COMP1321 Digital Infrastructure Richard Henson February 2016

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

The Client-Server Model n Centralisation of organisational resources –client can still hold resources »a lot (fat client) »Not much (thin client) n Microsoft model: called a domain

Request and response 1.All network users use clients 2.Client requests information… 2. Server processes the request, sends a response back to the client

Networks need management… n Two types emerged –Client-Server Networks »networked computers either clients or servers –Peer-Peer Networks »networked computers all of equal status

Requirements of organisational networks n The server would be expected to offer the following to its users: –Network Access –Access to “restricted” files »users with permission directly access files on the server –Applications –Printing –Access to & The Internet

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

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

Peer-Peer networks n Also known as workgroups n No central server n Computer nodes can act as both clients and servers n No expensive powerful machine dedicated to providing services

Peer-Peer networks n All users: –have their own local storage capacity –bear the following responsibilities… »local security & network administration »granting access to their computer’s services and resources via the network

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

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

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

Windows Networks n Peer-peer networks: –workgroups –limited resource sharing ability n Client-server networks: –domains –access to domain via domain controller(s) n Enterprise networks –multiple domains logically linked in a hierarchy

Virtual (client) and Cloud (server) Networks n Extension of client-server model… –client-end less resource intensive –most of resources & processing at server end n Popular because clients need less CPU power & less maintenance –therefore lower cost…

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

What makes up a LAN (1)? n 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

What makes up a LAN (2)? n Software to (just a sample…) –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

Transfer of data through LANs (1) n All done through electrical signals n n 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)

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

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

Network Adaptors n Generally fit inside the computer: –either as a separate card –or on the motherboard… –have their own unique “MAC address” –use own software (firmware) »work with other connectivity software to control the sending and receiving of data

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

Client-Server Networks n A client requests services from a server n Client-server interprocess communication (IPC) fast and reliable n Types of clients: –computer workstation (“fat” client) –computer with limited local storage and processing (“thin” client) –printer with processing ability

Workstations n Designed to work with other computers on a peer-peer network –include the basic networking software required: »to allow connection to the network structure »to communicate effectively with other network nodes n All computers in a peer-peer network are workstations

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

Servers in small networks n Scenario: a single server is the central controlling point n 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

NetworkLayer Network Layer TransportLayer User Specifies Service NetworkService Network provides Service Network layer service definitions

Data Link Layer n n Responsible for error free transmission, using data frames n n A frame is a basic unit for network traffic, and has a highly structured format n n Mechanism: – –data from the upper layers (ie the network layer) is converted by the data link layer into frames – –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 also a flow control mechanism, although this may be done at the transport layer n n Bridges operate up to this level

Physical Layer n n Responsible for communicating with the network media n n Bits are converted into electrical signals and vice versa n n Issues include modulation of signals and timing n n 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 n n Repeaters work only at this level

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

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

IEEE 802 Specifications and Layers 1&2 of the OSI model n 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 n Definitions used by manufacturers for hardware and software of network interface cards –origin of the MAC address…

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

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

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

OSI layer software and Network cards n 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 n Cards for wired connections have connectors for cables: –usually IEEE –more rarely … IEEE n Wireless Cards –usually based on IEEE802.11

“ Binding ” Network Card Software n Data received by the network card needs to be passed on to level 3 software n Normally held on the computer hard disk n Configuration: –level 2 software needs to combine with level 3 –achieved through “ binding ” OSI Level 1/2 software OSI Level 3 software binding

IP addresses n For packets to move between devices, each device must have an IP address –e.g n Three ways to allocate an IP address: –manually… just type it in –from DHCP server (between fixed range) –Through autoconfig (randomly allocated from a range of IP addresses)

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

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

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

Practical … LANs connectivity with Switches Simulation Practical … LANs connectivity with Switches Simulation Using Packet Tracer after the break…