AT&T 7400 series, 7500 series, 7100 series, 2500 series manual Layer 2 Protocols, Protocol States

1-14TRANSMISSION STATES

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types of trunk protocols (for example, primary rate interface [PRI] and 24th-channel signaling) use DS1 protocol at layer 1. (See the Trunking section of this chapter for an explanation of these trunk types.)

Inside the switch, data transmission appears in one of two forms. It can be raw bits (digital data), the physical layer protocols, like DCP and BRI having been stripped at the incoming port and inserted again at the outgoing port. Or it can be PCM-encoded analog signals (analog transmission via a modem), the signal having been digitized by a codec at the incoming port.

Layer 2 Protocols

At layer 2, the protocols are given below.

∙8-bit character code between the DTE and the DCE. Depending upon the type of equipment used, the code can be ASCII, EBCDIC, or any proprietary code set. ASCII code can be sent asynchronously (one character at a time), or synchronously (one transmission unit, or frame, at a time). EBCDIC is transmitted synchronously.

∙Digital multiplexed interface (DMI) proprietary family of protocols between the originating DCE and the destination DCE for digital transmission. (For a description of this protocol, see appendix D; DEFINITY Communications System and System 75 and System 85 DS1/DMI/ISDN PRI Reference, 555- 025-101; and Digital Multiplexed Interface [DMI] Technical Specification, Issue 3.2, 555-025-204.)

∙Voice-grade data between the originating DCE and the destination DCE for analog transmission.

Protocol States

Table 1-3summarizes the protocols used in switch communication at various points in the transmission stream. Figure 1-3illustrates these protocol-states for data transmission.

Table 1-3.Protocol States for Data Communication

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⎜

⎜

⎜

⎜

Protocols

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜Transmission

⎜

Incoming

⎜

OSI

⎜

DTE to

DCE to

Inside

⎜

⎜

⎜

⎜

Type

⎜

DCE

⎜

Layer

⎜

DCE

Switch Port

Switch

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

Analog

Modem

1

RS-232, RS-449, or V.35

⎜

analog

⎜

PCM

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

2

8- or 10-bit code

⎜

voice-grade data

⎜

voice-grade data

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

ADU

⎜

1

⎜

RS-232

ADU proprietary

raw bits (digital data)

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

2

asynch 8-bit code

⎜

asynch 8-bit code

⎜

DMI

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

Digital

Data Module

1

RS-232, RS-449, or V.35

⎜

DCP or BRI

⎜

raw bits (digital data)

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

2

8-bit code

⎜

DMI

⎜

DMI

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

DS1

⎜

1

⎜

any

DS1

PCM or

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜ raw bits (digital data)

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

2

⎜

8-bit code

DMI or voice-grade data

DMI or voice-grade data

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜

⎜