Networks A2


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A2 network talk

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Networks A2

  1. 1. Computer Networks (A2) ( PSC as an example) Charles Parish [email_address]
  2. 2. Topics we’ll cover <ul><li>Network protocol stack </li></ul><ul><li>Copper, fibre and wireless </li></ul><ul><li>Bus and star topologies </li></ul><ul><li>Hubs, bridges, switches and routers </li></ul><ul><li>Baseband v broadband </li></ul><ul><li>Ethernet </li></ul><ul><li>Addresses – MAC and IP (inc. network and host) </li></ul><ul><li>Packets and packet switching </li></ul><ul><li>TCP/IP </li></ul><ul><li>Subnets, subnet masks and gateways </li></ul><ul><li>Connecting to a WAN </li></ul>
  3. 3. OSI Protocol Stack Example Description OSI Level N Cables, connectors etc. Electrical properties of the cable Physical 1 Ethernet Transmit & receive packets Data Link 2 IP Packet routing Network 3 TCP Transport – guaranteed delivery of packets Transport 4 Authentication e.g. SSL Session 5 Data compression e.g. ASCII, MPEG Presentation 6 In practice these three layers combine in TCP/IP as ‘layer 7’ HTTP, SMTP Programs communicating over network e.g. SMTP, HTTP Application 7
  4. 4. Network Connectivity (1) <ul><li>Copper </li></ul><ul><ul><li>Coaxial: </li></ul></ul><ul><ul><ul><li>10 Mb ( mega bits per second ) max - bus configuration </li></ul></ul></ul><ul><ul><ul><li>Uses BNC bayonet connectors </li></ul></ul></ul><ul><ul><ul><li>Old technology </li></ul></ul></ul><ul><ul><li>Unshielded twisted pair (UTP) </li></ul></ul><ul><ul><ul><li>10Mb, 100 Mb, 1Gb, 10Gb - star configuration </li></ul></ul></ul><ul><ul><ul><li>Uses connectors similar to those for telephones (RJ45) </li></ul></ul></ul><ul><li>Good for internal cabling within buildings </li></ul>
  5. 5. Network Connections (2) <ul><li>Fibre Optic </li></ul><ul><ul><li>Two cores required for each connection (transmit / receive) </li></ul></ul><ul><ul><li>Usually bundled with multiple cores 8, 16, 24 etc. </li></ul></ul><ul><ul><li>Different grades e.g. 50-125, 62.5-125 </li></ul></ul><ul><ul><li>Multimode / Single mode </li></ul></ul><ul><li>Good for external or high security use as it: </li></ul><ul><ul><li>Is capable of high bandwidth </li></ul></ul><ul><ul><li>can run over long distances </li></ul></ul><ul><ul><li>Is not affected by electricity, water, lightning </li></ul></ul><ul><ul><li>Can’t be tapped </li></ul></ul>
  6. 6. Network Connections (3) <ul><li>Wireless </li></ul><ul><li>Up to 54 Mb 802.11g – faster coming </li></ul><ul><li>Flexible layout and no cabling </li></ul><ul><li>but </li></ul><ul><li>Security problems </li></ul><ul><li>Slow compared with cable and it’s a shared medium (hubs not switches) </li></ul><ul><li>Useful for laptops & PDAs </li></ul>
  7. 7. Network Devices (1) <ul><li>Hub </li></ul>Computer A sends data to computer D Data are broadcast to all computers
  8. 8. Network Devices (2) <ul><li>Bridge </li></ul><ul><ul><li>Two ports </li></ul></ul><ul><ul><li>Divides the traffic between two segments of a network </li></ul></ul>
  9. 9. Network Devices (3) <ul><li>Switch </li></ul>Computer A sends data to computer D Data are sent to computer D only
  10. 10. Network Devices (4) <ul><li>Router </li></ul>A router connects two (or more) different networks such as an Ethernet LAN and a Frame Relay (leased line) or ADSL (commonly called broadband) WAN Note: Some switches now also route between ethernet VLANs (virtual networks) or subnets. These are called layer 3 or routing switches. LAN WAN
  11. 11. Baseband v Broadband Long distances 1km max before repeater E.g. cable TV E.g. Ethernet Can have multiple channels One channel Transmission in one direction Transmission in both directions Analogue signalling Digital signalling Broadband Baseband
  12. 12. Network Technology <ul><li>Ethernet is most common LAN network technology </li></ul><ul><ul><li>Developed in early 1970s by Xerox at PARC </li></ul></ul><ul><ul><li>Specified in IEEE 802.3 series of standards </li></ul></ul><ul><ul><li>Hardware and software layers </li></ul></ul><ul><ul><li>Data sent in packets </li></ul></ul><ul><ul><li>Uses a Carrier Sense Multiple Access with Collision Detection ( CSMA/CD ) protocol </li></ul></ul><ul><li>Other common examples would be Appletalk, Bluetooth, X25 </li></ul>
  13. 13. Packet Encapsulation <ul><li>We have said that the network is a stack </li></ul><ul><li>Each layer is encapsulated (wrapped) in the layer below </li></ul><ul><li>So the application data is the data part of the TCP (socket –addressed) packet; the TCP packet is the data of the IP (IP addressed) packet, and the IP packet is finally the data part of the ethernet (MAC addressed) packet. </li></ul>
  14. 14. Ethernet Address (Layer 2) <ul><li>Every object on a network must have an address </li></ul><ul><li>In an ethernet network there is the MAC address: </li></ul><ul><ul><li>Media Access Control. This is hardware specific and 6 bytes (48 bits) long. It is usually written as 12 hexadecimal numbers. E.g. 00C36F1975BD </li></ul></ul>
  15. 15. Ethernet Packet (Layer 2) Destination Address Ethernet MAC address of the destination host, 6 bytes (48-bits) Source Address Ethernet MAC address of the source host, 6bytes (48-bits) EtherType Type of data encapsulated, e.g. IP, ARP, RARP, etc, 16-bits. Data Field Data area, 46-1500 bytes, which has CRC Cyclical Redundancy Check, used for error detection
  16. 16. Internet Protocol - Layer 3 <ul><li>Runs on top of various network technologies such as ethernet </li></ul><ul><li>IP address – Internet Protocol address. This is constant for a session but may vary each time you connect to the network. It is 4 bytes (32 bits) long and is written as 4 numbers, each between 0 and 255 and separated by dots e.g. </li></ul>
  17. 17. IP Packet
  18. 18. Transport Protocol – Layer 4 <ul><li>Each application that runs uses a TCP port . Each application has a standard port. E.g. </li></ul><ul><ul><li>FTP (file transfer) port 21 </li></ul></ul><ul><ul><li>SMTP (email) port 25 </li></ul></ul><ul><ul><li>HTTP (web) port 80 </li></ul></ul><ul><li>A socket is a combination of the address and the port e.g. </li></ul>
  19. 19. TCP Packet
  20. 20. TCP/IP subnets <ul><li>Address is made up of network and host addresses </li></ul><ul><li>E.g. an address with a subnet mask of has: </li></ul><ul><ul><li>Network address 192.168.130 (this is the subnet ) </li></ul></ul><ul><ul><li>Host address 156 </li></ul></ul><ul><li>If it is to access another subnet it must also point to a gateway in that subnet (e.g. </li></ul><ul><li>The owner of such a subnet has 254 effective host addresses (1 to 254) as 0 is the subnet address and 255 the broadcast address </li></ul>
  21. 21. Routers and VLANs
  22. 22. Some Network Operating Systems <ul><li>Unix </li></ul><ul><ul><li>Various ‘flavours’: Solaris, Linux, AIX </li></ul></ul><ul><li>Microsoft </li></ul><ul><ul><li>NT, Windows 2003 Server </li></ul></ul><ul><ul><li>Microsoft Active Directory </li></ul></ul><ul><li>Novell </li></ul><ul><ul><li>Netware 6.5 </li></ul></ul><ul><ul><li>Uses Novell Directory Services (NDS) to store objects </li></ul></ul>
  23. 23. Some Links <ul><li> IT definitions </li></ul><ul><li> Wikipedia – free encyclopaedia </li></ul><ul><li> JANET (Joint Academic Network) </li></ul><ul><li> Government links </li></ul><ul><ul><li>E.g. </li></ul></ul><ul><li> World Wide Web Consortium </li></ul><ul><li> UK internet registrations </li></ul>
  24. 24. A view of PSC Network