1. COMP1321 Digital Infrastructure
Richard Henson
February 2018

2. Week 15: Specifics of LANs
Objectives
Explain functions of client-server networks and network services
Define networking standards
Relate Lower OSI layers to technologies and naming systems

3. The Client-Server Model
Centralisation of organisational resources
client can still hold resources
a lot (fat client)
not much (thin client)
peer-peer network needs processing and storage on each device (!)
Microsoft LAN model: a domain

4. Advantages & disadvantages Client-Server v Peer-peer
In groups…
don’t look at next slides… yet!

5. Advantages of a client-server network v workgroup
Centralised:
security
access to resources
network administration
With more than about 10 users, much easier to manage than a workgroup. Can handle up to thousands of users

6. Disadvantages of client-server, compared to a workgroup
Expensive:
dedicated computer(s) not accessible to users
server operating system needed
network management required
Reduces user autonomy
If one server, and it goes down, the network ceases to function!!!

7. Request and response All network users use client devices/machines
Client requests information…
2. Server processes the request, sends a response back to the client

8. Requirements of organisational networks
The server would be expected to offer:
Network Access
Access to “restricted” files held on servers
only users with permission get access
Applications
Printing
Access to & The Internet

9. Servers in Larger Networks
To fulfill multiple requirements, larger networks have MANY servers
University ITS network: at least 50
Functions can be distributed servers e.g:
Login Server(s)
File and Print server
Applications Server
Internet Gateway

10. Login Servers (the most crucial!)
Dedicated to logging on users
database of usernames/passwords
Only allows a potential user to access the network if both username and password exactly correspond with entries in the database
Windows networks… called Domain Controllers

11. Windows LANs Client-server networks: Enterprise networks domains
access to domain via domain controller(s)
Enterprise networks
multiple domains logically linked in a hierarchy of LANs

12. Local (client) and Cloud (server) Networks
Cloud-based server
Extension of client-server model…
client-end less resource intensive
most of resources & processing at server end
Clients
less CPU & maintenance
therefore lower cost…
Cloud (somewhere in Internet)
client

13. Local Client/Cloud v conventional Local LAN
Advantages and disadvantages? 3 minutes….

14. What makes up a LAN (1)? Hardware:
computers and other network end devices
e.g. printers, web cameras
transmission media, e.g. cable, radio waves
network cards, and intermediate devices which link the network devices to the transmission media

15. What makes up a LAN (2)? Software to (e.g. …)
manage naming and routing
send/receive data
provide an even flow of data between devices
make sure sent data goes to the right place
provide a path for data through the network
make sure data is checked for corruption as it passes through the network
anything else that may need to be done to the data e.g. formatting, compression, encryption

16. Transfer of data through LANs (1)
All done through electrical signals
Medium/media transport(s) the signals
insulated copper wire (cheap but effective)
fibre optic cable (expensive, high volume)
wireless (microwaves that are sent out a specific frequency)

17. Transfer of data through LANs (2)
Cables designed from the start to transmit high volumes of digital data
Network cards provide the computer-medium interface:
control flow rate and error checking of data
send/receive data at high, and even higher… speeds

18. Network Media 3 main types: If a cabled LAN connection exceeds:
standardised copper cabling
standardised optical fibre cabling
“wifi” (wireless: e/m radiation of a standardised frequency)
If a cabled LAN connection exceeds:
100 metres (twisted pair cabling)
185 metres (coaxial cabling – rarely used now)
then a repeater (booster) is needed

19. Twisted Pair Cabling Consists of: RJ45 wiring
four pairs of colour coded copper wires in a plastic sheath
connectors for device at each end (RJ45)
RJ45 wiring
could be “straight through”
could be “cross over”

20. Cross-Over and Straight Through
“Cross over” when wires are “crossed” between RJ45 connectors
essential for direct communication between end (point) devices
“Straight through” for end-point connections to switches, routers, etc.

21. Network Adaptors Generally fit inside the computer:
either as a separate card
or on the motherboard…
have their own unique “MAC address”
connect externally via RJ45 socket
use own software (firmware)
work with other connectivity software to control sending & receiving of data

22. Network Software
On a peer-peer network, connectivity software is all that is needed…
If network is client-server…
complex “server” software is needed at the server end
“client” software as peer-peer at the client end

23. Servers High-powered computers
high storage capacity
a lot of memory
Provide network services which are access by users through clients
requires highly specialized software collectively called a Network Operating System (NOS)

24. Servers in small networks
Scenario: a single server (or cluster of servers) is the central controlling point…
The server also looks after security on the network:
only allows valid users to log on
only allows access to resources for users that have logged on
stores appropriate “user rights” for access to its files and directories

25. Architecture of Small Networks
Server(s) still need connecting to clients
Need IP addressing function (DHCP)
set of addresses set through router or server itself
Normal to use Ethernet switch (es) to connect to clients
structure called a VLAN

26. VLANs Segment of a LAN controlled by a switch
Router
(sets IP
Addresses)
Segment of a LAN controlled by a switch
addressing of data to/from VLAN using IP address
packets need routing
addressing between switch and its connected devices using MAC addresses
frames not packets…
more efficient…
no routing needed
IP packets
switch
MAC frames

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

28. Data Link Layer
Responsible for error free transmission, using data frames
A frame is a basic unit for network traffic, and has a highly structured format
Mechanism:
Down: data from the upper layers (ie the network layer) is converted by the data link layer into frames
Up: groups raw data bits received via the physical layer into frames, for passing on to the upper layers
may include an error recovery mechanism and flow control mechanism, but this could be done at the transport layer
Bridges & Ethernet Switches operate at this level

29. Physical Layer Responsible for communicating with the network media
Bits are 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 are deliberately omitted to allow future technology to be easily included
Repeaters work only at this level

30. Network Hardware
Nodes: computers/ other intelligent devices with MAC addresses
Repeaters: boost weak digital signals
Hubs: link devices through a cabling or wireless system
most hubs are also repeaters
Switches/Bridges
OSI level 2 devices that can process and filter the data in various ways, whilst hubs just send it on

31. More Network Hardware Switches Routers Gateways/Firewalls
OSI level 2/3 devices
also used to set up virtual LANs
Routers
Level 3 devices with routing protocols for network/Internet packet routing
Gateways/Firewalls
Level 4-7 devices with software allowing conversion between protocols & control of services

32. IEEE 802 Specifications and Layers 1&2 of the OSI model
Emerged from IEEE/OSI meeting: February ‘80
applied mainly to lower level OSI layers (1/2)
found it necessary to extend the data link layer into two parts
Essential for development of LANs
Definitions used by manufacturers for hardware and software of network interface cards
origin of the MAC address…

33. Effect of IEEE 802 on the OSI model
To cover engineering issues, IEEE divided the Data Link Layer into two sub-layers:
Layer 2 (upper): Logical Link Control – IEE & 802.2
Layer 2 (lower): Media Access Control – IEEE 802.3, 4, 5, 11, 12, etc.

34. Layer 2 (upper) Logical Link Control
Focuses on IEEE & 802.2
Controls transfer of data to the network layer
Uses logical interface points called SAPs (service access points)

35. Layer 2 (lower) Media Access Control
Direct communication with the network card
provides packets with MAC address
Focuses on IEEE 802.3, 4, 5, 11, 12…
Provides shared access for multiple network interface cards to the physical layer
Responsible for ensuring error-free communication across the network

36. OSI layer software and Network cards
Layer 1 and 2 software supplied with the network card
card itself should contain software (on ROM) that conforms to one of the sixteen IEEE 802 specifications
Cards for wired connections have connectors for cables:
usually IEEE 802.3
more rarely… IEEE 802.5
Wireless Cards
usually based on IEEE802.11

37. “Binding” Network Card Software
Data received by the network card needs to be passed on to level 3 software
Normally held on the computer hard disk
Configuration:
level 2 software needs to combine with level 3
achieved through “binding”
OSI Level 3
software
binding
OSI Level
1/2
software
Shows how OSI level 3 software must link in with level 2 software. Process known as binding.

38. IP addresses
For packets to move between devices, each device must have an IP address
e.g
Three ways to allocate an IP address to a Windows PC:
manually… just type it into client interface
from DHCP server/router (between fixed range)
through autoconfig (randomly allocated from a range of IP addresses)

39. Switches and IP addresses
Switches (and routers) link devices together
By default, a switch will create a virtual LAN (VLAN)
allows communication between devices on allocated IP address (e.g )
fine for small networks
regular cause of lack of connectivity!

40. Configuring Switches Come with default configurations for VLANs
May need changing…
IP address needs to be consistent with devices being connected
need IP addresses on the same subnet

41. Check your PC’s IP address
Access the Windows CLI
Type ipconfig
Where has IP address come from?
What is the IP address that it connects to?
Is it on the same subnet?

42. Practical… VLANs connectivity with Switches Simulation MORE Packet Tracer (after the break…)