ITU-T standard and connectors for access network

The major functions and services performed by the physical layer covers:
Providing a standardized interface to physical transmission media, including

• Mechanical specification of electrical connectors and cables, for example maximum cable length
• Electrical specification of transmission line signal level and impedance
• Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
• Specifications for IR over optical fiber or a wireless IR communication link

http://en.wikipedia.org/wiki/Layer_1

The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces.^[1]

V.35 is an ITU-T standard located on layer 1 on the OSI model. Max speed is 2 Mbit/s. Withdrawn ITU-T recommendation for 48 kbit/s data transmission over wideband circuits. The physical and electrical characteristics of this interface are now specified in ITU-T recommendation V.11.
http://en.wikipedia.org/wiki/List_of_ITU-T_V-Series_Recommendations

G.703 is an ITU-T standard for transmitting voice or data over digital carriers such as T1 and E1. G.703 provides specifications for pulse code modulation (PCM).
G.703 also specifies E0 (64kbit/s). For information about E0 audio see G.711.
G.703 is either transported over 75 ohm co-axial cable terminated in BNC or Type 43 connectors or 120 ohm twisted pair cables terminated in RJ48C jacks. The choice is carrier and region dependant.
http://en.wikipedia.org/wiki/G.703

X.21 (sometimes referred to as X21) is an interface specification for differential communications introduced in the mid 1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switching services, and test loops.
When X.21 is used with V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s.
http://en.wikipedia.org/wiki/X21

The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems.
http://en.wikipedia.org/wiki/D-subminiature#Network_ports