Quality of service (QoS), in the field of telephony, refers to the ability of a network to provide higher priority services for certain applications, by allocating dedicated bandwidth, controlling jitter and latency, and improving loss characteristics. At the same time, making sure that providing priority for one class of traffic does not make other flows fail.

There are many technologies available to achieve the necessary QoS for a given user and applications. Basically, there are 4 categories of QoS technologies: Queuing, Packet classification, Traffic shaping and policing, and Fragmentation. In the following table, we listed the main technologies under each category.

Queuing

Class-Based Weighted Fair Queuing (CB-WFQ): CB-WFQ is a variation of WFQ, that enables network admin to define a class based on criteria that exceed the confines of flow. For example, you may create a specific class for voice traffic to ensure its bandwidth usage over the network during periods of congestion.

Custom Queuing (CQ): CQ enables users to specify a percentage of available bandwidth to a particular protocol/application. Each queue is served sequentially in a round-robin fashion, transmitting a percentage of traffic on each queue before moving on to the next queue.

Fair Queuing (FQ): FQ uses multiple queues to separate flows and gives equal amounts of bandwidth to each flow. This prevents some applications such as file download from consuming all bandwidth.

Priority Queuing(PQ): PQ enables the network admin to configure four traffic priorities - high, normal, medium and low. Traffic in the high-priority queue will go-through first; then, packets in the next level of priority queue are transmitted. This will ensure mission critical traffic always given as much bandwidth as it needs, in the expenses of other applications.

Priority Queuing - Class-Based Weighted Fair Queuing: This queuing method can provide absolute priority to voice traffic over all other traffic, because you have many ways to guarantee strict priority delivery.

Random early detection or Random early discard (RED): RED is an active queue management algorithm. It is also a congestion avoidance algorithm. RED monitors the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped.

Weighted Fair Queuing (WFQ): WFQ uses multiple queues to separate flows and gives weighted amounts of bandwidth to each flow. This prevents some applications from consuming other's bandwidth and to achieve guaranteed data rate.

Weighted Random Early Drop/Detect (WRED): WRED, an extension to Random early detection (RED) with congestion avoidance capabilities, allows different queues to have different buffer occupation thresholds before random dropping starts, as well as different dropping probabilities. Packets are classified into these queues according to priority information such as IP precedence or DSCP (for DiffServ).

Packet Classification

Type of Service (ToS): ToS is an 8-bit field in IP header to provide the quality of service desired. The field was defined with two parts, a 3-bit precedence value and the ToS bits. The precedence value provides a form of priority queuing. The ToS bits specified how the network should make trade-offs between throughput, delay, reliability, and cost.

DiffServ: Diff-Serv is method for differentiating classes of service for Internet traffic, by using an 8-bit DS (Differentiated Services) field in the IP header that replaces the older ToS (Type of Service) field. There are two parts in the DS: 6-bit differentiated services codepoint (DSCP) and 2-bit currently unused. The DSCP field can define up to 64 discrete network service types.

IntServ: IntServ or integrated services is an architecture that specifies the elements to guarantee QoS on networks for real-time services such as video and sound. IntServ specifies a fine-grained QoS system, which is often contrasted with DiffServ's coarse-grained control system. "RSVP" is the underlying mechanism to signal it across the network.

Policy-based Routing: Policy-based routing (PBR) provides a mechanism for expressing and implementing forwarding/routing of data packets based on the policies defined by the network administrators. It complements the mechanism provided by routing protocols.

Resource Reservation Protocol (RSVP): RSVP enables endpoints to signal the network with the desired QoS for a particular application. RSVP defines how applications place reservations and how they can relinquish the reserved resources once the need for them has ended. RSVP operation will generally result in resources being reserved in each node along a path.

Traffic shaping and policing

Committed Access Rate (CAR): CAR can identify when traffic exceeds the defined thresholds and then decide to stop or slow-down traffic rate according to certain pre-set policy.

Generic Traffic Shaping(GTS): GTS can identify when traffic exceeds the defined thresholds and then buffer the excess traffic while waiting for the open interval to transmit he data.

Fragmentation

Multi-Class Multilink Point-to-Point Protocol (MCML PPP): MCML PPP is an architecture for providing integrated services over low-bitrate links. The main components are: a real-time encapsulation format for synchronous and synchronous low-bitrate links, a header compression architecture optimized for real-time flows, elements of negotiation protocols used between devices, and announcement protocols used by applications to allow this negotiation to take place. (RFC 2686)

Frame Relay Forum 12 (FRF.12): FRF.12 enables Frame Relay networks to provide multiple services with QoS. One of the main techniques used in this specification is frame fragmentation. It also defines the interworking between Frame Relay and ATM networks.

Maximum Transmission Unit (MTU): MTU refers to the size of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards or decided at connect time. A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other packets. You may set a small MTU at an interface or a protocol to force packet fragmentation.

Other

Compressed Real Time Transport Protocol (cRTP): Reduces the header overhead of IP/UDP/RTP datagrams by compressing the three headers to 2-4 bytes. This technology enhances QoS by reducing bandwidth usage of packet overhead. (RFC 3545)

Related Terms: VOIP, QoS, QoS Technologies, Fair Queuing, Weighted Fair Queuing, DiffServ, IntServ, RSVP, cRTP

Reference Links: http://www.javvin.com/protocolVOIP.html: VOIP Protocols