Layer 1 BRI
ISDN standards require the availability of three types of services for end-to-end connections by the local exchange carrier (LEC):
• Circuit-switched services on B channels
• Circuit-switched or packet-switched services on D channels
• Packet-switched services on B channels
238 Chapter 9: ISDN Technology Background
To use them, the ITU-T defined BRI and PRI access interfaces, and the operation speed and the number and type of channels. The BRI is also referred to as 2B+D, and typically operates over two 64-kbps B channels and one 16-kbps D channel. It can also be provisioned as 1B+D, and even 0B+D for low speed (9.6 kbps) data. As you can see, the D channel in BRI operates at 16 K, but in its S/T implementation, the speed is 192 Kbps. It looks like we are losing 48 Kbps. We're actually not. The remaining 48 Kbps can be used for out-of-band forward error correction (OOB FEC).
The following sections cover the elements of a typical ISDN BRI architecture:
• Reference points
• Interfaces
• BRI initialization
ISDN Devices
The hardware that is part of an ISDN service includes the following:
• TE1 — A device compatible with an ISDN network and that connects to NT type 1 or 2 devices. TE1 devices include ISDN routers, modems, and ISDN phones.
• TE2—A device not compatible with an ISDN network and that requires a terminal adapter to connect, such as a regular telephone's old-time terminal and other nonISDN devices.
• Terminal adapter (TA)—Converts non-ISDN signaling to ISDN signaling.
• NT1—Connects four-wire ISDN subscriber line wiring to the conventional two-wire local loop facility. In North America, it is common to find an NT1 at a user's location inside of a networking device (router or modem). In Europe and Asia, NT1 is not part of the ISDN device at the user's location because it belongs to the LEC.
• NT2—Directs traffic from different subscriber devices and from the NT1. The device performs switching and concentrating, and similar to the NT1, it converts wiring within the telephone carrier (four-wire) network to the (two-wire) local loop. The NT2 adds data link layer and network layer functionality to a NT1, and it is usually used with connecting private branch exchange (PBX) devices. CPE can be considered an NT2 device.
• Local Exchange (LE)—Includes Local Termination (LT) and Exchange Termination (ET) on the provider's site.
In the CPE site, the local loop is terminated by using an NT1, which is responsible for performance monitoring, timing, physical signaling, protocol conversion, power transfer, and multiplexing of the D and B channels, as shown in Figure 9-2.
The NT1 devices in Europe and Asia belong to the LEC. It is an external device for the router.
Figure 9-2 ISDN Devices and Access Points
Customer Premises
Cisco ISDN Router S U.S.and Canada
Local Loop ¡in U.S. and Canada
4-Wire,Twisted Pair
PBX, LAN iTerm nal Controller
2-Wire, Twisted Pair
Non-ISDN Terminal (TE2)
Usually Referred to as S/T Interface
Cisco ISDN Router Europe and Asia
I lb
S/T Interface
4-Wire,
Central Office (CO)
Local Loop ¡in U.S. and Canada
2-Wire, Twisted Pair
Central Office (CO)
|
1
|
LT
|
|
ET
|
|
|
-ISDN Switch
4-Wire,
|
NT1
|
|
LT (LE)
|
|
|
ET (LE)
|
|
|
V
|
ISDN Local Loop Reference Points: ISDN Interfaces
ISDN service providers (those providing the last mile or the local loop of the ISDN connection) refer to reference points when troubleshooting parts of the local loop. To connect devices that perform specific functions, the devices need to support specific interfaces, which can vary from CPE to CPE. As a result, the ISDN standards do not define interfaces in terms of hardware, but refer to them as reference points. However, for troubleshooting purposes, it is more reasonable to refer to interfaces. Every ISDN compatible router provides information about the type of connection or interface; and Cisco 77x, 80x, 25xx, and 40xx series routers are examples. These interfaces are as follows:
• R-reference point, or R-interface—This is between the non-ISDN device (TA2) and the TA. The TA allows the TE2 to appear to the ISDN network as an ISDN device.
• S/T reference point or S/T interface—This is usually considered together, where the S-reference point is between the TE1 and NT2, or between the TA and the NT2. The T-reference point is the four-wire (one pair for transmit and one for receive) interface between the customer site switching equipment (NT2) and the local loop termination (NT1). If there's no NT2 equipment, which is common, the UNI is usually called the S/T reference point. S/T is typical for Europe and Asia.
• U-reference point or U-interface—This is a two-wire facility that uses frequency-division multiplexing (FDM) and echo cancellation. It is typical for the U.S. and Canada, and the routers produced for these countries, to have a built-in U-interface. The NT1 device converts the four wire S/T interface to a two-wire U-interface. In Europe and Asia, the NT1 device is in the provider's central office (CO).
• V-reference point—This defines the interface in the LE between the LT and the ET. 
240 Chapter 9: ISDN Technology Background
The real structure of every ISDN design includes numerous implementations. For troubleshooting purposes, it is preferable to simplify the diagram of reference points and interfaces to the illustration (refer to Figure 9-2). In most common cases, consider the simpler structure. For residential and Centrex applications, the NT2 is absent.
NOTE The book Analyzing Broadband Networks, now in its third edition, provides some excellent examples of ISDN architecture and design.
Both the S/T and U-interfaces achieve full-duplex connections. The current wiring rules follow the ISO 8877 standard. RJ-45 is an 8-pin connector. Table 9-2 shows the wiring rules for the S/T interface and Table 9-3 shows the U-interface wiring rules. The polarity is important only for the S/T interface.
Table 9-2 ISDN RJ-45 Pins for the S/T Interface
|
Pin
|
TE Pin
|
NT Pin
|
Required
|
|
1
|
Power source 3(+)
|
Power sink 3 (+)
|
No
|
|
2
|
Power source 3(-)
|
Power sink 3 (-)
|
No
|
|
3
|
Transmit (+)
|
Receive (+)
|
Yes
|
|
4
|
Receive (+)
|
Transmit (+)
|
Yes
|
|
5
|
Receive (-)
|
Transmit (-)
|
Yes
|
|
6
|
Transmit (-)
|
Receive (-)
|
Yes
|
|
7
|
Power sink 2 (-)
|
Power source 2 (-)
|
No
|
8 Power sink 2 (+) Power source 2 (+) No
8 Power sink 2 (+) Power source 2 (+) No
Table 9-3 ISDN RJ-45 Pins for the U-Interface
Pin Function
1 Not used
2 Not used
3 Not used
4 U interface network connection (Tip)
5 U interface network connection (Ring)
6 Not used
7 Not used
8 Not used 
Chapter 3 covers the S/T interface and U-interface line codes and frame formats in greater detail. Table 9-4 describes the physical characteristics of both interfaces and their differences.
Table 9-4 Standards Defining the Physical Characteristics of the S/T and U-Interfaces
|
Reference Point
|
S or T or S/T Interface
|
U-Interface
|
|
Defining standard
|
CCITT I.430
|
ANSI T1.601
|
|
Devices
|
TE1/TA to NT
|
NT1 to LE
|
|
Distance
|
1 km
|
5.5 km
|
|
Physical configuration
|
Point-to-point or point-to-multipoint
|
Point-to-point
|
|
Bit rate
|
192 kbps
|
160 Kbps
|
|
User data rate
|
144 kbps
|
144 Kbps
|
|
Line code
|
Pseudo-ternary
|
2B1Q
|
|
Signaling rate
|
192 Kbaud
|
80 Kbaud
|
|
Maximum voltage
|
+- 750 mV
|
+- 2.5 V
|
|
Timing source
|
NT
|
LE
|
|
Number of wire pairs
|
2
|
1
|
|
Full-duplex method
|
One pair for each direction
|
Echo cancellation
|
|
Interleaving scheme
|
B1g D1B2S D1
|
B18B28 D2
|
|
Number of bits per frame*
|
48
|
240
|
|
Number of bits—user data
|
36
|
216
|
|
Number of bits—overhead
|
12
|
24
|
|
Number of frames per second
|
4000
|
666.666
|
*Referring to frames in the physical layer is not ISO compliant and could be confusing for network engineers; however, to be consistent with the ISDN standard, the adopted terminology is preferred for this and any other technology.
For more information, see ISDN Concepts, Facilities, and Services, Third Edition (McGraw-Hill, 1996).
Initializing BRI
Activation of the BRI is based on the exchange of INFO messages and on the performance of a straightforward handshake mechanism. I.430 defines 5 INFO (0-4) messages that perform different roles in the activation process. 
242 Chapter 9: ISDN Technology Background
As Figure 9-3 shows, the process starts with the TE sending INFO1 to the LE. The A bit that's set to 1 indicates that the line is activated. The INFO0, which is not shown in this figure, indicates "no signal."
Figure 9-3 Initializing the Physical Link
ISDN Switch LE
INFO 1
First Power_Up, First Connection, Alignment Lost
INFO 2
Sent by Network Will Provide TE with Appropriate Frame Synchronization B, D, A, E Set to "0"
INFO 3
Indicated that TE Has Frame Alignment Established
Operational B and D Channels
INFO 4
The Service Is Activated and Operational
I.430 Frame with Operational B and D Echoes D Channel. A Bit Set to "1"
Continue reading here: FCS Field
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