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.
We recommend that for voice-over-ATM configurations, you select timeout values
between 6 microseconds and 3 x 6 microseconds. Values within this range are generally
low enough to maintain a reasonable cell delay and high enough to take advantage of
the cell concatenation mechanism.
If traffic shaping is enabled on the egress router, the JunosE implementation of VCC cell
relay encapsulation preserves the spacing between cells.
Control Word Support
VCC cell relay encapsulation requires use of a control word. The control word contains
the T, E, C, and U bits, as well as an optional sequence number field.
The JunosE implementation of VCC cell relay encapsulation supports the T bit, which is
always set to indicate raw ATM cells, and the optional sequence number. The E, C, and
U bits have no meaning for VCC cell relay configurations because the router forwards a
complete ATM cell with as much header information as possible.
Unsupported Features
VCC cell relay encapsulation on JunosE routers does not support the following features:
•
Mapping multiple ATM VCs to a single MPLS pseudowire
•
Optional Martini one-to-one cell encapsulation method with cell headers removed
Related Topics
For information about AAL5 SDU encapsulation, see Encapsulation Methods for
Transport of ATM Over MPLS Networks—draft-ietf-pwe3-atm-encap-07.txt (April
2005 expiration)
•
•
For configuration examples of AAL5 SDU encapsulation using the
aal5all
encapsulation
keyword when you configure ATM subinterfaces, see Configuring Local ATM
Cross-Connects with AAL5 Encapsulation on page 534
•
For information about, VCC cell relay encapsulation, see Encapsulation Methods for
Transport of ATM Over MPLS Networks—draft-ietf-pwe3-atm-encap-07.txt (April
2005 expiration)
•
For VCC cell relay encapsulation configuration instructions, seeConfiguring an MPLS
Pseudowire with VCC Cell Relay Encapsulation on page 535
•
Control Word Support on page 518
519
Copyright © 2010, Juniper Networks, Inc.
Chapter 8: Layer 2 Services over MPLS Overview
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