The internet structure: TCP/IP stack, DNS, protocol layering, LANs and WANs, packet and circuit switching

The internet structure: The TCP/IP Stack DNS Protocol layering LANs and WANs Packet and circuit switching

1.3.3 Networks

Networks — Note 2 of 5

Candidates need to understand the purpose and benefits of layering protocols, particularly within the TCP/IP stack. Candidates need to know the different layers within the TCP/IP stack and the purpose of each. Candidates need to understand how data is transmitted on the Internet, the use of IP addresses and packets in the transfer of data. (NB Candidates are not expected to be familiar with the OSI model). Candidates are expected to understand the terms LAN and WAN. Candidates need to understand how the Domain Name System is used to find the IP of a URL is found. Candidates need to understand the purpose, function, benefits and drawbacks of both packet and circuit switching. Candidates need to be able to suggest packet or circuit switching for a given scenario.

What you need to know

The Internet is a global network of networks. Communication is organised into layers of protocols (the TCP/IP stack), domain names are resolved to IP addresses by DNS, and data is sent across networks using packet switching.

What to know

Benefits of layering: each layer can be developed and changed independently; it simplifies a complex problem; it allows standards to be set for each layer; manufacturers can specialise in one layer.
TCP/IP stack — APPLICATION layer: provides network services to applications using protocols such as HTTP, FTP and SMTP.
TCP/IP stack — TRANSPORT layer: TCP splits data into packets, numbers them, adds port numbers, checks they arrive correctly and requests retransmission of missing packets.
TCP/IP stack — INTERNET (network) layer: IP adds the source and destination IP addresses; routers use these to route packets between networks.
TCP/IP stack — LINK layer: handles physical transmission across the local network, adding MAC addresses and converting data into electrical, light or radio signals.

Key terms

Exam tipCommon mistake

Protocol layering: Network communication is divided into layers, each handling one part of the task using its own protocols, and each only interacting with the layers directly above and below.
DNSDomain Name System: A hierarchy of servers that converts domain names (e.g. www.ocr.org.uk) into IP addresses. If one DNS server does not know the address, the query is passed to another.
Common mistake: Saying DNS stores websites. DNS only maps domain names to IP addresses.
LAN: A network covering a small geographical area, such as a building, using hardware owned by the organisation. WAN: a network covering a large geographical area, connecting LANs using third-party infrastructure.
Packet switching: Data is split into packets, each containing a header (source and destination addresses, sequence number), payload and trailer (checksum). Packets may take different routes and are reassembled in order at the destination.
efficient use of network capacity, resilient if a link fails; but packets can arrive out of order or be delayed. Circuit switching: consistent, in-order delivery suited to real-time communication; but the line is reserved even when no data is sent.
Circuit switching: A dedicated path is set up between the two devices for the whole communication; all data follows the same path in order (e.g. traditional telephone calls).

Exam technique

Tip:You are NOT expected to know the OSI model, only the four-layer TCP/IP stack.