Juniper JUNOSE, Configuration Manual

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VCC Cell Relay Encapsulation
E Series routers support virtual channel connection (VCC) cell relay encapsulation
for ATM layer 2 services over MPLS. VCC cell relay encapsulation enables a router
to emulate ATM switch behavior by forwarding individual ATM cells over an MPLS
pseudowire (also referred to as an MPLS tunnel) created between two ATM VCCs, or
as part of a local ATM cross connect between two ATM 1483 subinterfaces on the
same router. The JUNOSe implementation conforms to the required N-to-1 cell mode
encapsulation method described in the Martini draft, with the provision that only a
single ATM virtual circuit (VC) can be mapped to an MPLS pseudowire.
VCC cell relay encapsulation is useful for voice-over-ATM applications that use ATM
Adaptation Layer 2 (AAL2)–encapsulated voice transmission.
AAL0 Raw Cell Mode
VCC cell relay encapsulation supports ATM Adaptation Layer 0 (AAL0) encapsulation,
also referred to as raw cell mode or null encapsulation. AAL0 is often used to carry
signaling ATM cells, which the router treats as raw cells.
When you create an ATM PVC as part of a VCC cell relay configuration, you must
use the
aal0 encapsulation.
Cell Concatenation Parameters
VCC cell relay encapsulation supports the concatenation (aggregation) of multiple
ATM cells in a single encapsulated packet that is transmitted on the MPLS pseudowire.
You can use the atm cell-packing and atm mcpt-timers commands to configure
the following parameters that control how the router performs cell concatenation:
■
Maximum number of ATM cells that the router can concatenate in a single packet.
■
Values (in microseconds) for each of the three ATM Martini cell packing timers
maintained on the router. These timers define the time threshold that the router
uses to concatenate ATM cells and transmit the cells in an MPLS packet on the
pseudowire.
■
Numeric identifier (1, 2, or 3) that indicates which of the three ATM Martini cell
packing timers you want to use to detect timeout of the cell collection threshold.
Based on this configuration, the router attempts to concatenate the specified
maximum number of ATM cells into an MPLS packet within the time interval allowed
by the ATM Martini cell packing timer you selected. When the timer detects that the
allotted time interval has expired, the router forwards the MPLS packet even if it
contains fewer than the specified maximum number of aggregated cells per packet.
Cell Concatenation and Latency
Cell concatenation increases network latency, which is undesirable for voice-over-ATM
applications. To minimize these effects, use care in choosing values for the ATM
Martini cell packing timers.
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ATM Layer 2 Services over MPLS
JUNOSe 11.0.x BGP and MPLS Configuration Guide