What a computer network is
Networks connect billions of devices so they can exchange data. Before studying protocols in detail, it helps to see the internet's overall shape: edges, cores, and the links between them.
Hosts, links, and switches
A network connects end systems, also called hosts, such as laptops, phones, servers, and sensors, through communication links and packet switches. Links include copper, fibre-optic cable, and radio. Packet switches come mainly in two kinds: routers, which forward packets between networks, and link-layer switches, which forward frames within a local network.
The network edge
At the edge, hosts connect through access networks: home broadband over DSL, cable, or fibre to the home; Ethernet and Wi-Fi in offices and campuses; and cellular networks such as 4G LTE and 5G for mobile devices. The access network links a host to its first router, the edge router, owned by an internet service provider (ISP).
The network core
The core is a mesh of routers and high-capacity links run by ISPs. Data is broken into packets that are forwarded hop by hop. Routers use store-and-forward transmission: a router must receive an entire packet before sending it on. Each router keeps a forwarding table mapping destination addresses to outgoing links.
Packet switching versus circuit switching
Traditional telephone networks used circuit switching, reserving a dedicated path and fixed capacity for each call, so capacity sits idle during silences. The internet uses packet switching: capacity is shared on demand, and packets from many users are interleaved. Packet switching is efficient for bursty data but can suffer queueing delays and losses when traffic exceeds capacity.
A network of networks
The internet is tens of thousands of interconnected networks. Access ISPs connect to regional ISPs, which connect to large tier-1 ISPs with global reach. Networks exchange traffic at internet exchange points (IXPs) and through peering agreements, often settlement-free, or by paying transit providers. Content providers such as Google and Netflix run their own global networks, connecting directly to many access ISPs.
Who makes the rules
Internet protocols are developed openly by the Internet Engineering Task Force (IETF) and published as Requests for Comments (RFCs); for example, the original IP specification is RFC 791 from 1981. Other bodies matter too: the IEEE standardises Ethernet and Wi-Fi (802.3 and 802.11), 3GPP standardises cellular networks, and ICANN coordinates domain names and IP address allocation.
A short history
Packet switching was developed independently in the 1960s by Paul Baran, Donald Davies, and Leonard Kleinrock. ARPANET sent its first message in 1969. Vint Cerf and Bob Kahn designed TCP/IP in the 1970s, and ARPANET switched to TCP/IP on 1 January 1983. Tim Berners-Lee's World Wide Web (1989-1991) and commercial ISPs in the 1990s turned the internet into a global public network.
Worked example
A phone on home Wi-Fi loads a video. Its frames travel by radio to the home access point, then over fibre to the ISP's edge router. From there, packets cross the ISP's core routers, maybe an IXP, and enter the content provider's network, reaching a server, possibly one placed inside the ISP for speed. Each router on the way stores each packet, looks up its forwarding table, and sends it onward.
Key takeaways
Networks connect hosts through links and packet switches: routers between networks, switches within them. Access networks connect hosts at the edge via broadband, Ethernet, Wi-Fi, and cellular. The core forwards packets hop by hop with store-and-forward routers and forwarding tables. Packet switching shares capacity on demand, unlike circuit switching's reserved paths. The internet is a hierarchy of ISPs linked at IXPs, plus large content networks. The IETF publishes protocols as RFCs; the IEEE, 3GPP, and ICANN play other roles. ARPANET began in 1969 and adopted TCP/IP in 1983.
Flashcard
End system such as a laptop or server
Check your understanding
Why is packet switching more efficient than circuit switching for web browsing?