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Wireless Communications Systems – class 12

Wireless Communications Systems – Class 12 (Study Guide Only)

  • Bluetooth Applications

  • Bluetooth Standards Documents

  • Protocol Architecture

  • Usage Models

  • Piconets and Scatternets

Radio Specifications

Baseband Specifications

  • Frequency Hopping

  • Physical Links

  • Packets

  • Error Correction

  • Logical Channels

  • Channel Control

  • Bluetooth Audio

  • Bluetooth Security

Link Manager Specifications

Logical Link Control and Adaptation Protocol

  • L2CAP Channels

  • L2CAP Packets

  • Signalling Commands

  • Quality of Service

Bluetooth Overview
Bluetooth is an always-on, short-range radio hook-up that resides on a microchip. It was initially developed by Swedish mobile-phone maker Ericsson in 1994 as a way to let laptop computers make calls over a mobile phone. Since then, several thousand companies have signed on to make Bluetooth the low-power short-range wireless standard for a wide range of devices.
The Bluetooth standards are published by an industry consortium known as the Bluetooth SIG (special interest group).
The concept behind Bluetooth is to provide a universal short-range wireless capability. It uses the 2.4-GHz band, available globally for unlicensed low-power uses.
Bluetooth is intended to support an open-ended list of applications including data, audio, graphics, and even video. Bluetooth can provide consumers with the ability to do the following:

  • Make calls from a wireless headset connected remotely to a cell phone.

  • Eliminate cable linking computers to printers, keyboards, and the mouse.

  • Hook up MP3 players wirelessly to other machines to download music.

  • Set up home networks so that normal household equipment such as air conditioning, lightning, oven controls etc. can be remotely monitored and controlled.

  • Call home from a remote location to turn appliances on or off, set alarms and monitor activity.

Bluetooth Applications
Bluetooth is designed to operate in an environment of many users. Up to eight devices can communicate in a small network called a piconet. Ten of these piconets can coexist in the same coverage range of the Bluetooth radio.
To provide security, each link is encoded and protected against eavesdropping and interference.
Using short range wireless connectivity, Bluetooth can provide support for three general applications areas:

  • Data and voice access point: Bluetooth facilitates real-time voice and data transmissions by providing wireless connection of portable stationary communications devices.

  • Cable Replacement: Connections are instant and are maintained even when the devices are not within LoS. The range of each radio is approximately 10 m but can be extended to 100 m with an optional amplifier.

  • Ad hoc networking: A device equipped with a Bluetooth radio can establish instant connection to another radio as soon as it comes into range.

Other Bluetooth Application examples are:

  • Three in one phone: Phone works as an intercom (office), fixed line (home), or mobile (when on the move).

  • Internet Bridge: Use a portable PC to surf in the internet anywhere, whether the connection is wireless through a mobile phone (mobile) or through a wired connection (PSTN, ISDN, LAN, xDSL).

  • Briefcase e-mail: Notification of received e-mail message using the mobile phone.

  • Automatic Synchronization: of desktop computer, portable PC, notebook and mobile phone.

  • Cordless Desktop: Connect your desktop/laptop computer cordlessly to printers, scanners, keyboard, mouse, and the LAN.

Bluetooth Standards Documents
Bluetooth standards represent a formidable bulk. There are well over 1500 pages, divided into two groups:

  • Core Specifications: Describes the details of the various layers of the Bluetooth protocol architecture, from the radio interface to link control.

  • Profile Specifications: Are concerned with the use of Bluetooth technology to support various applications. Each profile specification discusses the use of the technology defined in the core specifications to implement a particular usage model. It includes a description of which aspects of the core specifications are mandatory, optional, and not applicable.

Protocol Architecture
Bluetooth is defined as a layered protocol architecture consisting of core protocols, cable replacement, telephony control protocols, and adopted protocols as shown in Figure 1.

Figure 1: Bluetooth Protocol Stack.

The core protocols form a five-layer stack consisting of the following elements:

  • Radio: Specifies the details of the air interface, including frequency, the use of frequency hopping, modulation scheme, and transmit power.

  • Baseband: Concerned with connection establishment within a piconet, addressing, packet format, timing, and power control.

  • Link Manager Protocol (LMP): Responsible for linking setup between Bluetooth devices and ongoing link management. (this includes security aspects such as authentication and encryption, plus the control and negotiation of baseband packet sized).

  • Logical Link Control and Adaptation Protocol (L2CAP): Adapts upper-layer protocols to the baseband layer. L2CAP provides both connectionless and connection-oriented services.

  • Service Discovery Protocol (SDP): Device information, services, and the characteristics of the services can be queried to enable the establishment of a connection between two or more Bluetooth devices.

RFCOMM is the cable replacement protocol included in the Bluetooth specification. RFCOMM presents a virtual serial port that is designed to make a replacement of cable technologies as transparent as possible.
RFCOMM provides for binary data transport and emulates EIA-232 control signals over the Bluetooth baseband layer. EIA-232 (formerly known as RS-232) is a widely used serial port interface standard.
Bluetooth specifies a telephony control protocol. TCS BIN (telephony control specification-binary) is a bit oriented protocol that defines the call control signalling for the establishment of speech and data calls between Bluetooth devices.
The adopted protocols are defined in specifications issued by other standards-making organizations and incorporated into the overall Bluetooth architecture.
The Bluetooth strategy is to invent only necessary protocols and use existing standards whenever possible. Some of the adopted protocols are:

  • PPP (point to point protocol)


  • OBEX (object exchange protocol)

  • WAE/WAP (wireless application environment and protocol).

Usage Models
In essence, a usage model is a set of protocols that implement a particular Bluetooth-based application. Each profile defines the protocol and protocol features supporting a particular usage model. The highest priority usage models are:

  • File Transfer: Supports the transfer of directories, files, documents, images, and streaming formats. This usage model also includes the capability to browse folders on a remote device.

  • Internet Bridge: With this usage model, a PC is wirelessly connected to a mobile phone or cordless modem to provide dial-up networking and fax capabilities.

  • LAN access: This usage model enables devices on a piconet to access a LAN.

  • Synchronization: This model provides a device-to-device synchronization of personal information management (PIM) information.

  • Three in one phone: Telephone handsets that implement this usage model may act as a cordless phone connecting to a voice BS, as an intercom device for connecting to other telephones, and as a cellular phone.

  • Headset: The headset can act as a remote device’s audio input and output interface.

Piconets and Scatternets
The basic unit of networking in Bluetooth is a piconet, consisting of a master and from one to seven active slave devices. The radio designated as the master makes the determination of the channel (Frequency Hopping Sequence) and phase (timing offset, i.e., when to transmit) that shall be used by all devices on this piconet.
The radio designated as master makes this determination using its own device address as a parameter, while the slave devices must tune to the same channel and phase. A slave may only communicate with the master and may only communicate when granted permission by the master.
A device in one piconet may also exist as a part of another piconet and may function as either a slave or master in each piconet as illustrated in Figure 2. This form of overlapping is called a scatternet.

Figure 2: Master/Slave Relationship
Figure 3 shows the piconet/scatternet architecture in comparison with other forms of wireless networks. The advantage of the piconet/scatternet scheme is that it allows many devices to share the same physical are and make efficient use of the bandwidth. The physical area and total bandwidth are shared by the scatternet. The logical channel and data transfer are shared by a piconet.

Figure 3: Wireless Network Configurations.
Radio Specifications
The Bluetooth radio specification is a document that gives the basic details of radio transmission for Bluetooth devices. Table 1 summarizes the key parameters:




Up to 7 simultaneous links in a logical star



Peak data rate

1 Mbps

RF bandwidth

220 kHz (-3dB), 1 MHz (-20dB)

RF band

2.4-GHz ISM band

RF carriers


Carrier spacing

1 MHz

Transmit power

0.1 W

Piconet access


Frequency hop rate

1600 hops/s

Scatternet access


Table 1: Bluetooth Radio and Baseband Parameters.
Bluetooth makes use of the 2.4-GHz band within the ISM1 band. In most countries, the bandwidth is sufficient to define 79 1-MHz physical channels. Power control is used to keep the devices from emitting any more RF power than necessary. The power control algorithm is implemented using the link management protocol between a master and the slaves in a piconet.
One important aspect of the radio specifications is the definition of three classes of transmitters based on output power:

  • Class 1: Outputs 100 mW (+20 dBm) for maximum range, with a minimum of 1 mW (0 dBm). In this class power control is mandatory, ranging from 4 to 20 dBm. This mode provides the greatest distance.

  • Class 2: Outputs 2.4 mW (+4 dBm) at maximum, with a minimum of 250 W (-6 dBm). Powr control is optional.

  • Class 3: Lowest power. Nominal output is 1 mW.

Modulation for Bluetooth is Gaussian FSK, with a binary one represented by a positive frequency deviation and a binary zero represented by a negative frequency deviation from the centre frequency. The minimum deviation is 115 kHz.

The main characteristics of the receivers include the following:

  • Sensitivity Level:    The receiver must have a sensitivity level for which the bit error rate (BER) 0.1% is met. For Bluetooth this means an actual sensitivity level of -70dBm or better.

  • Interference Performance:   The interference performance on Co-channel and adjacent 1 MHz and 2 MHz are measured with the wanted signal 10 dB over the reference sensitivity level. On all other frequencies the wanted signal shall be 3 dB over the reference sensitivity level.

  • Out-of-Band blocking:     The Out of band blocking is measured with the wanted signal 3 dB over the reference sensitivity level. The interfering signal shall be a continuous wave signal. The BER shall be less than or equal to 0.1%.

  • Intermodulation Characteristics:   The reference sensitivity performance, BER = 0.1%, shall be met under the following conditions.

  • The wanted signal at frequency with a power level 6 dB over the reference sensitivity level.

  • A static sine wave signal at with a power level of –39 dBm.

  • A Bluetooth modulated signal at with a power level of -39 dBm.

Such that:

where n can be 3, 4, or 5. The system must fulfil one of the three alternatives.

  • RSSI: Receiver Signal Strength Indicator (Optional):    A transceiver that wishes to take part in a power-controlled link must be able to measure its own receiver signal strength and determine if the transmitter on the other side of the link should increase or decrease its output power level. A Receiver Signal Strength Indicator (RSSI) makes this possible.

The way the power control is specified is to have a golden receive power range. This golden receive power is defined as a range with a lower and higher threshold levels and a high limit. The lower threshold level corresponds to a received power between -56 dBm and 6 dB above the actual sensitivity of the receiver. The upper threshold level is 20 dB above the lower threshold level to reach an accuracy of 6 dB. The instructions to alter the TX power are carried in the LMP link

Baseband Specifications
The Baseband is the physical layer of the Bluetooth. It manages physical channels and links apart from other services like error correction, data whitening, hop selection and Bluetooth security.
The Baseband layer lies on top of the Bluetooth radio layer in the Bluetooth stack. The baseband protocol is implemented as a Link Controller, which works with the link manager for carrying out link level routines like link connection and power control.
The baseband also manages asynchronous and synchronous links, handles packets and does paging and inquiry to access and inquire Bluetooth devices in the area. The baseband transceiver applies a time-division duplex (TDD) scheme. (alternate transmit and receive). Therefore apart from different hopping frequency (frequency division), the time is also slotted.
Frequency Hopping
Frequency hopping in Bluetooth serves two purposes:

  • Provides resistance to interference and multipath effects

  • Provides a form of multiple access among co-located devices in different piconets.

The total bandwidth is divided into 79 physical channels, each of bandwidth 1 MHz. FH occurs by jumping from one channel to another in a pseudorandom sequence. The hopping sequence (PN) is shared by all of the devices on a single piconet2.

The hop rate is 1600 hops per second, so that each physical channel is occupied for a duration of 625-s. Each of the 625-s time period is referred to as a slot, and these are numbered sequentially.
Bluetooth radios communicate using a time division duplex discipline (TDD). Because more than two devices share the piconet medium, the access technique is TDMA. Thus, piconet access can be characterized as FH-TDD-TDMA.
Using TDD prevents crosstalk between the transmit and receive operations in the radio transceiver, which is essential if a one-chip implementation is desired.
The FH sequence is determined by the master in a piconet and is a function of the master’s Bluetooth address. Because different piconets in the same area will have different masters, they will use different hop sequences. Thus, most of the time, transmission on two devices on different piconets in the same area will be on different physical channels.
Occasionally, two piconets will use the same physical channel during the same time slot, causing a collision and lost data. However, because this will happen infrequently, it is readily accommodated with forward error correction and error detection/ARQ techniques. Thus, a form of CDMA is achieved between devices on different piconets in the same scatternet; this is referred to as FH-CDMA.
Physical Links
Two types of links can be established between a master and a slave:

  • Synchronous connection oriented (SCO): Allocates a fixed bandwidth between a point-to-point connection involving the master and a single slave. The master maintains the SCO link by using reversed slots at regular intervals. The basic unit of reservation is two consecutive slots (one in each transmission direction). The master can support up to three simultaneous SCO links while a slave can support two or three SCO links. SCO packets are never retransmitted.

  • Asynchronous connectionless (ACL): A point-to-multipoint link between the master and all the slaves in the piconet. In slots not reserved for SCO links, the master can exchange packets with any slave on a per-slot basis, including a slave already engaged in an SCO link. Only a single ACL link can exists. For most ACL packets, packet retransmission is applied.

The packet format for all Bluetooth packets consists of three main fields:

  • Access Code (72 bits): Used for timing synchronization, offset compensation, paging and inquiry.

  • Header (54 bits): Used to identify packet type and to carry protocol control information.

  • Payload (0 to 2745 bits): If present, contains user voice and data and, in most cases, a payload header.

There are three types of access code:

  • Channel access code (CAC): Identifies the piconet (unique for a piconet).

  • Device access code (DAC): Used for paging and its subsequent responses

  • Inquiry access code(IAC): Used for inquiry purposes.

The header format for all Bluetooth packets consists of six fields:

  • AM_ADDR(3 bits): Contains the “active mode” address3 of one of the slaves (there are at most seven devices in a piconet). A transmission from the master to a slave contains that slave’s address; a transmission from a slave contains it address. The 0 value is reserved for a broadcast from the master to all slaves in the piconet.

  • Type: Identifies the type of packed. Four type codes are reserved for control packets common to both SCO and ACL links. The remaining packet types are used to convey user information.

  • Flow: Provides a 1-bit flow control mechanism for ACL traffic only. When a packet with Flow = 0 is received, the station receiving the packet must temporarily halt the transmission of ACL packets on this link. When a packet with Flow = 1 is received, transmission may resume.

  • ARQN: Provides 1-bit acknowledgement mechanism for ACL traffic protected by a CRC. If the reception was successful, an ACK (ARQN = 1) is returned; otherwise a NAK (ARQN = 0) is returned. When no return message regarding acknowledge is received, a NAK is assumed implicitly. If a NAK is received, the relevant packet is retransmitted.

  • Header error control (HEC): An 8-bit error detection code used to protect the packet header.

Payload Format:
For some packet types, the baseband specification defines a format for the payload field. For voice payloads, no header is defined. For all of the ACL packets and for the data portion of the SCO packet, a header is defined. For data payloads, the payload format consists of three fields:

  • Payload header: An 8-bit header is defined for a single-slot packet, and a 16-bit header is defined for a multi-slot packet.

  • Payload body: Contains user information

  • CRC: A 16-bit CRC code is used on all data payloads except the AUX1 packet (which carries 30 information bytes with no CRC or FEC which is typically used for high-speed data.4)

The payload header, when present consists of three fields:

  • L_CH: Identifies the logical channel.

  • Flow: Used to control flow at the L2CAP level. (This is the same on/off mechanism provided by the flow field in the packet header for ACL traffic).

  • Length: The number of bytes of data in the payload, excluding the payload header and CRC.

Error Correction
Bluetooth makes use of three error correction schemes:

  • 1/3 rate FEC

  • 2/3 rate FEC

  • ARQ

These error correction schemes are designed to satisfy competing requirements. The error correction scheme must be adequate to cope with the inherently unreliable wireless link but must also be streamlined and efficient.

Logical Channels
Bluetooth defines five types of logical data channels designed to carry different types of payload traffic.

  • Link Control (LC): Used to manage the flow of packets over the link interface. The LC channel is mapped onto the packet header. This channel carries low-level link control information like ARQ, flow control, and payload characterization.

  • Link Manager (LM): Transports link management information between participating stations.

  • User asynchronous (UA): Carries asynchronous user data.

  • User isochronous (UI): Carries isochronous5 user data.

  • User synchronous (US): Carries synchronous user data.

Channel Control
The operation of a piconet can be understood in terms of the states of operation during link establishment and maintenance (Figure 4). There are two major states:

  • Standby: The default state. This is a low-power state in which only the native clock is running.

  • Connection: The device is connected to a piconet as a master or a slave.

Figure 4: Bluetooth state transition diagram.

In addition, there are seven interim sub-states that are used to add new slaves to a piconet. To move from one state to the other, either commands from the Bluetooth link manager or internal signals in the link controller are used. The sub-states are as follows:

  • Page: Used by the master to activate and connect to a slave. Uses the slave device access code (DAC)

  • Page scan: Device is listening for a page with its own DAC.

  • Master response: A device acting as a master receives a page response from a slave. The device can now enter the connection state or return to the page state to page for other slaves.

  • Slave response: A device acting as a slave responds to a page from a master. If connection setup succeeds, the device enters the connection state; otherwise it returns to the page scan state.

  • Inquiry: Device has issued an inquiry, to find the identity of the devices within range.

  • Inquiry scan: Device is listening for an enquiry

  • Inquiry response: A device that has issued an inquiry receives an inquiry response.

Bluetooth Audio
The baseband specifications indicate that either of two voice encoding schemes can be used:

  • Pulse Code Modulation (PCM)

  • Continuously Variable Slope Delta (CVSD)

Bluetooth Security
The Bluetooth baseband specifications defines a facility for link security between any two Bluetooth devices, consisting of the following elements:

  • Authentication.

  • Encryption (Privacy).

  • Key management and usage.

The security algorithms make use of four parameters:

  • Unit address: The 48-bit device address, which is publicly known.

  • Secret authentication key: A secret 128-bit key

  • Secret privacy key: A secret key of length from 4 to 128 bits

  • Random number: A 128-bit random number derived from a pseudorandom generation algorithm executed in the Bluetooth unit.

The two secret keys are generated and configured with the unit and not disclosed. The purpose of authentication is to verify the claimed identity of one of the two Bluetooth devices involved in an exchange. User information can be protected by encryption of the packet payload.

Link Manager Specifications
The Link Manager carries out link setup, authentication, link configuration and other protocols. It discovers other remote LM’s and communicates with them via the Link Manager Protocol (LMP). To perform its service provider role, the LM uses the services of the underlying Link Controller (LC).
The Link Manager Protocol essentially consists of a number of PDU (protocol Data Units), which are sent from one device to another. LM PDUs are always sent as single-slot packets and the payload header is therefore one byte.
The procedures defined for LMP are grouped into 24 functional areas, each of which involves the exchange of one or more messages. Table 2 lists these areas, together with the PDUs involved in each area.



General response

Accepted, not_accepted

Security Service


au_rand, sres


in_rand, au_rand, sres, comb_key, unit_key

Change link key


Change current link key

temp_rand, temp_key, use_semi_permanent_key


Clock offset request

clkoffset_req, clkoffset_res

Slot offset information


Timing accuracy

Information request



Station Capability

LMP version

version_req, version_res

Supported features

features_req, features_res

Mode Control

Switch master/slave role


Name request

name_req, name_res



Hold mode

hold, hold_req

Sniff mode

sniff, sniff_req, sniff_res

Park mode

park_req, park_set, set_broadcast_window,

modify_beacon, unpark_PM_ADDR_req,


Power control

incr_power_req, decr_power_req, max_power,


Channel quality-driven

Change between DM and DH

auto_rate, preferred_rate

Quality of service

quality_of_service, quality_of_service_req

SCO links

SCO_link_req, remove_ SCO_link_req

Control of multi-slot packet

max_slot, max_slot_req

Paging scheme

page_mode_req, page_scan_mode_req

Link supervision


Table 2: LMP PDU’s.

Logical Link Control and Adaptation Protocol (L2CAP)
In the IEEE 802 specification, L2CAP provides a link layer protocol between entities across a shared-medium network. It also provides a number of services and relies on the baseband layer for flow and error control.
L2CAP makes use of ACL links; it does not provide support for SCO links. L2CAP provides 2 alternatives of service to upper-layer protocols using ACL links:

  • Connectionless service: This is a reliable datagram style of service.

  • Connection-mode service: This is similar to the service offered by HDLC. A logical connection is set up between two users exchanging data. Flow and error control are provided.

L2CAP provides three types of logical channels:

  • Connectionless: Supports the connectionless service. Each channel is unidirectional and is used for broadcast from the master to multiple slaves.

  • Connection-oriented: Supports connection oriented service. Each channel is full duplex. A quality of service flow specification is assigned in each direction.

  • Signalling: Provides fro the exchange of signalling messages between L2CAP entities.

L2CAP Packets
Figure 5 shows the format of L2CAP packets:

Figure 5: L2CAP Packet Format. (a) Connectionless PDU. (b) Connection-oriented PDU. (c) Signalling command PDU. (d) Command Format.
In the connectionless PDU, the following fields are used:

  • Length: Length of the information payload plus PSM fields in bytes

  • Channel ID: A value of 2 indicating the connectionless channel.

  • Protocol/Service multiplexer (PSM): Identifies the higher layer recipient for the payload in this packet.

  • Information payload: Higher layer user data. This field may be up to 65533 bytes in length.

Connection oriented packets have the same format as connectionless packets, but without the PSM field. The PSM field is not needed because the CID identifies the upper layer recipient of the data. The information payload field may be up to 65535 bytes is length.

Signalling commands packets have the same header format as the connection-oriented packets. In this case the CID value is 1, indicating the signalling channel. The payload of a signalling packet consists of one or more L2CAP commands, each of which consists of four fields:

  • Code: Identifies the type of command

  • Identifier: Used to match a request with its reply. The requesting device sets this field and the responding device uses the same value in its response. A different identifier must be used for each original command.

  • Length: Length of the data field for this command in bytes

  • Data: Additional data, if necessary, relating to this command.

Signalling Commands
There are eleven commands in five categories (Table 3). The command reject command can be sent in response to any command to reject it. Reasons for rejection include invalid CID or length exceeded.

  • Connection Commands: Used to establish a new logical connection.

  • Configure Commands: Are sent to establish an initial logical link transmission contract between two L2CAP entities and to renegotiate this contract whenever appropriate

  • Disconnection Commands: Are used to terminate a logical channel

  • Echo commands: Are used to solicit a response from a remote L2CAP entity. These commands are typically used for testing the link or passing vendor-specific information using the optional data field.

  • Information Commands: Are used to solicit implementation-specific information from a remote L2CAP entity.





Command reject



Connection request

PSM, Source CID


Connection response

Destination CID, Source CID, Result, Status


Configure request

Destination CID, Flags, Options


Configure response

Source CID, Flags, Results, Options


Disconnection request

Destination CID, Source CID


Disconnection response

Destination CID, Source CID


Echo request

Data (optional)


Echo response

Data (optional)


Information request

Info Type


Information response

Info Type, Result, Data (optional)

Table 3: L2CAP signalling command codes.

Quality of Service (QoS)
The QoS parameter in L2CAP defines a traffic flow specification based on RFC 1363.6 In essence, a flow specification is a set of parameters that indicate a performance level that the transmitter will attempt to achieve.
The flow specification consists of the following parameter:

  • Service type: This parameter indicates the level of service for this flow. A value of 0 indicates that no traffic will be transmitted on this channel. A value of 1 indicates a best effort service. The device will transmit data as quick as possible with no guarantees about performance. A value of 2 indicates a guaranteed service (The sender will transmit data that conform to the remaining QoS parameters).

  • Token rate (bytes/second): See below (bucket size)

  • Token bucket size (bytes): This parameter, along with the Token rate parameter defines a token bucket scheme that is often used in QoS specifications. The advantage of this scheme is that it provides a concise description of the peak average traffic load the recipient can expect.

  • Peak bandwidth (bytes/second): Limits how fast packets may be sent back-to-back from applications.

  • Latency (microseconds): Is the maximum acceptable delay between transmission of a bit by the sender and its initial transmission over the air.

  • Delay variation (microseconds): is the difference between the maximum and minimum possible delay that a packet will experience.

All Materials in this document where extracted and summarized from:


Stallings, W (2005) Wireless Communications and Networks, 2nd Ed., Pearson Prentice Hall, ISBN: 0-13-196790-8

Compiled and typed by: Dr. Jose A. Santos – January 2003.

1 Industrial Scientific and Medical

2 This will be referred to as FH channel for clarity purposes.

3 Temporary address assigned to this slave in this piconet.

4 Check Stallings Pg 497, Table 15.5

5 The term isochronous refers to blocks of data that recur with known periodic time.

6 A proposed flow specification, RFC 1363, September 1992.


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