History of WAN – Part 2

(This is one of a series of articles on the history of WANs. View previous posts in this series here and here.)


The evolution of private WANs: LANs and WANs converge

Plain, old, slow, regulated, expensive but quality WAN service
In the mid to late 1970s and 1980s, enterprises created wide area networks with interconnecting sets of dedicated leased circuits using T1s and later T3s.  These Time Division Multiplex (TDM) point to point leased circuits were originally developed for digital voice (telecom) but served reasonably well for data communication (datacom).  The circuits were deterministic but expensive per bit.   The payloads were fixed and cells were the rule since the bandwidth was low and large frames could potentially cause substantial voice blocking and jitter.  What we now refer to as plain old telephone service (POTS) provided by phone switch telephone network providers (PSTN) is remembered for its high quality voice and data services over long distances.  For example, Sprint marketed the quality of their long distance voice service with the ability to audibly hear a pin drop over long distances phone calls.  The quality of the network was supported end to end.   If you sent a packet into the network at one side, it was coming out the other side with extremely high confidence. You paid for a TDM time slot in the network and you got it if you needed to use it.  On the other hand if you did not need to use the network, you paid for it anyway.  “Use it or lose it” was the arrangement. This and others factors resulted in the networks being reliable but, by today’s standards, relatively expensive.

Enterprise shift from deterministic, hierarchical, centralized mainframe systems to decentralize, fast, inexpensive probabilistic LAN networks
Meanwhile at the enterprise, in the 1980s- 1990s the enterprise data centers were evolving away from hub and spoke mainframe centric datacom networks toward local area networks (LANs) which utilized common shared network infrastructure with much higher speeds and much lower cost per bit. The economy of collectively using shared resources started driving a new generation of applications and data movement models such as client server and peer to peer. Technologies such as CDMA, Ethernet, LAN hubs, and LAN switches demonstrated that, for most uses, probabilistic LAN technology was good enough for many enterprises’ needs in terms of quality and the cost were low enough where they could afford to provide it to more user and for more diverse uses.

CoS/QoS improves probability, but more bandwidth is easier
Local area network technologies were enhanced to make them relatively higher probabilistic.  Methods such as token passing, priority queue (PQ) tagging and non-blocking layer 2 switches were developed.  For many enterprises, most local area networks issues of low quality service on the network could be dealt with the simple approach of just adding more bandwidth to eliminate congestion points. In the 1990s, it was common for LAN technology to grow in capacity by 10 times every few years. Local area networks were able to increase in speed more rapidly, as compared to wide area networks, because the shorter distances permitted the use of cheaper high speed copper technologies that are not viable over the longer haul wide area networks.


LAN economics and WAN worlds converge, WANs become more probabilistic
In the 90s and early 2000s, the worlds of enterprise telecom wide areas networks and datacom local area networks started to converge with the emergence of CLEC and the ending of the days of regulation.  The change was accelerated by the wide adoption of the Internet Protocol (IPv4) as the de facto standard for all networks LAN or WAN for OSI layer 3 and up over alternatives such as SNA, IPX, OSI, and other alternatives. New packet switched WAN technologies were developed, such as ATM, SMDS, Frame Relay, and eventually MPLS that were friendlier to variable packet sizes and higher speed with less cost per bit but are probabilistic as compared to the deterministic leased circuit services used prior.

The capacity of the service providers access points greatly increased by the adoption of fiber service within provider’s infrastructure.  These speed improvements were brought to some enterprises customer premises where customers could gain access to OC3 (155 Mbps), OC12 (633 Mbps) and OC24 (2.4 Gbps) but the cost of these services to the enterprises was very high as compared to the equivalent speed improvements in available to LAN. The availability of these high speed optical circuits was limited to a small set of geographic locations.  At the remote offices the WANs stayed slow and expensive.

But at least they have Service Level Agreements….  Or did they?

More to come…


(Note to the international reader: In this section we refered to historical patterns as they pertain to the United States market.  Many of these same patterns may have or may not have happened in other national markets.  The trends notes are relevant to the overall evolution of networking and the underlying approach to technology.)


Categories: Software Defined WAN (SD-WAN)