Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Quality of Service

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MPLS

Multiprotocol label switching is the most advanced QoS measure available for enter-

prise VoIP networks. MPLS bears great similarity to ATM signaling but borrows

heavily from RSVP. Unlike ATM, which incurs a 25% overhead on TCP/IP traffic

(called the ATM “cell tax”), MPLS doesn’t use its own framing format, just its own

labeling format. This gives it the ability to work with Ethernet and other non-ATM

switching technologies.

MPLS’s role is in carrier-grade networks or extremely large enterprise networks with

tens of thousands of nodes. Carrier-grade networks can use a mixture of MPLS, Diff-

Serv, and RSVP all carried on a legacy ATM topology even as they seek to migrate

away from ATM. MPLS can be supported by just about any modern topology.

The purpose of MPLS labels is to identify the paths and priorities associated with

each packet. The paths correspond to the media channel of the VoIP call, while the

priorities respond to the QoS level of service negotiated for those channels, just like

RSVP.

But like DiffServ, MPLS can use a dumb network core. If a packet is carrying a label,

all a router has to do is send it along the labeled path, rather than making a redun-

dant assessment of the packet’s payload. With MPLS, specialized, narrow-function

circuitry can handle ...