Layer Functions And Technology - Panasonic DX-2000 Service Manual

10.2 Layer Functions and Technology

10.2.1 MAC (Media Access Control)
The MAC address is burnt into each network card for establishing addresses for nodes on the network. These addresses are
hexadecimal in nature and are unique for each card. The First three bytes from the left end identify the manufacturer's code that
must be approved by IEEE (Institute of Electrical and Electronics Engineers). The Remaining three bytes on the right half should be
kept in a unique manner. For Ethernet connections, multiple stations share the topology, therefore, the identification packet from
each station should be unique.
10.2.2 Network Control
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
If a node is trying to make a link to the network, transmission from another station is prohibited and halted until the data transfer is
completed and the link is off. CSMA/CD and Token Passing are typical techniques used to control the connection. The General
sequence is as follows:
Wait for the next available timing to send,
Send out a frame,
Perform collision sensing simultaneously,
Retry to send the same frame up to 16 times if necessary.
(Sequence goes by a binary exponential back-off algorithm to avoid periodical incident)
802.3 (Ethernet) frame format
Most significant bit
Pre-amble Destination MAC address
8 byte 6 byte
Destination MAC address: 6 byte (uni-cast or broadcast)
If all "1" on 6 byte (FF-FF-FF-FF-FF-FF) means broadcast frame, it is detected by an applicable node, it must be passed to the
upper layer.
Also, if the destination MAC address is not matched with that node, the frame is discarded at that node immediately.
Token Passing
Token passing utilizes a frame called a token, which circulates around the network. A computer that needs to transmit must wait
until it receives the token, at which time the computer is permitted to transmit. When the computer is done transmitting, it passes
the token frame to the next station on the network.
The first station that is powered up on a token-ring network automatically becomes the active monitor station. Its responsibility is to
announce itself to the next active downstream station as the active monitor station and request that station to announce itself to its
next active downstream station. After each station announces itself to its next active downstream neighbor, the announcing station
becomes the nearest active upstream neighbor (NAUN) to the downstream station. After each station becomes aware of its NAUN,
the beaconing process continues every seven seconds.
A computer in the ring captures the token, if it has data to transmit, it holds the token and transmits a data frame. This data frame is
passed to each computer in the ring, which checks whether it is the intended recipient of the frame. When the frame reaches the
destination address, the destination PC copies the frame to a receive buffer, updates the frame status field of the data frame and
puts the frame back on the ring.When the computer that originally sent the frame receives it back from the ring, it acknowledges a
seccessful transmission, takes the frame off the ring, and places the token back on the ring.
Token Frame indicates that the network is available for transmission.
Data Frame indicates that the network is busy processing a transmission.
XX-XX-XX-XX-XX-XX
Manufacturer ID
MGCS:080023
Source MAC address
6 byte
325
Unique value
Least significant bit
Data type Data
2 byte 46 1500 byte
FCS
4 byte