SMDS - Switched Multimegabit Data Service - Yenra

SMDS was an early public packet-switched WAN service that promised LAN-like metropolitan and wide-area connectivity before ATM, MPLS VPNs, and Carrier Ethernet took over

SMDS - Switched Multimegabit Data Service
SMDS - Switched Multimegabit Data Service

SMDS, or Switched Multimegabit Data Service, was a high-speed, connectionless, packet-switched WAN and metropolitan-area service offered by some carriers in the 1990s. It was intended to give enterprises a public-network way to interconnect LANs at multimegabit speeds without buying a full mesh of private leased lines. In an era when 10 Mbps Ethernet was common and wide-area links were expensive, that was a serious promise.

The original article described Telecom Italia's use of AT&T BNS 2000 Broadband Switches to build C-LAN, a nationwide broadband service network for business customers. C-LAN used Frame Relay and SMDS connectivity to support banking, manufacturing, transportation, research, and enterprise data applications, while offering a migration path toward ATM services.

What SMDS Offered

SMDS tried to make a carrier data service feel more like a LAN. RFC 1209 describes SMDS as a connectionless public packet-switched service with datagram transfer, high throughput, low delay, large user-information fields, individual and group-addressed packets, and service-provider screening features that could create closed user groups.

That combination was attractive because customers could connect multiple sites into a logical private network across a public carrier infrastructure. Instead of ordering a dedicated private line between every pair of offices, an enterprise could attach sites to the SMDS cloud and let the provider switch packets among them according to addressing and access rules.

Technical Shape

SMDS was based on cell relay technology and Bellcore technical advisories derived from IEEE 802.6 Distributed Queue Dual Bus (DQDB). Cisco documentation from the era describes SMDS access through DS-1 or DS-3 facilities and an SMDS CSU/DSU, with support for protocols such as IP and IPX. IP over SMDS used LLC/SNAP encapsulation and ARP mapping between IP addresses and SMDS addresses.

Important SMDS ideas included:

The Telecom Italia C-LAN Context

Telecom Italia's C-LAN was positioned as a way for businesses to link multiple corporate networks over a public service and operate them as a seamless virtual private network. The AT&T BNS 2000 switches supported Frame Relay and SMDS, with management through the StarKeeper II Network Management System for fault, performance, configuration, security, and billing functions.

The announcement also reflected the market mood in 1995. European telecommunications liberalization was approaching, enterprises wanted "LAN-like" public WAN services, and carriers needed platforms that could carry existing protocols while moving customers toward newer broadband packet services. Frame Relay and SMDS were part of that migration story, while ATM was presented as the next step.

SMDS Versus Frame Relay And ATM

SMDS, Frame Relay, and ATM overlapped but had different models:

Frame Relay won more enterprise WAN mindshare because virtual circuits matched carrier operations and customer designs well. ATM became important in carrier cores, DSL aggregation, and some enterprise backbones. SMDS remained more of a transitional and regional service, remembered by network engineers but rarely seen after Ethernet, MPLS, and IP services matured.

Why SMDS Disappeared

SMDS did not fail because the problem was unimportant. It disappeared because other technologies solved the problem with better economics, broader vendor support, and cleaner operational models. Switched Ethernet became the enterprise LAN standard. Carrier Ethernet gave service providers a familiar Ethernet handoff for metro and wide-area services. MPLS VPNs gave enterprises scalable any-to-any private IP reachability. Internet VPNs and later SD-WAN made public broadband usable for many business paths.

The result is that SMDS became unnecessary. Enterprises could get higher speeds, simpler equipment, better tooling, and broader provider availability from Ethernet, MPLS, and IP-based services. Carrier networks also standardized around packet, label-switched, and optical transport platforms that did not require customers to understand SMDS-specific addressing or access behavior.

Modern Equivalents

The role SMDS once aimed to fill is now split among several services:

Design Lessons

SMDS is historically useful because it shows what enterprises wanted before the modern WAN stack existed: LAN-like connectivity, private groups over a public provider network, high-speed access, multicast or group reachability, protocol flexibility, and manageable service operations.

Those requirements still apply. A modern WAN refresh should ask:

SMDS is no longer a service most organizations will buy or operate. Its legacy is the concept of a carrier-managed packet cloud that could make distant LANs feel connected. Today that idea lives on in MPLS VPNs, Carrier Ethernet, EVPN, SD-WAN overlays, and cloud networking, all of which keep chasing the same goal: flexible, reliable, private connectivity without a dedicated circuit between every pair of sites.

References