Traffic Types and Wireless Media

Four ways to address a message

Every packet is meant for one host, some hosts, or all hosts. Real-life versions:

• Unicast: a letter to one person. Web browsing, email, file transfers. Almost all traffic.
• Broadcast: the fire alarm in a building. Everyone in the building hears it, and it stops at the walls. On a network the "walls" are routers: they never forward a broadcast. ARP ("who has this IP?") and DHCP Discover ("is there a DHCP server?") are broadcasts, because the sender does not yet know who to ask. A subnet is therefore also a broadcast domain.
• Multicast: a radio station. The sender transmits once, and only receivers that tuned in get it. Efficient for video streams and for routing protocols talking to each other (OSPF uses 224.0.0.5 and 224.0.0.6). IPv4 multicast addresses are the Class D range, 224.0.0.0 through 239.255.255.255.
• Anycast: one phone number for a pizza chain that always rings the nearest store. Many servers share one address and the network delivers each request to the closest one. DNS root servers and content delivery networks use it so everyone gets a nearby, fast answer.

Unicast     one   -> one
Broadcast   one   -> everyone on the segment (routers stop it)
Multicast   one   -> the group that subscribed
Anycast     one   -> the nearest of several sharing an address

One more fact that surprises people: IPv6 has no broadcast at all. It uses multicast for everything broadcast used to do.

Wireless media, briefly

• 802.11 Wi-Fi uses unlicensed spectrum at 2.4, 5, and 6 GHz. There is a whole unit on it later.
• Cellular (4G LTE, 5G) is the carrier's network. Used for phones, as a backup WAN link, and for sites where no wire is available.
• Satellite reaches anywhere with a view of the sky. Traditional geostationary satellites sit about 36,000 km up, so a signal goes up and back down twice (you to satellite to ground station and back), which adds roughly 500 to 600 ms of delay. That is why voice calls over satellite feel awkward. Low-earth-orbit constellations fly much lower and cut the delay dramatically. Rain and snow can degrade either.

How the exam asks it

• "Delivered to the closest of many servers sharing one address": anycast.
• "Sent to every host on the local segment, not forwarded by routers": broadcast.
• "Only hosts that joined the group receive it": multicast.
• "Remote site with high latency on voice calls": satellite.
• "Which protocol has no broadcast": IPv6.

What to memorize

• Unicast one, broadcast all (local only), multicast the group, anycast the nearest.
• Multicast range 224 to 239. Satellite adds about half a second.