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The Open Systems Interconnection (OSI) Reference Model of the International
Organization for Standardization (ISO) describes the structure of networks. It
defines the seven layers shown at the left side of Fig. 2 on page 9. The lower three
layers are relevant to the demand priority protocol.
Layer 3: Network Layer (NL). The network layer is responsible for passing a
packet of data through an internetwork, which can consist of many individual LANs
and even wide area links.
Layer 2: Data Link Layer (DLL). The data link layer provides the transmission of
data between two nodes on the same network. It receives a packet from Layer 3,
the network layer, and adds source and destination addresses and other information to it. The DLL consists of two sub-layers: the logical link control (LLC) sublayer and the media access control (MAC) sublayer.
The LLC sublayer links the network layer to the MAC. There are standard LLCs,
enabling different protocols to link successfully. In 100VG-AnyLAN, the LLC can
be either an IEEE 802.2 Class I LLC, supporting Type 1 unacknowledged, connectionless-mode transmission, or a Class II LLC, supporting Type 2, connectionmode transmission.
When a frame is ready for transmission in an end node, it is sent from the LLC
sublayer to the MAC sublayer where the appropriate Ethernet or token ring MAC
frame is built. The frame is then passed to the physical layer’s PMI (physical
medium independent) sublayer.
When a data packet is received from the physical layer, the MAC sublayer reassembles the MAC frame and performs various checks for errors in the received
frame. Only valid frames are sent on to the LLC sublayer in an end node.
Frames received by the hub’s RMAC sublayer are forwarded to the addressed
destination (if it can be determined) and to all promiscuous ports regardless of
error condition (there is no LLC in a hub). Frames containing errors are marked
with an invalid packet marker.
Layer 1: Physical Layer (PHY). The physical layer defines the procedures and
protocols associated with the physical transmission of bits (such as cable interfaces, data signal encoding, and connector types and pinouts). It consists of two
sublayers: the PMI sublayer and the physical medium dependent (PMD) sublayer.
The PMI sublayer includes provisions for quartet signaling, data ciphering, 5B/6B
encoding, the addition of preambles, and start and end frame delimiters (SFD and
EFD). See the articles on pages 18 and 27 for details.
During transmission, the PMI sublayer accepts data from the MAC sublayer and
prepares the packet for transmission. It converts the octet data into quintets which
are separated into four streams. In each stream, each quintet is ciphered and then
encoded as a 5B/6B sextet. The PMI adds physical layer headers and trailers to
each data stream.
When receiving data packets, the PMI removes physical layer headers and trailers
and then passes the packet onto the MAC layer. It decodes each received 5B/6B
sextet, deciphers the resulting quintet, merges the four deciphered quintet
streams, and converts the result into a single octet stream for delivery to the MAC
sublayer.
The PMI sublayer connects with the PMD sublayer through the medium independent interface (MII).
The PMD sublayer includes channel data packet multiplexing (for 2-pair STP and
fiber-optic cabling only), NRZ encoding, link medium operation, and link status
control. It connects with the physical medium (the cable) through the medium
dependent interface (MDI).
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