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6 Further System Fundamentals (HL)

6 Further System Fundamentals (HL). 6.4 Further Network Fundamentals. 6-4-2 Packet Switching. Public data networks. PDNs are usually based around the telephone system.

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6 Further System Fundamentals (HL)

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  1. 6 Further System Fundamentals (HL) • 6.4 Further Network Fundamentals

  2. 6-4-2 Packet Switching

  3. Public data networks • PDNs are usually based around the telephone system. • They send information by splitting it into packets, each of which has a header which contains various ‘labels’ and the data itself.

  4. Packets • They are usually fixed in length. • The protocol in use determines the exact characteristics of the packets. • Packet headers contain source address, destination address, ‘time-to-live’ (TTL) data, packet number (position in the whole message sequence) and parity bit.

  5. Packet switching • A local PAD (Packet Assembly-Disassembly) unit splits up the data file and sends off the packets. • Nodes on the network can look at the destination address and pass the packet on. • There may be many nodes in a network and if one becomes busy then alternative routes are used to route the packets (packet switching).

  6. Packet switching • Essentially, packets find their own way across the internet - each router pushes it on in the best direction currently available at that instant. • They may take different routes, before being reassembled at the destination. • The TTL is a number that is decremented by each router.

  7. Advantage • Packet switching is robust (if a link is broken, another is used), • Delays in reassembly are usually small and tolerable for email/text download. • It is less useful for real-time applications e.g. video streaming (the choice of protocols improves this).

  8. The role of routers • These direct the network traffic. • Traffic management involves redirecting packets if necessary to balance the load. • Router programming is very complex - algorithms evaluate the shortest, cheapest route and the availability of nodes.

  9. Errors • Packets with a TTL of 0 are dropped (not passed on, assumed to have been misrouted). • The parity bit is checked on arrival. • Errors can occur from collisions or interference. • If a packet is not received or incorrect, retransmission is requested.

  10. Footnote • Classic phone connections use circuit switching - a dedicated ‘permanent’ connection between the nodes.

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