Digital Convergence (Traffic Prioritization)

Network Computing Issue March 2003
By:- Harbhajan Kaur Julka
Digital convergence makes sense when voice and video traffic over IP have better than
“best effort” packet delivery with guaranteed QoS.
The convergence of voice and video will be very soon. The circuit-switched voice and
video traffic “overlay” networks to packet-based technology will take place. Factors like
bandwidth, network delay, jitter and packet loss are the 4 indicators to evaluate a
network’s service levels. If the network is barely meeting the service levels, then more
bandwidth is required. Distribute the bandwidth using subnets, VLANs, trafficprioritization equipment and content-delivery technologies to help meet the service levels
and keep voice and video traffic flowing. In general, Bandwidth should be as large as
possible while minimizing delay, jitter and packet loss.
The audio and video data is transmitted over UDP, which, unlike TCP, does not
retransmit lost packets. There is no place for out-of-sequence or lost packets. Long
network delays result in stopgap conversations with extensive silent periods between
sentences. Network jitter, the end-to-end delay and sequential packet delivery, results in
jerky video and stuttering audio. The same result occurs when intermediate devices drop
or lose packets because of congestion.
Bandwidth alone will not ensure the smooth delivery of real-time voice and video
packets, especially considering IP’s best-effort nature over Ethernet. Upgrading to the
Layer 3 (Network Layer) switches and traffic prioritization strategies such as RSVP
(Resource Reservation Protocol), DiffServ (Differentiated Services) and MPLS
(Multiprotocol Label Switching) can be helpful.
RSVP is designed to clear a path for audio and video traffic to reduce packet delay and
loss. This flow-based communication protocol, which signals a switch or router to reserve
bandwidth for a real-time transmission, such as a VoIP (voice over IP) call, allows a node
to specify an end-to-end delay, such as 100 ms.
Implementing QoS with MPLS is building a circuit-switched network that can create endto-end circuits over any type pf Layer 2 transport medium. In an MPLS network, a LER
(Label Edge Router) assigns incoming packets a “label”. Packets then travel along an
LSP (label Switch Path) that makes forwarding decisions based on the label’s contents.
At each hop along the path, an LSR (Label Switch Router) strips off the existing label
and applies one with new forwarding instructions. Network operators establish LSPs to
route around network congestion or create IP tunnels for network-based VPNs.
Hence, packet loss and delay, jitter can all be reduced by increasing the bandwidth over a
network and properly distributing the bandwidth using the traffic prioritization equipment
like RSVP, MPLS.