UNI   - User-to-Network Interface

UNI (You-Knee) - The interface between any of:

o User ATM equipment (e.g. hosts) and private or public ATM network 
  equipment (e.g. ATM switches)

o CPE (Customer Premises Equipment, e.g. private ATM switch) and 
  public network equipment (e.g. provider ATM switch)

The ATM Forum has defined two contexts in which the UNI appears:

1) Public UNI, connecting ATM equipment within a private network--either
   hosts or switches--with public network provider (carrier) networks. 

2) Private UNI, used exclusively to connect hosts to switches where both 
   are managed by the same administrative entity.

Note that switch-to-switch interconnection is defined by the NNI
(Network-to-Network Interface) in the case where the interconnection is
not between a private and public network (i.e. the interconnection resides
wholly within either a private or a public network). 

These concepts are illustrated below:

              Private                      Public
                UNI                         UNI
    ATM-User-----------Private ATM Switch---------Public ATM Network
                                                    /
                                                   /
    ATM-User-----------Public UNI-----------------/


Specification of the ATM Forum private and public UNIs involves ITU
(International Telecommunications Union) B-ISDN Protocol Reference Model
(B-ISDN PRM) layers one, two, and three; i.e. the physical media (Layer 1)
over which the ATM protocol layer (Layer 2) is carried, and the signaling
AAL (layer 3) necessary to implement the UNI. 

The current published ATM Forum UNI specification is version 3.0.  An
erratum was produced by the ATM Forum Technical Committtee. This erratum
specifies changes to the ATM Forum UNI 3.0 specification to align it with
current ITU work involving Q.2931.