Page 227 - Asterisk™: The Future of Telephony

While this will certainly increase the chance of a VoIP packet passing quickly through
each link, it does not guarantee anything.

Guaranteed Service
The ultimate guarantee of QoS is provided by the PSTN. For each conversation, a 64
Kbps channel is completely dedicated to the call; the bandwidth is guaranteed. Simi-
larly, protocols that offer guaranteed service can ensure that a required amount of
bandwidth is dedicated to the connection being served. As with any packetized net-
working technology, these mechanisms generally operate best when traffic is below
maximum levels. When a connection approaches its limits, it is next to impossible to
eliminate degradation.

MPLS
Multiprotocol Label Switching (MPLS) is a method for engineering network traffic
patterns independent of layer-3 routing tables. The protocol works by assigning short
labels (MPLS frames) to network packets, which routers then use to forward the packets
to the MPLS egress router, and ultimately to their final destinations. Traditionally,
routers make an independent forwarding decision based on an IP table lookup at each
hop in the network. In an MPLS network, this lookup is performed only once, when
the packet enters the MPLS cloud at the ingress router. The packet is then assigned to
a stream, referred to as a Label Switched Path (LSP), and identified by a label. The label
is used as a lookup index in the MPLS forwarding table, and the packet traverses the
LSP independent of layer-3 routing decisions. This allows the administrators of large
networks to fine-tune routing decisions and make the best use of network resources.
Additionally, information can be associated with a label to prioritize packet forwarding.

RSVP
MPLS contains no method to dynamically establish LSPs, but you can use the Reser-
vation Protocol (RSVP) with MPLS. RSVP is a signaling protocol used to simplify the
establishment of LSPs and to report problems to the MPLS ingress router. The advant-
age of using RSVP in conjunction with MPLS is the reduction in administrative
overhead. If you don’t use RSVP with MPLS, you’ll have to go to every single router
and configure the labels and each path manually. Using RSVP makes the network more
dynamic by distributing control of labels to the routers. This enables the network to
become more responsive to changing conditions, because it can be set up to change the
paths based on certain conditions, such as a certain path going down (perhaps due to
a faulty router). The configuration within the router will then be able to use RSVP to
distribute new labels to the routers in the MPLS network, with no (or minimal) human
intervention.

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