The physical layer specifies the characteristics of the “wire” that is used to connect the machines in a network. It also specifies the way in which the bits are encoded. An examples of such a standards is X 21.

A repeater is e level one device as it simply regenerates the bits that it receives and passes them along the “wire”.

Media

• Tapes,
• Twisted Pair
• Coaxial Cable
• Fiber Optics
• Microwave
• Satellite

ISDN

• [A] 4kHz analog telephone channel;
• [B] 64kbps digital PCM channel for voice or data
• [C] 8 or 16 kbps digital channel
• [D] 16 or 64 kbps digital channel for out-of-band signalling
• [E] 64 kbps digital channel for internal ISDN signalling
• [H] 384, 1536, or 1920 kbps digital channel

1. {\bf Basic Rate:} 2B + 1D
2. {\bf Primary Rate:} 23B + 1D (US and Japan) or 30B + 1D (Europe)
3. {\bf Hybrid:} 1A + 1C