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Networking Basics for A/V
Published in Al Kovalick, Video Systems in an IT Environment, 2013
Methods for QoS classification techniques are based on inspecting some parameter in the packet stream to differentiate and segment it to provide the desired level of service. For example, if the stream is going to a well-known UDP port address (say a video stream), then the router may decide to give this packet a high priority. Sorting on port numbers is not the preferred way to classify traffic, however; it breaks the law of independence of stack layers. The generally accepted classification methods in use today are based on tags. The three most popular means are as follows: Ethernet frame tagging. This is based on an IEEE standard (802.1D-1998) for prioritizing frames and therefore traffic flows. It has limited use because it is a layer 2 protocol and cannot easily span beyond a local LAN. For A/V use in small LANs, this type of segmentation is practical, and many routers and switches support it.Network level ToS tagging. The type of service (ToS) is an 8-bit field in every IP packet used to specify a flow priority. This layer 3 field has a long history of misuse but was finally put to good use in 1998 with the introduction of IETF’s Differentiated Services (Diffserv) model as specified by RFC 2475. Diffserv describes a method of setting the ToS bits (64 prioritized flow levels) at the edge of the network as a function of a desired flow priority, forwarding each prioritized IP packet within the network (at each router) based on the ToS value, and traffic shaping the streams so that all flows meet the aggregate QoS goals. Diffserv is a class of service method to manage data flows. It is a stateless methodology (connectionless) and does not enforce virtual paths as MPLS does.MPLS tagging. This technique builds virtual circuits (VCs) across select portions of an IP network. Virtual circuits appear as circuit switched paths, but they are still packet/cell switched. VCs are called label switched paths (LSPs). MPLS is an IETF-defined, connection-oriented protocol (see RFC 3031 and others). It defines a new protocol layer, let us call it “layer 2.5,” and it carries the IP packets with a new 20-bit header, including a label field. The labels are like tracking slips on a pre-addressed envelope. Each router inspects the label tags and forwards the MPLS packet to the next router based on a forwarding table. Interestingly, the core MPLS routers do not examine the IP address, only the label. The label carries all the information needed to forward IP packets along a path across a MPLS-enabled network. Paths may be engineered to provide for varying QoS levels. For example, a path may be engineered for low delay and a guaranteed amount of bandwidth. MPLS operation is outlined in a later section.
Voice over Wi-Fi
Published in Ali Youssef, Douglas McDonald II, Jon Linton, Bob Zemke, Aaron Earle, Wi-Fi Enabled Healthcare, 2014
Ali Youssef, Douglas McDonald II, Jon Linton, Bob Zemke, Aaron Earle
The figure shows three distinct markings: Assured Forwarding (AF), Expedited Forwarding (EF), and Class Selector (CS). The DiffServ tag lives in the ToS field. This field is 1 byte in length. The first six bits represent the DSCP and the last two are the ECN or explicit congestion notification. The DSCP tag is split into two parts: the per hop behavior and drop precedence. As each part has 3 bits, they both have possible values of 0 to 7 hence, the 64 possible DSCP values of 0 to 63. Notice that all of the values are even as the odd ones are omitted from the standard. If this is a new concept to you, it should seem a little confusing. Because IP Precedence is the senior interpretation of the ToS byte, DSCP needed to provide reverse compatibility. With DSCP, the ToS field is now called the DS field or Differentiated Services field. It has 6 bits to define 64 different values but still provide the proper semantics to the 3 bit IP Precedence field. This was accomplished by splitting the 6 bits DS field into two fields of 3 bits, which are called the CS Class Selector and Drop Precedence, respectively. The first 3 bits, which are the CS or Per Hop behavior, map directly to the IP Precedence values or 0 to 7. For example, a voice packet will likely receive a DSCP value of 46, which is 101110 in binary. The first 3 bits could be interpreted as IP Precedence 5 or 101 in binary. I should note that 6 and 7 were only used for network control traffic and were not configurable. So, an IP Precedence value of 5 would be the highest configurable priority and would be consistent with the proper designation for voice traffic. Now we can look at the inverse of this scenario and see if it holds true as well. If a device that is configured to support IP Precedence only transmits a packet onto the network, what would the DSCP value be? It would look like this: 101000 to the computers, or 40 to us. DSCP 40 has the alternate name of CS5. A class selector (CS#) is the DSCP interpretation of an IP Precedence value. So, if you see these values on the network it is safe to say that a device is likely using IP Precedence. DSCP is designed to provide additional granularity over IP Precedence. This extra granularity is called the Drop Precedence. In most networks, the easiest way to ensure that VoIP continues to work as expected is to add more bandwidth. If adding bandwidth is not an option then a network engineer must decide which packets get dropped and which do not. Hopefully, you are getting the idea behind the drop precedence value by now. Drop precedence is only used on Assured Forwarding (AF), also known as classes 1 to 4. The higher the value the more likely it is to be dropped. VoIP traffic will typically be marked as EF (DSCP 46) or CS5 (DSCP 40). Many smart phone apps mark the voice traffic incorrectly as CS7 (DSCP 56) or CS6 (DSCP 48). It is important to understand how the IP ToS/DS field is affected every step of the way through a network. In addition it is equally important to understand how the packets are treated or “queued” at each of those steps. This is defined as a policy map and usually involves selecting queuing methods.
Model of Assessing the Quality Indicators of the Service Process in Transport
Published in IETE Technical Review, 2023
To determine the optimal quality of service from an economic point of view (the amount of service of their devices or other indicators of the quality of service), it is necessary to use the following objective functions. The minimum value of this objective function, under certain restrictions, is the optimal value of service quality indicators from an economic point of view. The limiting condition of the objective function is the condition t > tδ. This implies another condition P < Pδ. Deterioration of the quality of service to a certain – the so-called Pδ marginal value leads to the fact that this type of service becomes unclaimed.