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environments is implemented using ATM switches, utilizing in most cases SONET
(STS-3) for transport and management over fiber optic interfaces.
There are two functional ATM User-to-Network Interfaces: the public-UNI and the
private-UNI. The UNI defines the connection between the DTE (user) and the ATM
switch (network). The difference mostly has to do with the type of physical interface
and its application. Private UNIs are defined for LAN applications that have public
UNI support for transmission interfaces that are compatible with PSTN transports
(such as T-carrier and SONET). Along with the UNI, there are also three other ATM
interface types:
• Network-to-Network Interface (NNI)—This is the interconnection point
between two ATM switches.
• Data Exchange Interface (DXI)—This is a serial (or equivalent) interface
between a router/terminal, which sends DXI-formatted information over a
DSU/CSU. The DSU/CSU then performs the data segmentation and formats
the data into ATM cells for transmission over a compatible transport interface
connected to an ATM network.
• Frame User Network Interface (FUNI)—This provides the capability for DTEs
to transmit FUNI frames directly to an ATM switch, which does the
frame-to-cell conversion necessary for data transmission over the ATM
network.
Most common ATM implementations utilize single- or multimode fiber optic cable
and SONET/SDH STS-3 transport for public/private UNI and NNI connections. Table
5.4 lists all the defined ATM public and private UNIs.
Table 5.4. Common ATM Transport Implementations
Transport Medium Application Operating Rate NNI or UNI
ATM over DS1 (Category 3) 1.544 Public
ATM over UTP (Category 3) 26.6 Private
ATM over DS3 (Category 3) 44.736 Public
ATM over STP (Category 3) 155.52 Private
ATM over SONET (SMF/MMF) 155.52 Public/Private
ATM over SONET UTP (Category 3) 155.52 Private
ATM over SONET UTP (Category 5) 155.52 Private
ATM over SONET (SMF) 622.08 Public/Private
SMF = Single Mode Fiber
MMF = Multimode Fiber