Download Network Troubleshooting

SECTION II
TROUBLESHOOTING LOCAL-AREA NETWORKS
322
ATM
10
622 Mbit/s. The cells are transmitted in a continuous stream of ordinary ATM
data cells, OAM cells and idle/unassigned cells. Every 27th cell at most may be a
physical layer cell, which is either an idle cell inserted when no ATM user data
cells are queued for transmission, or a PL-OAM cell. The latter type is used to
carry out the operation monitoring functions that are otherwise accomplished
by SDH/SONET headers. At least one PL-OAM cell is required for every 513 cells.
ATM Cell Streams over V.35, EIA/TIA 449/530, HSSI, and E1
The ATM Forum has specified a cell-based transmission convergence sublayer
based on ITU-T Recommendation I.432 for “clear channel” interfaces. This term
designates all interfaces that are capable of transporting any data stream without imposing bit stream encoding and framing restrictions. Examples include
V.35, EIA/TIA 449/530, EIA/TIA 612/613 (High Speed Serial Interface or HSSI)
and unframed E1. Any other clear channel interface can also be used, however.
The cells are transferred in a continuous stream (of ordinary ATM cells, OAM
cells, and idle/unassigned cells). No F1 or F2 OAM functions are specified for
monitoring the network. F3 OAM functions can be optionally implemented using
special physical layer OAM cells to monitor processes at the level of transmission paths. In this case, the following parameters can be analyzed:
• The number of included cells (NIC) per OAM cell: 128
• The number of cells for which transmission error values are calculated
(Monitoring Block Size or MBS): 16
• The number of blocks monitored per OAM cell: 8
• The number of monitored blocks received by the remote station: 8
The cell rate is decoupled from an interface’s data rate by inserting idle/
unassigned cells. The remote station is synchronized with the individual ATM
cells using the Header Error Control (HEC) mechanism described in the next
section. Cells may be transmitted either in scrambled or in unscrambled form.
10.1.6.5
ATM Cell Streams over FDDI Infrastructures (TAXI)
TAXI is a special variant of cell-based ATM transmission is TAXI. The TAXI
interface was defined to support the use of existing FDDI infrastructures to
transport ATM cells. In this way FDDI rings can be converted into ATM networks
while conserving most of the existing FDDI hardware. The name TAXI originated with the first commercially available chipset for FDDI-based ATM. The
fiber optic media and signal characteristics specified for TAXI communication
are exactly the same as those defined in the FDDI standard ISO 9314-3. The ATM
cells are 4B/5B-encoded and transmitted without any additional framing. In 4B/
5B encoding, 4 bits of data are transmitted as symbols of 5 bits. This is due to the
requirement that no more than three ‘1’ bits occur in a row. The bit sequence
1111 is coded as 11101, for example. Of the 32 possible 5-bit symbols, only 16 are