Which Sublayer Defines The Use Of Logical Interface Points, Called Service Access Points (Saps)?

Connection-Oriented Service

Cablevision Networking Protocols

Walter Ciciora , ... Michael Adams , in Modern Cable Boob tube Applied science (Second Edition), 2004

Differences Between Connection-Oriented and Connectionless Service

In summary, connection-oriented services, an example being the public switched telephone network, dedicate a communications path to 2 users for the duration of a communications session. The session may last less than a second, or it may final for days. When communication is desired, a circuit must be fix upwards that is defended to that communicating pair for the elapsing. At the end of the session, the excursion is torn down, releasing whatever common resources for other users. The device at the center of a connection-oriented service is called a excursion switch.

In contrast, connectionless circuits are established on an advertisement hoc basis when they are needed for an individual parcel of data. As soon as the bundle passes, the resources are available for other users. Each parcel of information has a destination address. That destination address is read by the router, which and so consults a routing table to learn the best route on which to ship the packet at that moment.

To the user of a connection-oriented service, information technology is necessary to establish the excursion (e.g., by placing a phone phone call), wait for the connection to be established, so use information technology. The filibuster, once the circuit is established, is brusk and predictable. A user of a connectionless service is not aware of the establishment of a path for each packet, that function being handled automatically in protocol. So far as the user is concerned, the advice path is always available to him. On the other paw, because of route diversity, sequential packets may travel on different paths; and since some routes may be faster than others, information technology is non guaranteed that packets will get in on a regular basis or even in the right society. It is the responsibility of the protocol to rearrange packets into the right order and to present them to the using function correctly. (With reference to the OSI model of Figure five.2, this function would be handled by the send layer.)

Connectionless services use system resource more efficiently considering no resource are tied up between users who have established a communications path they are not actively using at the moment. On the other hand, the user of a connectionless service "thinks" a communications channel is available at all times, though he or she may experience delays. Of course, many of the delays are small in human terms, and packets received out of club may exist properly ordered without user awareness.

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Network Services and Layered Architectures

Jean Walrand , Pravin Varaiya , in High-Performance Communication Networks (Second Edition), 2000

2.3.1 Connection-Oriented Service

When a network implements a connection-oriented service, it delivers messages from the source to the destination in the correct guild. Thus, the information transfer in a connection-oriented service appears to have place over a dedicated transmission line, except for the variability in the transmission filibuster of different packets. A connection-oriented service is required by user applications that expect reliable and ordered transmissions of messages. A CBR or VBR bit stream is delivered by a connectedness-oriented service.

A connection-oriented service involves three phases: a connexion setup stage, a data transfer stage, and a connection teardown phase.

The quality of service (QoS) in some connexion-oriented network services specifies whether the transmission is error gratis and may assign some priority level to packets with the understanding that the network will attempt to transmit high-priority packets before depression-priority packets. Thus the delays are likely to be smaller in a high-priority connection-oriented service than in a low-priority connection-oriented service.

Some networks allow a more detailed QoS specification. That specification includes the delay, delay jitter, and packet mistake rate. It also includes a description of the amount of traffic that the service can ship. These more detailed specifications are required by existent-time and interactive applications, equally nosotros explained in section 2.2.

When requesting a connection-oriented service in these networks, at the connection setup fourth dimension the user specifies the QoS that is required by the user awarding. The network tin then determine whether information technology has sufficient resource available to handle that connexion with the requested QoS, and the network can then set aside these resources for the connection. In social club to undertake these tasks, the network retains "state" information nigh existing connections. How these tasks can be carried out is discussed in Chapters viii and 9.

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Congestion Control and Resource Allocation

Larry L. Peterson , Bruce S. Davie , in Computer Networks (Fifth Edition), 2012

Connectionless Flows

For much of our discussion, nosotros assume that the network is essentially connectionless, with whatever connectedness-oriented service implemented in the transport protocol that is running on the terminate hosts. (We explicate the qualification "substantially" in a moment.) This is precisely the model of the Internet, where IP provides a connectionless datagram delivery service and TCP implements an end-to-end connexion abstraction. Notation that this assumption does not concur in virtual circuit networks such as ATM and X.25 (see Section 3.1.2). In such networks, a connectedness setup message traverses the network when a excursion is established. This setup message reserves a set of buffers for the connectedness at each router, thereby providing a form of congestion control—a connexion is established only if enough buffers can exist allocated to it at each router. The major shortcoming of this arroyo is that information technology leads to an underutilization of resource—buffers reserved for a particular excursion are non available for use by other traffic fifty-fifty if they were not currently being used past that circuit. The focus of this chapter is on resource allocation approaches that apply in an internetwork, and thus we focus mainly on connectionless networks.

We need to qualify the term connectionless because our classification of networks as being either connectionless or connexion oriented is a fleck as well restrictive; there is a gray area in between. In detail, the assumption that all datagrams are completely independent in a connectionless network is also strong. The datagrams are certainly switched independently, but it is usually the case that a stream of datagrams betwixt a detail pair of hosts flows through a particular set of routers. This idea of a period —a sequence of packets sent betwixt a source/destination pair and following the same road through the network—is an important brainchild in the context of resource allocation; it is one that nosotros will use in this chapter.

One of the powers of the flow abstraction is that flows can exist defined at different granularities. For instance, a flow can be host-to-host (i.eastward., have the same source/destination host addresses) or procedure-to-process (i.east., have the same source/destination host/port pairs). In the latter case, a flow is substantially the same as a channel, as nosotros have been using that term throughout this book. The reason we introduce a new term is that a flow is visible to the routers inside the network, whereas a channel is an cease-to-terminate abstraction. Figure 6.2 illustrates several flows passing through a series of routers.

Considering multiple related packets flow through each router, it sometimes makes sense to maintain some state data for each menstruum, information that can be used to brand resource allocation decisions about the packets that vest to the flow. This land is sometimes chosen soft state; the primary divergence between soft state and difficult state is that soft state need not e'er be explicitly created and removed by signalling. Soft state represents a middle ground between a purely connectionless network that maintains no country at the routers and a purely connectedness-oriented network that maintains hard state at the routers. In general, the correct operation of the network does not depend on soft state being present (each packet is still routed correctly without regard to this country), but when a package happens to vest to a catamenia for which the router is currently maintaining soft state, then the router is amend able to handle the parcel.

Note that a menses tin be either implicitly defined or explicitly established. In the former case, each router watches for packets that happen to be traveling between the aforementioned source/destination pair—the router does this by inspecting the addresses in the header—and treats these packets as belonging to the same flow for the purpose of congestion control. In the latter case, the source sends a flow setup bulletin across the network, declaring that a flow of packets is well-nigh to commencement. While explicit flows are arguably no dissimilar than a connexion across a connectedness-oriented network, we phone call attention to this case because, even when explicitly established, a menses does not imply any end-to-end semantics and, in detail, does not imply the reliable and ordered delivery of a virtual circuit. Information technology simply exists for the purpose of resource allocation. Nosotros will see examples of both implicit and explicit flows in this affiliate.

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Protocol Layers

Pierre Duhamel , Michel Kieffer , in Joint Source-Channel Decoding, 2010

The Ship Layer (UDP/RTP)

The protocols involved in the transport layer manage the finish-to-end communication. The TCP is used in many situations. It provides a connection-oriented service with reliable transmission since TCP packets that are lost or erroneous are transmitted over again. In our situation, the respective filibuster introduced past the protocol mechanisms (acknowledgment, retransmissions, sliding window) cannot be tolerated. Therefore, information technology is necessary to use a less constrained protocol (nonreliable), the UDP. This simple protocol is often associated to another one to obtain an efficient connection between the network and the multimedia awarding: the RTP.

The RTP takes care of IP data with real-fourth dimension constraints. It is directly associated with the current application, and some of its data are used at the upper layer. Information technology allows to rearrange the received packets in sequential order, to command the safe reception of the packets, to synchronize the decoded bitstreams (time markers), and to identify the information types. All these are more often than not additional data (other than what is normally named the bitstream) generated by the application layer. In our example of video transmission, the RTP encapsulates the NAL packets generated by the H.264/AVC encoder (see Affiliate three). The data are placed in the payload field, and a header containing the boosted data, useful at RTP level, is added in front of each packet. The RTP format is detailed in Appendix A.

Finally, the UDP packets encapsulate the data from the RTP packets. The corresponding data are placed in a payload field, and a header contains the data useful for the UDP, such as Server Port, Terminal Port, and the size of the UDP packet, as detailed in Appendix A.

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Protocols and Communications Models

Edward Insam PhD, BSc , in TCP/IP Embedded Cyberspace Applications, 2003

The IEEE 802 Model

In February 1980, the IEEE created the 802 Networks Standards Committee to operate as a major working group in local surface area networks. Their remit was to create, maintain and encourage the use of generic networking standards. The series of standards adult by the committee are known every bit 802.X. They are too known equally ISO 8802. The number 802 only relates to the second month of year 80, the standard style of defining IEEE committees. All of the current 802 Standards are obtainable from the IEEE web site www.ieee.org, although not all standards have been finalized. The work of the 802 group is organized into sub-committees, designated as 802.X sub-committees. where the 'X' corresponds to one of many project numbers. Some of these sub-committees define standards, others serve every bit an informational function.

The IEEE 802 model relates only to the two everyman layers of the OSI model, the physical and information link layers. However, it does not exactly match the OSI model. The IEEE 802 architecture defines three layers that are roughly equivalent to the OSI data link layer: These are named logical link control (LLC), the optional bridging and the medium admission control (MAC) layers. Figure three-3 shows the standard classifications and the relationship betwixt IEEE 802 and OSI.

The logical link control layer (LLC-802.two)

This layer describes methods for establishing link services betwixt a number of ports or nodes, and is mutual to all the physical implementations. The layer provides a number of services:

•: unacknowledged connectionless service,
•: acknowledged connection-oriented service,
•: best-selling connectionless service.

In a like style to the OSI terminology, a connectionless service is where data is sent out without establishing a connexion outset. An acknowledged service is where reception of a transmission is acknowledged by a local feedback machinery (usually by the transmission of another frame in the opposite management). The need for local acquittance is more than relevant in the case of wireless networking (radio or optical), as wired networks are reliable enough not to necessitate local forms of acknowledgment. LLC likewise defines the utilise of logical interface points, called service access points (SAP) that other computers can use to transfer information from the LLC layer to the upper OSI layers. Note that these services are as well mentioned in the OSI send and session layers. IEEE 802 provides for elementary forms of acknowledgment and session control at the lower levels, but it accepts that they may be duplicated at the higher layers. The LLC layer is mutual to all implementations of IEEE 802, thus making it easier to supersede a wired node with say a wireless node within the same local network.

Bridging layer

This is an optional layer that covers routing, bridging and internet work communications. This is an intermediate layer not always present in pocket-size implementations. This is fully described in IEEE 802.1.

The media access command layer

This roughly corresponds to the lower one-half of the OSI data link layer. It provides shared access for multiple devices within the physical layer. Information technology is responsible for the generation and reception of frames, validation of data integrity via checksums, addressing and other functions that control access to the network itself. MAC is not defined in a single document, but is defined every bit part of the document describing the underlying physical structure. In other words, every document discussing a detail engineering also discusses its ain method of MAC implementation. In society to ensure compatibility betwixt the different MAC implementation, a set up of standard layer interfaces have been designed at the software function level.

The physical layer (PHY)

This describes each specific hardware implementation, its operation and its methods for managing the medium. Different media may be used within the same standard; for example, IEEE 802.11 describes wireless local area networks using radio systems at different frequencies, and using different modulation methods, and also equivalent methods using infrared optical engineering science.

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Multimedia Networks and Communication

Shashank Khanvilkar , ... Ashfaq Khokhar , in The Electrical Technology Handbook, 2005

7.2.1 Multimedia Classification

From a networking perspective, all media types tin can be classified as either real-time (RT) or non real-time (NRT), as shown in Effigy 7.two. RT media types require either hard or soft bounds on the cease-to-finish packet delay/jitter, while NRT media types, like text and epitome files, practise not take any strict delay constraints but may take rigid constraints on mistake.

There are basically two approaches to mistake control (Leon-Garcia and Widjaja, 2000):

1.: Fault detection followed past Automatic Retransmission reQuest (ARQ): This approach requests retransmission of lost or damaged packets. It is used by Transport Command Protocol (TCP), a transport layer protocol in the TCP/IP protocol stack, to provide reliable connection-oriented service. Applications that require an error-free delivery of NRT media typically use TCP for transport.
ii.: Forward fault correction (FEC): This second approach provides sufficient redundancy in packets and then that errors can be corrected without the need for retransmissions. It tin exist used by User Datagram Protocol (UDP), another transport layer protocol in the TCP/IP protocol stack that provides connectionless unreliable service. Applications that commutation error-tolerant media types (both RT and NRT) typically use UDP for transport because information technology eliminates time lost in retransmissions. Leigh et al. (2001) accept conducted experiments using FEC along with UDP over a global high-bandwidth advice network, STARTAP.

The RT media types are further classified equally detached media (DM) or continuous media (CM), depending on whether the data is transmitted in discrete quantum equally a file or message or continuously equally a stream of letters with intermessage dependency. The real-fourth dimension discrete type of media has recently gained high popularity considering of ubiquitous applications like MSN/Yahoo messengers (which are error intolerant) and instant messaging services like stock quote updates (which are mistake tolerant).

The RT continuous type of media tin further exist classified equally delay tolerant or delay intolerant. We cautiously use the term delay tolerant to signify that such media type tin can tolerate college amounts of delay than their filibuster-intolerant counterparts without significant operation degradation. Examples of RT, continuous, and delay-intolerant media are audio and video streams used in audio or video conferencing systems, and remote desktop applications. Streaming audio/video media used in applications similar Internet Webcast are examples of delay-tolerant media types. Their delay dependency is significantly diminished by having an adaptive buffer at the receiver that downloads and stores a certain portion of the media stream before starting playout. The entire classification has been carefully illustrated in Figure 7.ii.

We now talk over some common media types and their defining characteristics in terms of bandwidth usage, error requirements, and existent-time nature.

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Video Communication Networks

Dan Schonfeld , in Handbook of Image and Video Processing (Second Edition), 2005

3.6 Asynchronous Transfer Mode Networks

ATM, also known as cell relay, is a method for information transmission in small stock-still-size packets called cells based on asynchronous time-division multiplexing. ATM technology was proposed as the underlying foundation for the Broadband Integrated Services Digital Network (B-ISDN). B-ISDN is an aggressive very high data charge per unit network that will replace the existing telephone system and all specialized networks with a single integrated network for data transfer applications such equally video on demand (VoD), circulate television, and multimedia communication. These lofty goals not withstanding, ATM technology has institute an important niche in providing the bandwidth required for the interconnection of existing local area networks (LAN); east.grand., Ethernet.

The ATM cells are 53 bytes long of which 5 bytes are devoted to the ATM header and the remaining 48 bytes are used for the payload. These small fixed-sized cells are ideally suited for the hardware implementation of the switching mechanism at very high data rates. The information rates envisioned for ATM are 155.5 Mbps (OC-3), 622 Mbps (OC-12), and ii.v Gbps (OC-48). ²²

The B-ISDN ATM reference model is shown in Fig. 7. Information technology consists of several layers: physical layer, ATM layer, ATM Adaptation Layer (AAL), and upper layers. ²³ This layer can be further divided into the physical medium dependent (PMD) sublayer and the transmission convergence (TC) sublayer. The PMD sublayer provides an interface with the physical medium and is responsible for transmission and synchronization on the physical medium (e.thou., SONET or SDH). The TC sublayer converts betwixt the ATM cells and the frames—strings of bits—used by the PMD sublayer. ATM has been designed to be contained of the transmission medium. The data rates specified at the physical layer, however, require category five twisted pair or optical fibers. ²⁴

The ATM layer provides the specification of the cell format and jail cell transport. The header protocol defined in this layer provides generic flow control, virtual path and aqueduct identification, payload blazon, prison cell loss priority, and header error checking. The ATM layer is a connection-oriented protocol that is based on the creation of end-to-end virtual circuits (channels). The ATM layer protocol is unreliable—acknowledgments are not provided—since it was designed for utilize of real-time traffic such as audio and video over fiber optic networks that are highly reliable. The ATM layer nonetheless provides quality of service (QoS) guarantees in the form of cell loss ratio (CLR), bounds on maximum prison cell transfer delay (MCTD), prison cell delay variation (CDV)—known as well equally delay jitter. This layer also guarantees the preservation of cell order along virtual circuits.

The structure of the ATM accommodation layer (AAL) is illustrated in Fig. 8. This layer can be decomposed into the partition and reassembly sublayer (SAR) and the convergence sublayer (CS). The SAR sublayer converts between packets from the CS sublayer and the cells used by the ATM layer. The CS sublayer provides standard interface and service options to the various applications in the upper layers. This sublayer is too responsible for converting between the bulletin or data streams from the applications and the packets used by the SAR sublayer. The CS sublayer is further divided into the common role convergence sublayer (CPCS) and the service specific convergence sublayer (SSCS).

Initially four service classes were divers for the AAL (Class A-D). This classification has subsequently been modified by the label of 4 protocols: Class A is used to correspond existent-fourth dimension (RT) constant chip-rate (CBR) connection-oriented (CO) services handled past AAL-ane. This class includes applications such as circuit emulation for uncompressed audio and video transmission. Grade B is used to define real-fourth dimension (RT) variable bit-rate (VBR) connection-oriented (CO) services given by AAL-2. Among the applications considered past this class are compressed audio and video transmission. Although the aim of the AAL-2 protocol is consequent with the focus of this presentation, we shall not discuss it in detail since the AAL-2 standard has not yet been divers. Classes C and D support not-real-time (NRT) variable bit-rate (VBR) services corresponding to AAL-3/4. ²⁵ Class C is further restricted to not-existent-fourth dimension (NRT) variable bit-rate (VBR) connectedness-oriented (CO) services provided by AAL-v. ²⁶ It is expected that this protocol will be used to transport IP packets and interface to ATM networks. A summary of the ATM accommodation layer service classes and protocols is presented in Tabular array iv.

Tabular array 4. ATM Adaptation layer service classes

Parameters	Service Classes
Parameters	Course A	Class B	Class C	Class D
Timing compensation	Required		Not Required
Bit rate	Constant		Variable
Connection way		Connexion Oriented		Connectionless
Applications	Voice/Video	VBR	Frame Relay	SMDS
	Circuit Emulation	Video/Audio		Data Transfer
AAL type	AAL-1	AAL-ii	AAL-3/4 AAL-v	Aal-3/4

Every bit is apparent from the preceding word, the main methods available for video communications over ATM are based on AAL-1 and AAL-v. The remainder of this section shall therefore focus on the mapping of the MPEG-ii transport stream to the ATM Application Layer (AAL)—AAL-1 and AAL-5.

3.6.1 Asynchronous Transfer Mode Awarding Layer-1

The AAL-1 protocol is used for transmission of existent-time (RT) constant bit-rate (CBR) connection-oriented (CO) traffic. This awarding requires transmission at constant charge per unit, minimal delay, insignificant jitter, and low overhead.

Transmission using the AAL-i protocol is in 1 of ii modes: unstructured data transfer (UDT) and structured data transfer (SDT). The UDT mode is provided for data streams where boundaries need non exist preserved. The SDT way is designed for letters where message boundaries must be preserved.

The CS sublayer detects lost and misinserted cells that occur due to undetected errors in the virtual path or channel identification. It also controls incoming traffic to ensure transmission at a abiding rate. This sublayer too converts the input messages or streams into 46-47 bytes segments to be used past the SAR sublayer.

The SAR sublayer has a 1-byte protocol header. The convergence sublayer indicator (CSI) of the odd numbered cells forms a data stream that provides a iv-bit synchronous residual timestamp (RTSP) used for clock synchronization in SDT fashion [21]. The timing information is essential for the synchronization of multiple media stream as well equally for the prevention of buffer overflow and underflow in the decoder. The sequence count (SC) is a modulo-8 counter used to detect missing or misinserted cells. The CSI and SC fields are protected by the circadian redundancy bank check (CRC) field. An even parity (P) chip covering the protocol header affords additional protection of the CSI and SC fields. The AAL-1 SAR sublayer protocol header is depicted in Fig. 9. A respective glossary of the AAL-1 SAR sublayer protocol header is provided in Table 5.

Tabular array v. AAL1 SAR-PDU header glossary

Acronym	Function
CSI	Convergence sublayer indicator
SC	Sequence count
CRC	Cyclic redundancy cheque
P	Parity (even)

An additional 1-byte pointer field is used on every even numbered cell in STD mode. ²⁷ The arrow field is a number in the range of 0 to 92 used to indicate the starting time of the start of the side by side message either in its own prison cell or the one post-obit it in order to preserve message boundaries. This arroyo allows messages to exist arbitrarily long and need not align on cell boundaries. In this presentation, still, nosotros shall restrict ourselves to operation in the UDT mode for information streams where boundaries need not be preserved and the pointer field will be omitted.

As we have indicated earlier, the MPEG-2 systems layer consists of 188-bytes stock-still-length TS packets. The CS sublayer straight segments each of the MPEG-2 TS packets into four 47-bytes fixed-length AAL-1 SAR payloads. This approach is used when the cell loss ratio (CLR) that is provided past the ATM layer is satisfactory.

An alternative optional approach is used in noisy environments to improve reliability by the use of interleaved Reed-Solomon (128,124) forward error correction (RS-FEC). The CS sublayer groups a sequence of 31 distinct 188-bytes fixed-length MPEG-two TS packets. This group is used to form a matrix written in standard format (row-by-row) of 47 rows and 124-bytes in each row. Four bytes of the Reed-Solomon (128,124) FEC are appended to each row. The resulting matrix is equanimous of 47 rows and 128-bytes in each row. This matrix is forwarded to an interleaver that reads the matrix in transposed format (column-by-column) for transmission to the SAR sublayer. The interleaver assures that a cell loss would be limited to the loss of a single byte in each row, which can exist recovered by the FEC. A balmy filibuster equivalent to the processing of 128 cells is introduced by the matrix formation at the transmitter and the receiver. An illustration of the germination of the interleaved Reed-Solomon (128,124) FEC TS packets is depicted in Fig. ten.

Whether the interleaved FEC of the TS packets is implemented or direct manual of the TS packets is used, the AAL-1 SAR sublayer receives 47-bytes fixed-length payloads that are appended past the 1-byte AAL-1 SAR protocol header to grade 48-bytes fixed-length packets. These packets serve equally payloads of the ATM cells and are attached to the 5-bytes ATM headers to incorporate the 53-bytes fixed-length ATM cells. An illustration of the mapping of MPEG-2 systems layer TS packets into ATM cells using the AAL-one protocol is depicted in Fig. 11.

3.6.two Asynchronous Transfer Way Application Layer-5

The AAL-5 protocol is used for non-real-time (NRT) variable flake-rate (VBR) connection-oriented (CO) traffic. This protocol likewise offers the option of reliable and unreliable services.

The CS sublayer protocol is composed of a variable-length payload of length not to exceed 65,535 bytes and a variable-length trailer of length 8 to 55 bytes. The trailer consists of a padding (P) field of length 0 to 47 bytes called to make the entire message—payload and trailer—be a multiple of 48 bytes. The user-to-user (UU) direct information transfer field is bachelor for higher layer applications (e.thousand., multiplexing). The common part indicator (CPI) field designed for estimation of the remaining fields in the CS protocol is currently not in use. The Length field provides the length of the payload (not including the padding field). The standard 32-fleck cyclic redundancy cheque (CRC) field is used for error checking over the entire message—payload and trailer. This fault checking capability allows for the detection of missing or misinserted cells without using sequence numbers. An illustration of the AAL-5 CPCS protocol trailer is depicted in Fig. 12. A corresponding glossary of the AAL-5 CPCS protocol trailer is provided by Table half-dozen.

Table vi. AAL5 CPCS-PDU trailer

Acronym	Role
P	Padding
UU	User-to-user directly information transfer
CPI	Common part indicator field
Length	Length of payload
CRC	Cyclic back-up check

The SAR sublayer but segments the message into 48-byte units and passes them to the ATM layer for manual. Information technology also informs the ATM layer that the ATM user-to-user (AAU) bit in the payload type indicator (PTI) field of the ATM cell header must exist ready on the last cell in order to preserve message boundaries. ²⁸

Encapsulation of a unmarried MPEG-2 systems layer 188-bytes stock-still-length TS parcel in ane AAL-5 CPCS packet would innovate a pregnant amount of overhead due to the size of the AAL-five CPCS trailer protocol. The transmission of a single TS packet using this approach to the implementation of the AAL-5 protocol would require five ATM cells in comparison to the four ATM cells needed using the AAL-1 protocol. More than one TS parcel must be encapsulated in a unmarried AAL-5 CPCS package in order to reduce the overhead.

The encapsulation of more than one TS bundle in a single AAL-5 CPCS packet is associated with an inherent packing jitter. This will manifest itself as delay variation in the decoder and may affect the quality of the systems clock recovered when i of the TS packets contains a programme clock reference (PCR). To alleviate this problem the number of TS packets encapsulated in a unmarried AAL-v CPCS packet should be minimized. ²⁹

The preferred method adopted by the ATM Forum is based on the encapsulation of two MPEG-2 systems layer 188-bytes TS packets in a unmarried AAL-5 CPCS packet. The AAL-5 CPCS packet payload consequently occupies 376 bytes. The payload is appended to the 8-byte AAL-v CPCS protocol trailer (no padding is required) to grade a 384-byte AAL-five CPCS package. The AAL-5 CPCS packet is segmented into exactly viii 48-byte AAL-5 SAR packets, which serve as payloads of the ATM cells and are attached to the 5-byte ATM headers to comprise the 53-byte fixed-length ATM cells. An illustration of the mapping of two MPEG-2 systems layer TS packets into ATM cells using the AAL-five protocol is depicted in Fig. 13.

The overhead requirements for the encapsulation of two TS packets in a unmarried AAL-5 CPCS packet are identical to the overhead needed using the AAL-i protocol—both approaches map two TS packets into eight ATM cells. This arroyo to the implementation of the AAL-v protocol is currently the nigh popular method for mapping MPEG-ii systems layer TS packets into ATM cells.

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Fibre Aqueduct Standard

Stephen R. Guendert , in Handbook of Fiber Optic Data Advice (Fourth Edition), 2013

eight.2.three.9.5 Grade 4

A unlike, but no less significant problem with Class 1 is that it only allows dedicated connection from a unmarried source to a single destination, at the total channel bandwidth. In many applications, information technology is useful to classify a fraction of the resources between the N_Ports to be used, and then that the remaining portion tin be allocated to other connections. Class iv is a connection-oriented service similar Form 1, but the master deviation is that it allocates only a fraction of the bachelor bandwidth of the path through the fabric that connects 2 N_Ports. Virtual circuits (VCs) are established between two N_Ports with guaranteed QoS, including bandwidth and latency. In Class 4, a bidirectional circuit is established, with i VC in each direction, with negotiated QoS guarantees on bandwidth and latency for transmission in each management'due south VC. A source or destination N_Port may support up to 254 simultaneous Class four circuits, with a portion of its link bandwidth dedicated to each i. Form iv does not specify how information is to exist multiplexed between the different VCs or how information technology is to be implemented in the fabrics—these functions are determined past the implementation of the cloth supporting Form four traffic.

Like Class 1, Class iv guarantees in-club delivery frame delivery and provides acquittance of delivered frames, just now the fabric is responsible for multiplexing frames of unlike VCs. Grade 4 service is mainly intended for multimedia applications such as video and for applications that classify an established bandwidth past section within the enterprise. Form 4 was added in the FC-PH-two standard.

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Standards and Protocols in Data Communications

William Shay , in Encyclopedia of Data Systems, 2003

XI. Transmission Control Protocol/internet Protocol

The Cyberspace connects many devices, each of which runs a protocol known as transmission command protocol/internet protocol (TCP/IP). TCP and IP correspond roughly to layers 4 and three of the OSI model, respectively, although they are not role of the OSI model. They were adult along with the Department of Defence force's (DoD) Avant-garde Research Projects Agency (ARPA) project and have become DoD standards. TCP/IP is probably the most widely implemented protocol in the world and runs on almost anything from PCs to supercomputers.

The TCP/IP pair of protocols is part of a protocol collection called the TCP/IP protocol suite (Fig. 6 ). TCP provides connectedness-oriented services for college layer applications and relies on IP to route packets through the network in order to brand those connections. These applications, in plough, provide specific services for Internet users. For example, simple mail transfer protocol (SMTP) defines the protocol used for the delivery of mail messages over the Internet. The TELNET protocol allows users to log in to remote computers via the Internet. The file transfer protocol (FTP) allows Internet users to transfer files from remote computers. The domain name system (DNS) provides a mapping of host names to Internet addresses.

TCP is a connexion-oriented transport protocol designed to provide reliable communications over dissimilar network architectures. Similar to its counterpart in the OSI model, TCP implements flow command and mistake detection algorithms. Its predecessor in the original ARPANET was the network control protocol (NCP), which was designed to run on top of a reliable network. ARPANET was sufficiently reliable, but every bit it evolved into an internetwork, reliability was lost. Consequently, the transport protocol was forced to evolve equally well. NCP, redesigned to run over unreliable networks, became TCP. The user datagram protocol (UDP) provides a connectionless way of communication over dissimilar networks.

The IP is a layer 3 protocol designed to provide a bundle commitment service between 2 sites. Information technology is commonly used with TCP. Figure vii shows how it works with TCP. Two sites (A and B) need a connection-oriented service requiring the manual of some data. To begin, the TCP protocol at site A creates a TCP segment containing the user's data and "sends:" the segment to site B. If all goes well site B will acknowledge what it receives. From the TCP'south point of view it has made a directly connection with site B (dotted line). IP, still, intercepts the segment and creates an IP packet containing the TCP segment and, amidst other things, the destination address. It sends the packet to a router. Each router in the Internet executes routing algorithms that determine where a packet is sent side by side. When the packet somewhen arrives at site B, the TCP segment is extracted and the transmission is consummate.

Of interest is that no router knows the complete path that the packet takes. Instead, each one knows only, that given a destination, what router is the next link toward that destination. At that place are many routing algorithms, and there are difficulties in dealing with developing congestion or packets that broadcast among routers without making progress toward their concluding destination. In fact, IP will not guarantee delivery of whatever parcel, having the power to simply discard a parcel that has been circulating for too long. However, that is not ordinarily a trouble since TCP provides the acquittance and resending mechanisms that respond correctly if expected information does non arrive.

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Embedded Calculating

Marilyn Wolf , in High-Performance Embedded Calculating (Second Edition), 2014

i.2.4 Radio and networking

Combined wireless/network communications

Modern communications systems combine wireless and networking. Equally illustrated in Effigy 1.nine, radios bear digital information and are used to connect to networks. Those networks may be specialized, as in traditional cell phones, but increasingly radios are used as the physical layer in Net protocol systems.

Networking

The Open Systems Interconnection (OSI) model [Sta97A] of the International Standards System (ISO) defines a seven-layer model for network services:

1.: Physical: The electric and physical connection
ii.: Data link: Access and error command across a single link
three.: Network: Basic end-to-end service
iv.: Transport: Connection-oriented services
5.: Session: Activity control, such as checkpointing
6.: Presentation: Information exchange formats
7.: Application: The interface between the application and the network

Although it may seem that embedded systems would be also simple to require utilize of the OSI model, the model is in fact quite useful. Fifty-fifty relatively elementary embedded networks provide concrete, data link, and network services. An increasing number of embedded systems provide Net service that requires implementing the total range of functions in the OSI model.

Internetworking standard

The Internet is one example of a network that follows the OSI model. The Internet Protocol (IP) [Los97, Sta97A] is the fundamental protocol on the Net. IP is used to internetwork between different types of networks. IP sits at the network layer in the OSI model. It does not provide guaranteed terminate-to-end service. It instead provides best-try routing of packets. College-level protocols must be used to manage the stream of packets between source and destination.

Wireless

Wireless data communication is widely used. A data receiver, for example, must perform several tasks:

•: They must demodulate the point down to the baseband.
•: They must detect the baseband signal to place bits.
•: They must correct errors in the raw scrap stream.

Software radio

Wireless data radios may be congenital from combinations of analog, hardwired digital, configurable, and programmable components. A software radio is, broadly speaking, a radio that can be programmed; the term software-defined radio (SDR) is ofttimes used to mean either a purely or partly programmable radio. Given the clock rates at which today's digital processors operate, CPUs are used primarily for baseband operations. Some processors tin can run fast plenty to be used for some of the radio-frequency processing.

Software radio tierdue south

The SDR Forum, a technical grouping for software radio, defines five tiers of software-divers radio [SDR05]:

•: Tier 0: A hardware radio cannot exist programmed.
•: Tier 1: A software-controlled radio has some functions implemented in software but operations like modulation and filtering cannot be altered without changing hardware.
•: Tier 2: A software-defined radio may use multiple antennas for different bands just the radio tin cover a wide range of frequencies and utilize multiple modulation techniques.
•: Tier three: An platonic software-defined radio does not use analog amplification or heterodyne mixing earlier A/D conversion.
•: Tier 4: An ultimate software radio is lightweight, consumes very little power, and requires no external antenna.

Digital demodulation

Demodulation requires multiplying the received point past a bespeak from an oscillator and filtering the result to select the lower-frequency version of the indicate. The bit detection process depends somewhat on the modulation scheme, but digital advice mechanisms oftentimes rely on phase. Loftier-data-charge per unit systems often employ multiple frequencies arranged in a constellation. The phases of the component frequencies of the signal tin be modulated to create unlike symbols.

Error correction

Traditional error correction codes can be checked using combinational logic. For instance, a convolutional coder can be used as an error correction coder. The convolutional coder convolves the input with itself according to a chosen polynomial. Effigy 1.10 shows a fragment of a trellis that represents possible states of a decoder; the label on an edge shows the input flake and the produced output bits. Whatever bits in the manual may have been corrupted; the decoder must determine the nigh likely sequence of data bits that could take produced the received sequence.

Several more powerful codes that require iterative decoding have recently get popular. Turbo codes use multiple encoders. The input information is encoded by two convolutional encoders, each of which uses a dissimilar simply generally unproblematic code. I of the coders is fed the input data directly. The other coder is fed a permuted version of the input stream. Both coded versions of the data are sent beyond the aqueduct. The decoder uses 2 decoding units, one for each code. The two decoders are operated iteratively. At each iteration, the ii decoders swap likelihood estimates of the decoded $.25; each decoder uses the other'south likelihoods every bit a priori estimates for its own adjacent iteration.

Depression-density parity cheque (LDPC) codes also require multiple iterations to determine errors and corrections. An LDPC code tin be defined using a bipartite graph like that shown in Effigy 1.11; the codes are chosen low-density because their graphs are thin. The left-hand nodes are chosen message nodes and the correct-hand nodes are cheque nodes. Each check node defines a sum of message node values. The bulletin nodes define the coordinates for codewords; a legal codeword is a set of message node values that sets all the check nodes to 1. During decoding, an LDPC decoding algorithm passes letters betwixt the message nodes and bank check nodes. Ane approach is to pass probabilities for the data bit values as letters. Multiple iterations should cause the algorithm to settle onto a good estimate of the data chip values.

Radios and protocols

A radio may simply act as the physical layer of a standard network stack, just modern networks and radios are designed to take reward of the inherent characteristics of wireless networks. For example, traditional wired networks have simply a express number of nodes connected to a link just radios inherently broadcast; circulate can be used to improve network control, error correction, and security. Wireless networks are generally ad hoc in that the members of the network are non predetermined and nodes may enter or leave during network operation. Ad hoc networks require somewhat different network control than is used in fixed, wired networks.

The next example looks at a prison cell telephone communication standard.

Example 1.ane

CDMA2000

CDMA2000 [Van04] is a widely used standard for spread-spectrum-based cellular telephony. It uses direct sequence spread-spectrum transmission, which multiplies the data to be transmitted with a loftier-frequency, pseudorandom fleck sequence. The pseudorandom sequence spreads the data over a broad range of frequencies. The data appears every bit dissonance unless the receiver knows the pseudorandom sequence. Several radios tin apply the same frequency ring without interfering because the pseudorandom codes permit their signals to be separated.

A simplified diagram of the system looks similar this:

The spreader modulates the information with the pseudorandom code. The interleaver transposes coded information blocks to make the code more resistant to burst errors. The transmitter's power is controlled and then that all signals take the same strength at the receiver.

The concrete layer protocol defines a set of channels that can carry data or control. A forward channel goes from a base station to a mobile station, while a reverse channel goes from a mobile station to a base of operations station. Airplane pilot channels are used to larn the CDMA signal and provide phase information; they also allow the mobile station to estimate the channel'south characteristics. A number of different types of channels are defined for information, control, power command, etc.

The link layer defines medium admission control (MAC) and signaling link access control (LAC). The MAC layer multiplexes logical channels onto the physical medium, provides reliable send of user traffic, and manages quality of service. The LAC layer provides a diversity of services: authentication, integrity, partitioning and reassembly, etc.

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