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Networks – basics

Networks – basics. A network consists of: Transmission media (wire, cable, …). Hardware devices (routers, switches, …). Software components (protocol stacks, drivers, …). Terminology: Host: Computers and other devices that use a network.

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Networks – basics

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  1. Networks – basics • A network consists of: • Transmission media (wire, cable, …). • Hardware devices (routers, switches, …). • Software components (protocol stacks, drivers, …). • Terminology: • Host: Computers and other devices that use a network. • Node: Any computer or switching device attached to a network. • Subnet: Set of interconnected nodes. • Design issues: Performance, scalability, reliability, security, mobility, quality of service, multicasting.

  2. Networks – types Range Bandwidth (Mbps) Latency (ms) LAN 1-2 kms 10 – 1000 1 – 10 WAN worldwide 0.010 – 10000 [1] 100 – 500 MAN 2-50 kms 1 – 150 10 Wireless LAN 0.15-1.5 km 2 – 54 [2] 5 – 20 Wireless WAN worldwide 0.010 – 2 100 – 500 Internet worldwide 0.010 – 2 100 – 500 [1]: OC-192 over ATM: http://newsroom.cisco.com/dlls/innovators/switching/eugene_wang_profile.html [2]: IEEE 803.11a: http://www.wi-fiplanet.com/tutorials/article.php/2109881

  3. Networks – packet transmission • Message: Sequence of data items of arbitrary length. • Messages subdivided into packets. • Switching schemes: • Broadcast. • Circuit switching. • Packet switching. • ATM / Frame relay.

  4. Protocols – basics • Protocol: Set of rules and formats to be used for communication between processes in order to perform a given task. • Should include specification of: • Sequence of messages that must be exchanged. • Format of the data in the messages. • Implemented by a pair of software modules in the sending and receiving computers.

  5. Protocols – layers Message received Message sent Layer n Layer 2 Layer 1 Communication Sender Recipient medium

  6. Protocols – encapsulation and headers

  7. Protocols – the ISO Open Systems Interconnection (OSI) model

  8. Protocols – internetwork layers

  9. A 1 B 2 Hosts Links 4 3 or local C networks 5 D 6 E Routers Protocols – network layer routing (WAN) Packet delivery: Datagram or virtual circuit? Routing algorithms, adaptive routing (congestion control)

  10. Internet protocols – Internetworking • Internetwork: Network which integrates a number of different subnets. • Needs: • Unified internetwork addressing scheme (Internet: IP addresses) • Protocol defining format of internetwork packets and specifying rules for handling (Internet: IP protocol). • Interconnecting components that route packets to their destinations (Internet: Internet routers).

  11. Message Layers Application Messages (UDP) or Streams (TCP) Transport UDP or TCP packets Internet IP datagrams Network interface Network-specific frames Underlying network Internet protocols –the TCP/IP protocol suite

  12. Application message port TCP header TCP IP header Ethernet header IP Ethernet frame Internet protocols – encapsulation and headers

  13. Internet protocols – IP • Internet Protocol. • Transmits datagrams from one host to another, if necessary via intermediate routers. • Unreliable, best-effort delivery semantics. • Address resolution: Conversion of Internet addresses to network addresses (for a given network). • Routing: Each router in the Internet implements IP-layer software to provide a routing algorithm.

  14. Internet protocols – IP packet layout and addressing

  15. Internet protocols – TCP and UDP • UDP features: • Transport-level replica of IP. • No guarantee of delivery. • No setup cost, no acknowledgement messages. • Message size up to 64 kbytes. • TCP features: • Reliable delivery. • Arbitrarily long sequences of bytes. • Connection-oriented. • Mechanisms: Sequencing, flow control, retransmission, buffering, checksum.

  16. Ethernet (IEEE 802.3) – Basics • Carriers Sense Multiple Access with Collision Detection (CSMA/CD) • Xerox – Ethernet • Random Access • Stations access medium randomly • Contention • Stations contend for time on medium

  17. Ethernet – ALOHA • Sender • Go ahead and send! • Retransmit if no ACK • Problems • Collisions • Low utilization (18%) • Slotted ALOHA is an improvement (max utilization 37%)

  18. Ethernet – CSMA • Carrier Sense Multiple Access • Observations • Propagation time is much less than transmission time • All stations know that a transmission has started almost immediately • Sender • First listen for clear medium (carrier sense) • If medium idle, transmit • If two stations start at the same instant, collision • Wait reasonable time • Retransmit if no ACK • Max utilization depends on propagation time (medium length) and frame length

  19. Ethernet – CSMA/CD • Carrier Sense Multiple Access – Collision Detection • Observation: With CSMA, collision occupies medium for duration of transmission • Sender • If medium idle, transmit • If busy, listen for idle, then transmit • Station listens whilst transmitting • If collision detected, transmit jam signal, then cease transmission • After jam, wait random time then start again • Binary exponential back off

  20. Ethernet –CSMA/CDOperation

  21. Ethernet – Collision Detection • Bus • Collision produces much higher signal voltage than signal • Collision detected if cable signal greater than single station signal • Star • Activity on more than one input is collision • Special collision presence signal

  22. Summary • Networks. • Protocols. • Internet protocols (TCP/IP). • Ethernet

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