12-6
C
HAPTER
12: ATM, LAN E
MULATION
,
AND
V
IRTUAL
LAN
S
LE Service normally resides on a central ATM switch, such as the CELLplex
7000, but may reside on an ATM end station instead.
An ELAN is assigned a name (
ELAN name
) which is communicated to
each LEC as the LEC joins the ELAN.
More than one emulated LAN can operate on the same ATM network.
However, each of the emulated LANs is independent of the others and
users cannot communicate directly across emulated LAN boundaries.
LAN Emulation Client
The LEC is the LE connection service located in an ATM edge device which
represents
the device’s LAN users to the ATM network. A LEC has a
unique
LEC ID
as well as an
ATM address
by which it is known in the
emulated LAN. It handles the forwarding of its LAN users’ data frames
over the ATM network to their destination, a task which also includes
ascertaining the destination LEC address and setting up the connection
between them.
Also provided is a MAC-level emulated Ethernet service interface to
higher level software which implements the
LAN Emulation User to
Network Interface
(LUNI).
LAN Emulation Server
The LES is the central “directory” service of an emulated LAN to which a
LEC can turn to look up the ATM address of another LEC. The LES
directory contains a table of LAN station MAC addresses together with
the ATM addresses of the LECs that represent them. In order to transmit a
data frame to a particular destination MAC address, the LEC sends the
data frame to the LEC that represents that destination MAC address. If
the LEC does not already know the destination LEC’s address, it can send
the destination MAC address to the LES to look it up (
resolve)
. To
populate the LES directory, the LECs may
register
the MAC addresses of
LAN stations they represent with the LES.
Broadcast and
Unknown Server
(BUS)
The BUS is the LE connection service which handles ATM traffic other
than direct transmissions between LECs. It handles the following:
Q
Data sent by a LEC to the broadcast MAC address
Q
All multicast traffic
Q
Initial unicast frames which are sent by a LEC before the data direct
virtual connection to the ATM address has been resolved
abtthgde.book Page 6 Tuesday, June 23, 1998 10:29 AM
