The ISDN specifications identify the various functions that must be performed to support customer premises equipment (CPE). ISDN uses the term function group to refer to a set of functions that a piece of hardware or software must perform. Because the ITU wanted several options for the customer, it defined several different function groups. Because the function groups might be implemented by separate products, possibly even from different vendors, the ITU needed to explicitly define the interfaces between the devices that perform each function. Therefore, ISDN uses the term reference point to refer to this interface between two function groups.
- Function group – A set of functions implemented by a device and software.
- Reference point – The interface between two function groups, including cabling details.
Most people understand concepts better if they can visualize or actually implement a network. A cabling diagram is helpful for examining the reference points and function groups. The cabling diagram for several most used examples is here:
Router A is ordered with an ISDN BRI U interface; the U implies that it uses the U reference point, referring to the I.430 reference point for the interface between the customer premises and the telco in North America. No other device needs to be installed; the line supplied by the telco is simply plugged into the router’s BRI interface.
Router B uses a BRI card with an S/T interface, implying that it must be cabled to a function group NT1 device in North America. An NT1 function group device must be connected to the telco line through a U reference point in North America. When using a router BRI card with an S/T reference point, the router must be cabled to an external NT1, which in turn is plugged into the line from the telco (the U interface).
A router can connect to an ISDN service with a simple serial interface, as shown with Router C on the picture. Router C must implement an ISDN function group called TE2 (Terminal Equipment 2) and connect directly to a device called a terminal adapter using the R reference point.
The following tables summarize the types shown in the picture above.
|TE1||Terminal Equipment 1||ISDN-capable four-wire cable. Understands signaling and 2B+D. Uses an S reference point.|
|TE2||Terminal Equipment 2||Equipment that does not understand ISDN protocols and specifications (no ISDN awareness). Uses an R reference point, typically an RS-232 or V.35 cable, to connect to a TA.|
|TA||Terminal adapter||Equipment that uses R and S reference points. Can be thought of as the TE1 function group on behalf of a TE2.|
|NT1||Network Termination Type 1||CPE equipment in North America. Connects with a U reference point (two-wire) to the telco. Connects with T or S reference points to other CPE.|
|NT2||Network Termination Type 2||Equipment that uses a T reference point to the telco outside North America or to an NT1 inside North America. Uses an S reference point to connect to other CPE.|
|NT1/NT2||–||A combined NT1 and NT2 in the same device. This is relatively common in North America.|
And here are the reference points:
|Reference Point||What it connects between|
|R||TE2 and TA|
|S||TE1 or TA and NT2|
|T||NT2 and NT1|
|U||NT1 and telco|
|S/T||TE1 or TA, connected to an NT1when no NT2 is used. Alternatively, the connection from a TE1 or TA to a combined NT1/NT2.|
The ITU planned for multiple implementation options with BRI because BRI would typically be installed when connecting to consumers. PRI was seen as a service for businesses, mainly because of the larger anticipated costs and a PRI’s larger number of B channels. So the ITU did not define function groups and reference points for ISDN PRI!
Encoding and Framing
For any physical layer specification, the line encoding defines which energy levels sent over the cable mean a 1 and which energy levels mean a 0. For instance, an early and simple encoding scheme simply used a +5 volt signal to mean a binary 1 and a -5 volt signal to mean a binary 0. Today, encoding schemes vary greatly from one Layer 1 technology to another. Some consider a signal of a different frequency to mean a 1 or 0. Others examine amplitude (signal strength), look for phase shifts in the signal, or look for more than one of these differences in electrical signals.
ISDN PRI in North America is based on a digital T1 circuit. T1 circuits use two different encoding schemes-Alternate Mark Inversion (AMI) and Binary 8 with Zero Substitution (B8ZS). You will configure one or the other for a PRI; all you need to do is make the router configuration match what the telco is using. For PRI circuits in Europe, Australia, and other parts of the world that use E1s, the only choice for line coding is High-Density Bipolar 3 (HDB3).
PRI lines send and receive a serial stream of bits. So how does a PRI interface know which bits are part of the D channel, or the first B channel, or the second, or the third, and so on? In a word-framing.
Framing, at ISDN’s physical layer, defines how a device can decide which bits are part of each channel. As is true of encoding, PRI framing is based on the underlying T1 or E1 specifications. The two T1 framing options define 24 different 64-kbps DS0 channels, plus an 8-kbps management channel used by the telco, which gives you a total speed of 1.544 Mbps. That’s true regardless of which of the two framing methods are used on the T1. With E1s, framing defines 32 64-kbps channels, for a total of 2.048 Mbps, regardless of the type of framing used.
The two options for framing on T1s are to use either Extended Super Frame (ESF) or the older option-Super Frame (SF). In most cases today, new T1s use ESF. For PRIs in Europe and Australia, based on E1s, the line uses CRC-4 framing or the original line framing defined for E1s. You simply need to tell the router whether to enable CRC-4 or not.
As soon as the framing details are known, the PRI can assign some channels as B channels and one channel as the D channel. For PRIs based on T1s, the first 23 DS0 channels are the B channels, and the last DS0 channel is the D channel, giving you 23B+D. With PRIs based on E1 circuits, the D channel is channel 15. The channels are counted from 0 to 31. Channel 31 is unavailable for use because it is used for framing overhead. That leaves channels 0 through 14 and 16 through 30 as the B channels, which results in a total of 30B+1D.
ISDN BRI uses a single encoding scheme and a single option for framing. Because of this, there are no configuration options for either framing or encoding in a router.
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