Every now and then, when teaching students about connectivity architecture and business trends, I have to describe the differences between public networks as they existed pre-internet and the way networks exist today, in the internet era.
It always is messy, as we use both cabled internet data infrastructure, alongside with mobile, satellite and older voice networks, as well as various types of local access and indoor networks. Looking just at the global backbone networks will illustrate the differences.
The public switched network was built to support voice calls, and was centralized and hierarchical, connecting voice switches (Class 4 for wide area transport; class 5 for local distribution and end user connections). Traffic origination was the class 5 switches.
The packet switched network underpinning the internet is a distributed network featuring routers and servers connected over optical wide area transport networks supporting any sort of media type.
Traffic origination is generally content or app servers interacting with end user locations.
So one big difference is that the PSTN was built to connect the voice switches, while the global packet network is built to support servers and computing devices (data centers on one hand and personal computers or other devices on the other).
Where the PSTN used core network active elements, the packet network is more organized around edge elements, with the wide area network basically consisting of routers that forward the traffic.
So one difference between the PSTN and internet networks is that while traffic on the PSTN was mostly generated by telcos, a majority of traffic on the internet is generated by app providers and large data centers. By Cisco estimates, more than half of global wide area network traffic flows between data centers.
The older PSTN was hierarchical, deterministic and well-structured. The internet-supporting packet networks are flatter, more distributed, less deterministic and heterogeneous.
Fixed network access networks remain more recognizable, connecting end user customers with a traffic aggregation point or point of presence (generically, class 5 switch or equivalent for PSTN; ISP data center for packet and internet traffic). The difference for mobile and all other fixed networks is the use of radios for access, rather than cabling of some sort.
It is the fixed network core and “long haul” networks that are different. The PSTN core network connects class 4 and then class 5 switches. The packet network connects points of presence and data centers. The modern mobile network is essentially a packet network with wireless radio tails.
Satellite networks also have been different, as there is really no distinction between the core transport and access networks. Ground stations transmit and receive, with satellites essentially acting as signal relay and retransmission elements.
But the physical architectures (network elements) arguably pale in importance compared to the logical architectures (protocols and how data moves through the networks).
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