1. COMP1321 Digital Infrastructure Week 13
Richard Henson
January 2019

2. Week 13: WANs, LANs, Open Systems Interconnect
Objectives:
Naming systems for LANs and WANs
Need for a standard
OSI model and why seven layers
Roles of individual layers
Simplifying the OSI model

3. Intro (& some revision?)
Remember the legendary Carrie-Ann Philbin?

4. Network Topologies Modern LANs usually some kind of star arrangement
at one time, a central spine was popular

5. Network Topologies WANs usually some kind of mesh arrangement
Need to ensure that an alternative route is available in case the preferred route cannot carry a signal

6. Device Naming No duplicate names allowed Early efforts
essential to be logical
enforced by “not null” in database field
Early efforts
based on telephone networks
Town… then number within that town
e.g. Whitehall 1212 (Scotland Yard)
development of an all number system
Known as STD code (e.g for Worcester)
no two telephone numbers alike…

7. Naming and Computer Networks
Within a network…
same principle… no duplicates
Names used to navigate data round the network
naming needs to be designed into any network communications protocol
e.g. MAC address, IP address

8. WANs, LANs and getting the message through (Routing)
In principle, routing is the same whether LAN or WAN
In practice…
LANs infrastructure owned and managed by organisation themselves (or a third party on their behalf)
WAN infrastructure managed by a specialist organisation and offered as a service

9. Naming and Addressing Words used almost interchangeably…
“address” tends to be numerical
“name” tends to be based on words
Fulfill essentially the same purpose:
unique network identity for each device
provides location for data to be sent to
provides source location for data sent

10. LAN naming systems IEEE responcible for naming
802 spec based on Feb’ 1980 start
defined and used “hardware” address of any networkable device
MAC address
Data sent as “frames” (not packets!)
Typical MAC address:
xx.xx.xx.xx.xx.xx (where x= a hex number)

11. Packet-based LAN naming
Associated with Internet (TCP/IP)
NetBIOS names
WINS names
Active Directory naming…
IP addresses

12. IP address systems Originates from TCP/IP naming system
typical IPv4 name:
x.x.x.x
where x = a number, 0 to 255 decimal
now IPv6
IPv4 running out of unique numbers!

13. WAN and LAN combination
Organisational data often travels through LANs and WANs…
Navigation may need a combination of naming systems…
dealt with through software that will deliver via a unique name

14. Communications Protocols and Software Layers
Rapidly got very complicated
Different computer manufacturers through their protocol was best
Prevented communications development via computer for many years

15. The OSI Seven Layer Protocol
Software for a WAN communication protocol needs to cover SEVEN (!) layers
software has to manage a diverse range of newtork issues and concepts
compromise between IBM and Internet community, but other players involved

16. The OSI Model (a 1978 compromise…)
Benefits of OSI compatible software:
other manufacturers products would be able to communicate with their own
consumer would no longer be “locked in” to specific vendor products
vendors would be able to produce products that work at specific layers only
specialise and hence produce better products

17. Layer Communication (Sending)
Each layer in the OSI model considers itself to be talking to a peer layer in another computer
adds/removes its own “header” (formatting info)
e.g. application layer
adds a header to the user data on screen
passed to the presentation layer as a single block e.g. presentation layer
adds its header to the block of data
passed on to session layer as a single block…
and so on…

18. The OSI reference model
Transmit
Station
Receive
Station AH
DATA PH SH TH NH LH LT
Physical
Layer
Data link
Network
Transport
Session
Presentation
Application
Physical
Layer
Data link
Network
Transport
Session
Presentation
Application
DATA AH PH SH TH NH LT LH
Initially, the diagram shows the seven network software layers in the sending computer. It then shows how extra header data is added to screen data by each of the network layers until it is send out onto the physical network via the physical layer.
It then shows all this data arriving at the receiving computer and the header data being stripped away layer by layer until what appears on the screen is just the data that was originally on the sending computer’s screen.
Link

19. Layer Communication (Receiving)
Each layer in the OSI model strips away its own header
e.g. physical layer
removes header from data block
passed to the data link layer
e.g. data link layer
removes header to the block of data
passed on to network layer
and so on…

20. The OSI model Link Transmit Receive Station Station Physical LayerAH
DATA PH SH TH NH LH LT
Physical
Layer
Data link
Network
Transport
Session
Presentation
Application
Physical
Layer
Data link
Network
Transport
Session
Presentation
Application
DATA AH PH SH TH NH LT LH
Repeat of earlier slide
Link

21. Simplifying The OSI model
Layers can be sub-divided into two groups
The top 3 layers (interworking layers)
user applications and support services
The lower 4 layers (interconnection layers)
the network (and navigation of packets)
Memory aids:
PDNTSPA
Please Do Not Throw Sausage Pizza Away!

22. Interworking Layers All about servicing needs of users
support for the application layer…
includes presentation layer
and session layer

23. Application Layer
Interface for applications to use to gain access to network services:
Networked file transfer
Message handling
Database query processing
Controls generalised network access:
supports applications which exchange data
provides error & status information for applications
If network is peer-peer…
authenticates peer partners
determines if peers are ready to communicate

24. Presentation Layer Responsible (sending) for converting data from
application-specific format
to a generic (machine-independent) format that can be passed across a network
Receiving…
for converting incoming data from a generic format to one that makes sense to the receiving application
Also responsible for protocol conversion, encryption & decryption, and graphics commands
The redirector (software for handling service requests) also operates at this layer:
If a service cannot be resolved locally, it sends the request out to the network resource that can offer the required service

25. Session Layer
Sets up a logical connection between machines called a “session”, which allows networked resources to communicate
Manages the setting up of a user “session”, exchange of information, and “tear down” as the session ends
Manages issues such as who may transmit data at a certain time, and for how long, also ensuring that the system doesn’t “time out” after inactivity
Ensures data is routed to the correct application on the local machine
Synchronises services between tasks at each end of the communications channel in half duplex communications

26. Interconnection Layers
Concerned with packets of data
and navigating them through the network
Transport
Network
Data Link
Physical

27. The Four Layers Model Introduced with Unix (mid-1970s, pre-OSI)
based on Internet protocols…
“application”
“transport”
Shows four names in boxes: application, transport, network, physical
“network”
“physical”

28. TCP/IP Evolved with the Unix four layers… Application,
presentation, session
TCP IP
As previous slide but application, presentation, session all together in top layer
Next layer: TCP
Third layer: IP
Fourth layer: connecting with physical medium
Connecting with
physical medium

29. Transport Layer (from Unix)
Manages the transmission of level 4 data from sender to corresponding layer in receiver
segments data streams into chunks of a given packet size for the medium being used
checks for errors due to corruption, requests retransmission etc.
Gateways can operate at this layer

30. Transport Layer (from Unix)
Other roles:
managing flow control
providing acknowledgement of successful transmission of chunks of data
software multiplexing
routing in an Internetwork
Manages OSI levels 1-4 so messages travel between network nodes via pairs of “sockets”
After slide text: socket A (sender) in a box, arrow from box to socket B (receiver) in another box
socket A
(sender)
socket B
(receiver)

31. Transport layer Socket A Socket B End User Peer-to-Peer communications
Physical
Layer
Data link
Network
Transport
Upper
OSI
Layers
End User
Peer-to-Peer communications
Socket A
Socket B
Similar to previous diagrams showing all seven network layers. This time, transport layer on sender identified at socket A, and transport layer on receiver is socket B. Arrow linking to two together as “peer-peer” communication, Diagram also shows two intermediate routers, where data is not converted up the stack beyond the network layer, before forwarding.
Network A
Network B

32. End-end v logical neighbour communications
Top four OSI layers communicate logically with remote peer…
regardless of topology or distance
The lower layers all communicate physically with their nearest neighbour in a network
dependent on topology and routing to get the packets through

33. Network Layer Responsibilities: packet (IP) addressing and sequencing
determining to route from source to destination computer
Routers operate up to this layer
use IP addresses to navigate packets

34. Network Layer Functions
Provides messages with an address for delivery (e.g. IP address)
Translates logical network addresses/names into physical equivalents
Handles packet switching and routes packets to their destination on the local network
Controls network packet congestion
Ensures packets conform to the network's format

35. Network Layer Transport Layer Network Layer
User Specifies
Service
Transport
Layer
Network
Service
Network
Layer
Diagram shows the all important communication between transport layer (user oriented service) and network layer (network oriented service)
Network provides
Service
Network layer service definitions

36. Data Link Layer
Responsible for error free physical transmission of data, using frames
may include an error recovery mechanism and also a flow control mechanism, although this may be done at the transport layer
Mechanism (down):
data from the upper layers (i.e. the network layer) is converted by the data link layer into frames
Mechanism (up)
arranges raw data bits received via the physical layer into frames, for passing on to the upper layers
Bridges/Switches operate up to this level
Virtual LANs use frames
frames navigated through MAC addresses

37. Physical Layer Responsible for communicating with the network media
bits (0,1) converted into electrical signals and vice versa
Issues include modulation of signals and timing
Manages the interface between a computer and the network medium, but cable type and speeds of transmission no specified
Allows improved technology to be easily included
Repeaters work only at this level