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Chapter 1 Overview
24 Avaya M770 M-ACCF/SF ATM Access Modules User’s Guide
Physical Layer
The physical layer is responsible for transmitting and receiving ATM cells over a
physical medium. It is also responsible for checking the integrity of the bits being
transferred over a physical media, and for making sure that they are error-free.
The ATM access module is compliant with both SONET STS-3c and SDH STM-1
physical layer standards.
These standards are similar, and most devices allow you to use either framing
standard on each link in the ATM network.The same framing standard must be
used at each end of the link.
Many users prefer to use the same framing standard throughout their network (for
example SONET STS-3c).
The physical layer is sub-divided into:
• Path — SONET and SDH are capable of carrying traffic for a number of upper
layers, and ATM is only one of those layers. Each upper layer uses its own Path
through the SONET/SDH layer.
• Line — A line is the whole path between one ATM device and the adjacent ATM
switch or ATM end-station.
Extending VLANs into the ATM Network
You can use LAN Emulation to define and extend VLANs seamlessly through the
ATM network, as shown in the example in Figure 1.17.
Traffic from one Emulated LAN (ELAN) is not seen on another ELAN as they are
logically separate domains. For this reason, when you plan your network, you
should consider what ELANs you require, and how the VLANs will map to these
ELANs.
The ATM access module has a LEC for each of the Switch’s 16 VLANS, and each
VLAN/LEC can be mapped onto an ELAN. In this way, Ethernet traffic is mapped
to an ELAN by a VLAN-to-LEC association. The mapping of VLANs to ELANs is
shown in Figure 1.16.
When an Ethernet device attached to a Switch generates traffic, the Switch forwards
the frames to the appropriate port.
A unicast frame is only forwarded to a port if the address of the destination device
is known to be on that port and the destination port is in the same VLAN as the
source port. If a unicast frame is forwarded to the ATM port, the ATM port uses the
destination MAC address to identify the ATM connection to use.
A broadcast or multicast frame is forwarded to all ports in the same VLAN as the
source port. If a frame is received by the ATM port, the ATM port forwards it to the
BUS for the associated VLAN.