows all of the devices to
communicate on a single path/wire
• All send the signals on this wire
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The intended device will listen and receive the
data, the other machines ignore it
• This is easy for sending a packet A and F, but consider
what happens when the network gets bigger
F
A
Mobile Computing
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Open System Interconnection (OSI) Layers
• This is an agreed standard for protocols
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All devices use this as a standard, but certain protocols
may combine layers for efficiency
• Each layer in OSI can only communicate with the
layer above it or below it
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As the data moves down the stack the protocol from the
layer above goes into the data part of the layer below it
The opposite being true when receiving the packet
• Only the layer which added a piece of data can
remove and process on the receiving machine
Mobile Computing
Sender
Recipent
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Mobile Computing
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Open System Interconnection (OSI) Layers
Link for discussion of exactly
what each layer does:
http://webopedia.internet.co
m/quick_ref/OSI_Layers.asp
Mobile Computing
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OSI Layers
• This method allows a common interface for the
software
• The operating system, does not need to know
what network technology is used
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It just has to request access to the application layer
• As different technology is used this is changed at
the correct layer
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So from wired Ethernet to ieee 802.11b the technology
at layers 1 and 2 will need to be changed
All other layers will remain the same !
Mobile Computing
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Each device in a network has to have a
unique address
• This is like the post service to deliver mail you
need a unique address
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In networks this is mostly the Internet
Protocol (IP) addresses
• Open a command window on your machine and
type ipconfig
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This will show you your unique address
• Every device attached to the Internet has to have
a unique address for message delivery
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There are ways around this, but it is beyond this lecture
to cover these
Mobile Computing
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Remember though
• IP addresses are needed by the Internet as the
packets get routed
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As in they are moved beyond there local connection
If this is not true – then they are not routing
• On this basis a unique address is still needed if there are
multiple destinations but you may not need a IP address
• A hardware address is used for these communications MAC
• Point to point communications
 There is only one device sending one receiving you are
not routing anything only two devices can talk
therefore no unique address is required
Printer
Mobile Phone
Mobile Computing
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IP addresses
• Example output of ipconfig
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So this machines IP address is 194.66.179.218
Mobile Computing
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A common language must be used between
the devices
These are protocols
• Most commonly used is the reliable TCP/IP
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This is used for routing and transfer of the packets
• Internet Protocol (IP) gives each machine a unique address
• Transport Control Protocol (TCP) allows each packet to be
error checked and ensures that the packets arrive at the
correct rate (prevents buffer overflows)
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Both TCP and IP have a lot more functionality than this,
but is beyond the required information for this course
• For layers 1 and 2, different technologies are used
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Ethernet in the home and offices
Bluetooth in a Portable device
Asynchronous Transfer Mode (ATM) in phone networks or
Multimedia intensive environments
Mobile Computing
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Packet switched vs Circuit Switched networks
• Circuit switched is the same as the telephone
network
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You specify who you wish to talk to and then a circuit is
created between the two locations before
communications are allowed
Resources are reserved when the circuit is set up
• This is good for multimedia which may require defined
latency and throughput during the transmission
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All communication packets travel through this circuit
The packets should always arrive in the sequence sent
Mobile Computing
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Circuit Switched
Router
Router
Router
Router
192.168.0.2
Router
Router
192.168.0.3
Mobile Computing
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Packet switched vs Circuit Switched networks
• Packet Switched can be considered as sending a
letter in the post
• An address is put on the envelope and you put the
letter in the post box
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How the letter gets to its destination you do not care
No route is worked out first as the best between the sites
The packets can arrive out of sequence
• A sequence of packets can all take different routes to the same
destination
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No guarantees can be offered in terms of latency or
throughput
• This is changing with the newer but not widely used IPv6
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No delay in communications starting up
No network resources are wasted reserving routes before
• Larger amount of devices can use the same connections
 When one device is silent, another can communicate
Mobile Computing
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Packet Switched Diagram
Router
Router
Router
Router
192.168.0.2
Router
Router
192.168.0.3
Networking
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Networking
• So to send a packet of information you need
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Hardware to physically send the packet
• Ethernet, GSM, ATM, GPRS
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Software to encapsulate the data from the
application
• TCP/IP is the most common
 There are other protocols which replace TCP, like RTP
• Both the receiving and sending device need to have the
correct protocols agreed and ability to receive them
Networking
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Networking
• Using this technology will allow for information to be
sent from one person to another
• The increasing use of wireless technology has
introduced additional considerations like
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Potential high error rates on packets
Number of unique IP addresses
• Problems with the use of TCP error checking in a wireless
environment
• Here is a research paper on the subject of TCP across a
wireless link looking at the issues
 (http://www.cs.kau.se/~alfs/licmaterial/tcpreport.pdf#sear
ch=%22tcp%20wireless%22, 2005)
Networking
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Usefulness of Mobile Computing
• At some stage each device will need attaching into a
wider scale wired network
• This allows the transfer of the information from and
to the device
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Returning your emails you have written and placing them
onto the work network
Downloading updates to the software of your phone by using
Bluetooth via your home PC
• So mobile/wireless DO NOT replace wired
communications they enhance the functionality
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This is what this course is looking at
• How the technologies and which ones assist in differing
environments
• As stated in lecture 1 – no technology is perfect for all uses
they are designed with an intention of use and they are usually
good at that
Conclusion
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What we have discussed today
• OSI layer
• TCP/IP
• Packet switched networks
• Circuit switched networks
• Types of LAN