COMP 2221 Networks in Organisations Richard Henson February

COMP 2221 Networks in Organisations Richard Henson February 2014

Week 2: Standards and Computer Networks n Objectives – Explain functions of client-server networks and network services – Define networking standards – Explain how digital networks standards have helped make world-wide digital communications more effective

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 email & The Internet

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

Servers in Larger Networks n Larger networks have MANY servers – University ITS network: at least 50 n Functions can be distributed around different individual servers. Examples: – 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

Problems with this Scenario? n Discussion in Groups… Possible Solutions? n Further Discussion…

Peer-Peer networks 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 n

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

Advantages & disadvantages of Client-Server, compared to Peer-peer n In groups… n Don’t look at next slides!

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

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

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 Extension of client-server model… n Client-end less resource intensive n Most of resources & processing at server end n Popular because clients need less CPU power & less maintenance n – therefore lower cost…

Thin Client/Cloud Advantages and disadvantages? Another 3 minutes….

More about Standards n Definition: – “A standard is an established or accepted model” n Communication protocols… – “Elements of a communication system that are defined by an agreed set of rules, conditions, parameters or methods”

Type of Standards n De Facto – A product or service that is a standard by virtue of its widespread use by interested users n De Jure – The standard devised by a committee of the organisation or, a working group of a subcommittee of a committee of the organisation

Communication Protocols in 1977 n Lots of “proprietary standards” had arisen – IBM – Honeywell – ICL (UK) – Bull (France) – DEC n Each corporation thought theirs was best…

Historic Geneva Meeting (1978) All stakeholders in International communications protocols invited to conference by the Lake… n Had to agree to a hypothetical International communications protocol n No expectation that it would be implemented…. n

Open Systems Interconnect (OSI) Dilemma: all manufacturers wished to have their own communication models represented n Principle: n – “A new layer must be created for each new level of abstraction” n Result: they agreed only by developing a model based on… 7 software layers!

OSI Model – Benefits n All manufacturers… – target to aspire towards n Benefits of OSI compatible products: – 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

Layer Communication (Sending) n 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) n 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… n and so on…

The OSI reference model Transmit Station AH DATA Application Layer DATA AH PH AH DATA Presentation Layer DATA AH PH Session Layer DATA AH PH SH TH SH PH AH DATA Transport Layer DATA AH PH SH NH TH SH PH AH DATA Network Layer DATA AH PH SH NH TH SH PH AH DATA Data link Layer DATA AH PH SH Physical Layer SH PH AH DATA LH Receive Station LT Link LT TH TH TH NH NH LH

Layer Communication (Receiving) n n 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 n e. g. data link layer – removes header to the block of data – passed on to network layer n and so on…

The OSI reference model Transmit Station AH DATA Application Layer DATA AH PH AH DATA Presentation Layer DATA AH PH Session Layer DATA AH PH SH TH SH PH AH DATA Transport Layer DATA AH PH SH NH TH SH PH AH DATA Network Layer DATA AH PH SH NH TH SH PH AH DATA Data link Layer DATA AH PH SH Physical Layer SH PH AH DATA LH Receive Station LT Link LT TH TH TH NH NH LH

Simplifying The OSI model n 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) n Memory aids: – PDNTSPA – Please Do Not Throw Sausage Pizza Away!

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

The Four Layers Model n Introduced with Unix (mid-1970 s, pre-OSI) – based on Internet protocols… “application” “transport” “network” “physical”

TCP/IP n Evolved with the Unix four layers… Application, presentation, session TCP IP Connecting with physical medium

Transport Layer (from Unix) n 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. n Gateways can operate at this layer

Transport Layer (from Unix) n Other roles: – managing flow control – providing acknowledgement of successful transmission of chunks of data – software multiplexing – routing in an Internetwork n Manages OSI levels 1 -4 so messages travel between network nodes via pairs of “sockets” socket A (sender) socket B (receiver)

Transport layer Socket A End User Upper OSI Layers Transport Layer Peer-to-Peer communications Transport Layer Network Layer Data link Layer Physical Layer Network A Network B Socket B

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

Network Layer User Specifies Service Transport Layer Network Service Network Layer Network provides Service Network layer service definitions

Data Link Layer n n n Responsible for error free transmission, using data frames A frame is a basic unit for network traffic, and has a highly structured format 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 Bridges operate up to this level

Physical Layer n n n 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

Standards and the OSI reference model n OSI designed to promote the development of protocols… – that support open systems interconnection n Become an agreed standard in 1984 – ISO 7498 (the International Standard) – BSI 6568 (the identical British Standard) – CCITT recommendation X. 200

How Apple complied with OSI Level 7…. Apple. Share (files, printers, PCs) Apple. Talk Filing Protocol (AFP), Printer Access Protocol (PAP) Apple. Talk Session Protocol (ASP), Zone Protocol (ZIP), Apple. Talk Data Stream Protocol (ADSP) Apple. Talk Transport Protocol (ATP), Apple. Talk Echo Protocol (AEP), Name-Binding Protocol (NBP), Routing Table Maintenance Protocol (RTMP) Level 3…. Datagram Delivery Protocol (DDP) Local. Talk Link-Access Protocol (LLAP), Ethernet Link-Access Protocol (ELAP), Token ring Link-Access Protocol (TLAP), Other Link-Access Protocols Local. Talk, Ether. Talk, Token. Talk, or other Network Topologies Level 1…. STP, UTP, Coax, Fiber

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 sublayers: – Layer 2 (upper): Logical Link Control – IEE 802. 1 & 802. 2 – Layer 2 (lower): Media Access Control – IEEE 802. 3, 4, 5, 11, 12, etc.

Layer 2 (upper) Logical Link Control n Focuses on IEEE 802. 1 & 802. 2 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 n n 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

OSI layer software and Network cards n 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 Cards for wired connections have connectors for cables: – usually IEEE 802. 3 – more rarely… IEEE 802. 5 n Wireless Cards – usually based on IEEE 802. 11

“Binding” Network Card Software n OSI Level 3 software binding n OSI Level 1/2 software n 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”

Now for the practical… Group A: after break (10. 25) Group B: in one hour (11. 25) Self-Study: check out all the IEEE 802. x standards and decide which of these are most important in 2014… feedback next session so you’d better do this!