Understanding the network.pdf - Back to Home

• Flag—This is an 8-bit (01111110) field that indicates the start and end of the
frame.
• Address—This is a 16-bit field that contains the Service Access Point
Identifier (SAPI). This is a 6-bit subfield, which indicates which Layer 3
service is associated with the message data. The Command and Response
bit (C-R) indicates if the message data is a command or a response. The
Terminal Endpoint Identifier (variable) specifies the unique identifier used
between the ISDN device and the CO switch. One or more TEIs can be
assigned to an ISDN device. TEI addresses can either be dynamically
assigned when the interface first comes up or set manually. Dynamic TEI
assignment is the common practice. The Extended Addressing (EA) bits are
used for bit stuffing.
• Control—This is an 8-bit field that indicates the type of frame being
transmitted. There are three types:
o Information frames relay signaling or user data in a sequenced order.
o Supervisory frames relay flow control, acknowledgements,
retransmission requests, and so on. Like information frames, these
messages are sequenced.
o Unnumbered frames are used for transmitting data-link messages,
sending unsequenced data, and so on.
• Information—This is a variable length field that contains the ISDN Layer 3
signaling information.
• FCS—This field contains a 16-bit CRC of the messages used for data integrity
checking.
ISDN Layer 3
ISDN Layer 3 services provide the user-network signaling communications for other
functions that establish, monitor, and terminate ISDN connections. These functions
include voice call setup and teardown, virtual-circuit establishment and termination
for data transmissions, and the use of supplemental ISDN services. The "user" does
not necessarily mean an individual person, but rather an "interface" that is
establishing a connection over the ISDN network.
ISDN uses its own D channel Layer 3 protocol, which is defined in ITU-T
recommendations Q.930 (General Signaling, Q.931 (Call Control), Q.932
(Supplemental Services), and Q.933 (Frame-Mode Services). These documents
describe the framing and signaling messages used for various ISDN services. The D
channel protocol is used by the ISDN CPE device and the CO switch to exchange
signaling information. Signaling communications outside of the local loop (that is,
LE to LE) are done using SS7.