G is a series of recommendations (standards) from Study Group 15 (Transport, Access and Home) of the International Telecommunication Union (ITU-T). The G series concerns "Transmission systems and media, digital systems and networks".

Specification Range Specification Type
G.100 - G.199 International Telephone Connections and Circuits
G.200 - G.299 General Characteristics Common to all Analogue Carrier Transmission Systems
G.300 - G.399 Individual Characteristics of International Carrier Telephone Systems on Metallic Lines
G.400 - G.449 General Characteristics of International Carrier Telephone Systems on Radio-Relay or Satellite Links and Interconnection with Metallic Lines
G.450 - G.499 Coordination of Radiotelephony and Line Telephony
G.600 - G.699 Transmission Media and Optical Systems Characteristics
G.700 - G.799 Digital Terminal Equipments
G.800 - G.899 Digital Networks
G.900 - G.999 Digital Sections and Digital Line System
G.1000 - G.1999 Quality of Service and Performance – Generic and User-Related Aspects
G.6000 - G.6999 Transmission Media Characteristics
G.7000 - G.7999 Data Over Transport – Generic Aspects
G.8000 - G.8999 Packet over Transport Aspects
G.8000 - G.8099 Ethernet over Transport aspects
G.8100 - G.8199 MPLS over Transport aspects
G.8200 - G.8299 Quality and availability targets
G.8600 - G.8699 Service Management
G.9000 - G.9999 Access Networks