This enumerated value identifies whether this interface is running SONET or SDH framing at the medium (physical) layer. An admin cares because SONET and SDH use different terminology and slightly different framing/overhead conventions, and this object tells monitoring tools and admins which convention applies to interpreting the rest of this interface's counters and alarms. For example, an interface connected into a European carrier's network would report sdh(2) here, while a North American circuit would report sonet(1).