Understanding the network.pdf - Back to Home

• Physical layer signaling (PLS)
• Attachment unit interface (AUI)
• Physical medium attachment (PMA)
• Medium-dependent interface (MDI)
The PLS-to-MAC interface is responsible for transmitting and receiving bit data, and
performing the carrier sense and collision detection duties. The AUI interface passes
signals between the PLS and the PMA interface.
The AUI interface can be either integrated into the NIC or exist as an external
physical interface. Its purpose is to give end-stations the independence to connect
to different transmission media. The AUI interface has very exacting technical
specifications that must be adhered to for proper operation. The AUI interface
transmits data in a bitstream, 1 bit at a time, between the NIC and the media
attachment unit (MAU) at a 10MHz operating rate (10Mbps). As an external
interface, it uses a 15-pin female interface (on the NIC) and a 15-pin male on the
MAU. The connecting cable between the NIC and the MAU must be no longer than
50m. When an external AUI is used with an MAU in a bus topology, the MAU
represents the transmission medium interconnect. In multiport repeater or star
topology, the cabling between the NIC and the repeater port is the horizontal cable
connection. The cabling the MAU attaches to is referred to as the backbone cabling.
The backbone cabling has its own distance and connector requirements, which are
dependent on the type of transmission medium being used.
The MAU consists of the PLA and PMA interfaces. Substantial differences exist
between the MAUs used for baseband and broadband. Baseband Ethernet is what is
commonly used for LAN connectivity, whereas broadband implementations are used
almost exclusively for consumer networking over public cable systems. Broadband
Ethernet implementations have gained popularity in the last few years because they
allow cable systems to provide Internet access over a cable TV infrastructure. Both
broadband and baseband MAUs perform the same function, they just do it
differently.
With a baseband MAU, the PMA takes signals from the AUI, converts them
appropriately, and sends them on to the medium through the MDI, performing in
reverse when transmissions are being received. The PMA is also responsible for
detecting collisions, and it relays notification about them to the PLS over the AUI. In
addition, the PMA acts as a transmission regulator, suppressing transmission
activities when signals are being received and "listening" to the wire for collisions
and incoming transmissions. The PMA also enforces the 20-millisecond maximum
transmission period (a function known as jabber protection) rule, when long
datastreams need to be transmitted. The PMA functions remain the same regardless
of the MDI.