There are two type of links in an ATM network, known as UNI (User to Network Interface and NNI (Network to Network Interface).
The main two types of signalling used in the ATM networks use the NNI and UNI names to indicate where and how they are used. The Switch to Switch connections within a network are known as the PNNI signalling and the User to Network signalling is known as the UNI Signalling.
In order to assisted the signalling process, all switch and interfaces are given an address, with this address being based on the NSAP addressing scheme. Also all user interfaces (e.g. ATM NICs cards) are given an address on initial power up using the ILMI signalling process.
The ATM address, or NSAP is 20 octets (bytes) long and is specifically in two parts, the Initial Domain Part which is supplied by a standards body and the Domain
The Network Operator, when initialising with a new user will supply the Prefix to the user, and the user will add its own identification which is normally the MAC or Ethernet address.
The actual formats used are 20 octets long, however because they are written in Hexadecimal, the complete number is 40 digits long. The first two digits are the AFI - Authority and Format Identifier, followed by one of 3 formats
DCC - Data Country Code, and is assigned by the ITU-T, and is using the ISO country codes. The allocated number is the AFI (1octet) and the DCC (2 octets). Additional to the DCC field each member country will add a 2 to 4 octet prefix to identify multiple users in individual countries and is known as the CDP (Country Domain Part). This leaves between 6 - 8 octets of the HO-DSP for the use of the Network Operator.
ICD- International Code Designator is administered by the British Standards Institute, and the 10 bytes are available for use by the Network Operator. Used by Marconi Switches.
E.164 - Is based on the format used by the ITU -T for international telephone Numbers which are 8 octets long, this provides the E164 part of the NSAP address, the rest is available to the operator..
Finally, The HO-DSP (Higher Order DSP) together is available to the network operator and is assigned by the Standards Authority.
The ESI + SELR ( End System Identifier + Selector) for the user is the MAC address, but the equipment can be the serial number or simply allocated by the network operator. The Selector is the last two digits of the address and normally defaults to '00'. It is normally used to identify more than one virtual object configured within a physical switch or interface.
Associated with each NSAP or ATM Address format is an anycast address format.
The Anycast address allows more than one switch or virtual object to have the same address. The other 19 octets can chosen by the network operator.
A typical NSAP Address would look like this:
The first address is an individual address, and the second is an example of an Anycast address.
If you are intending building a private ATM network, then please consider if it could eventually connect to public ATM networks. If this is the case, then it is a good idea to obtain some public E.164 addresses, which would make the connection to public networks much easier.