3.2.2 Packet Networks
Packet networks offer integrated multiple service levels at a quality agreed upon by
the service provider and the client, based on a service level agreement (SLA). Packet
networks perform bandwidth management, buffering (queuing), scheduling, routing
and dynamic rerouting, policing, and traffic shaping.
Bandwidth management consists of functions such as the Call Admission Control,
Class of Service Routing, and Signaling. CAC is performed per service level.
Routing requires knowledge of the complete network topology in terms of available
bandwidth per service level and it executes shortest path algorithms (e.g., traveling
salesman algorithm). When the optimum route has been identified, it forms a routing
database. Routing also executes scheduling algorithms and simple or complex
queuing algorithms per SLA. Dynamic rerouting may also be performed during service.
Policing provides a provisioned mechanism at the entry point (edge) of the network
to verify and also control subscriber traffic parameters in terms of the SLA.
Traffic shaping is a buffered function that smoothes the traffic exiting the system
to ensure compliance with the agreed-upon profile, and by proper bandwidth management
it avoids congestion situations.
Packet networks assemble a number of client data bytes (or octets) into a payload
block, attach overhead bytes to it, and form a packet, the length of which is
measured in octets or bytes. Typically, there are two types of packet networks:
those that form a short fixed-length packet and those that form a variable-length
packet; packet lengths may vary from forty to many thousands octets.
Packet networks route bursty data and, therefore, it is doubtful that they can meet
the real-time requirements of voice and real-time (interactive) video without substantial
signal delays, unless excess network bandwidth capacity and sophisticated
protocols are used.
Besides the delay introduced during packet assembly and disassembly, large delays
are experienced during the switching process. Clearly, the more switching elements
on the path there are, the higher the overall latency is. Buffering and delays
impact network congestion. Thus, if the total packet rate is such that the node bandwidth
capacity is exceeded, then packets with the lowest priority are the first candidates
to be “dropped or stripped” and never be delivered.
Multiple service levels are supported with parallel overlay networks or with native
multiservice technologies like Asynchronous Transfer Mode (ATM). The overlay
is a purpose-built network to support multiple services. There are generally
classes of service (CoS) supported by this network:
- Guaranteed: synchronous, negligible jitter, bounded latency, and low packet
loss (such as TDM) - Predictable: isochronous, minimal oversubscribing (or overbooking), bounded
jitter, and bounded latency (such as ATM) - Shared: moderate oversubscribing, bandwidth-aware, low jitter, some latency
(such as frame relay—FR) - Best Effort: highly oversubscribed, high jitter, moderate latency (such as IP)
Multiple services arise from Multiple Protocol Label Switching (MPLS) and the
Differentiated Services Model (Diffserv). MPLS is a protocol encapsulation technique,
which, in addition, includes bandwidth-aware routing extensions and path
signaling specifications. MPLS enables packet networks to apply ATM traffic engineering
principles. Diffserv provides a model for multiple service levels, utilizing
enhanced IP and MPLS protocols to deliver multiple services over an IP backbone
network. Based on this model, packets can be classified and dynamically aggregated
into provisioned service levels (each with different QoS).
Typically, these service levels are instantiated over separate networks, perhaps
each based on a different technology platform (TDM, FR, ATM, and IP) and from a
different service provider, and, thus, each may be provisioned differently. Thus,
guaranteeing a “guaranteed service” over a path that crosses many service providers
may be an issue.
3.2.2 Packet Networks
Packet networks offer integrated multiple service levels at a quality agreed upon by
the service provider and the client, based on a service level agreement (SLA). Packet
networks perform bandwidth management, buffering (queuing), scheduling, routing
and dynamic rerouting, policing, and traffic shaping.
Bandwidth management consists of functions such as the Call Admission Control,
Class of Service Routing, and Signaling. CAC is performed per service level.
Routing requires knowledge of the complete network topology in terms of available
bandwidth per service level and it executes shortest path algorithms (e.g., traveling
salesman algorithm). When the optimum route has been identified, it forms a routing
database. Routing also executes scheduling algorithms and simple or complex
queuing algorithms per SLA. Dynamic rerouting may also be performed during service.
Policing provides a provisioned mechanism at the entry point (edge) of the network
to verify and also control subscriber traffic parameters in terms of the SLA.
Traffic shaping is a buffered function that smoothes the traffic exiting the system
to ensure compliance with the agreed-upon profile, and by proper bandwidth management
it avoids congestion situations.
Packet networks assemble a number of client data bytes (or octets) into a payload
block, attach overhead...
More >>
