3.2 LOCAL AREA NETWORKS

 

3.2  LOCAL AREA NETWORKS

1. Ethernet (802.3)

IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation, Intel Corporation, and Xerox. This was called Ethernet.

802.3 defines two categories: baseband and broadband, as shown in Fig. 3-1. The word base specifies a digital signal (in this case, Manchester encoding). The word broad specifies an analog signal (in this case, PSK encoding). IEEE divides the baseband category into five different standards (see Table 3-1): 10BASE-5 (Thick Ethernet), 10BASE-2 (Thin Ethernet), 10BASE-T (Twisted-pair Ethernet), 1BASE-5 (StarLAN), and 100BASE-T (Fast Ethernet). The first number (10,1,or100) indicates data rate in Mbps. The last number or letter (5,2,1,or T) indicates maximum cable length or the type of cable. The notation 10BASE-5 means that it operates at 10Mbps, uses baseband signaling, and can support segments of up to 500 meters. IEEE defines only one specification for the broadband category: 10BROAD36 (broadband Ethernet). Again, the first number (10) indicates data rate. The last number defines the maximum cable length (36). However, the maximum cable length restriction can be changed using networking devices such as repeaters or bridges.

Fig. 3-1  Two categories of 802.3

Table 3-1  The most common kinds of baseband 802.3 LANs

Access Method: CSMA/CD

Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other. Such overlaps, which turn the signals into unusable noise, are called collisions. As traffic increases on a multiple access link, so do collisions. A LAN therefore needs a mechanism to coordinate traffic, minimize the number of collisions that occur, and maximize the number of frames that are delivered successfully. The access mechanism used in an Ethernet is called carrier sense multiple access with collision detection (CSMA/CD, standardized in IEEE802.3).

CSMA/CD is the result of an evolution from multiple access (MA) to carrier senses multiple access (CSMA), and finally, to carrier sense multiple access with collision detection (CSMA/CD). The original design was a multiple access method in which every workstation had equal access to a link. In MA, there was no provision for traffic coordination. Access to the line was open to any node at any time, with the assumption that the odds of two devices competing for access at the same time were small enough to be unimportant. Any station wishing to transmit did so, and relied on acknowledgements to verify that the transmitted frame had not been destroyed by other traffic on the line.

In a CSMA system, my workstation wishing to transmit must first listen for existing traffic on the line. A device listens by checking for a voltage. If no voltage is detected, the line is considered idle and the transmission is initiated. CSMA cuts down on the number of collisions but does not eliminate them. Collisions can still occur. If another station has transmitted too recently for its signal to have reached the listening station, the listener assumes the line is idle and introduces its own signal onto the line.

The final step is the addition of collision detection (CD). In the CSMA/CD the station wishing to transmit first listens to make certain the link is free, then transmits its data, then listens again. During the data transmission, the station checks the line for the extremely high voltages that indicate a collision. If a collision is detected, the station waits a predetermined amount of time for the line to clear, then sends its data again.

Today the term Ethernet, CSMA/CD, and 802.3 are used interchangeably.

2. Token Bus (802.4)

Although 802.3 is widely used in offices, during the development of the 802 standard, people from General Motors and other companies interested in factory automation had serious reservations about it. For one thing, due to the probabilistic MAC Protocol, with a little bad luck a station might have to wait arbitrarily long to send a frame (i.e., the worst case is unbounded). For another, 802.3 frames do not have priorities making them unsuited for real-time systems in which important frames should not be held up waiting for unimportant frames.

A simple system with a known worst case is a ring in which the station take turns sending frames. If there are n stations and it takes T sec to send a frame, no frame will ever have to wait more than nT sec to be sent. The factory automation people in the 802 committee liked the conceptual idea of a ring but did not like the physical implementation because a break in the ring cable would bring the whole network down. Furthermore, they noted that a ring was a poor fit to the linear topology of most assembly lines. As a result, a new standard was developed, having the robustness of the 802.3 broadcast cables, but the known worst-case behavior of a ring.

This standard, 802.4, describes a LAN called a token bus. Physically, the token bus is a linear or tree-shaped cable onto which the stations are attached. Logically, the stations are organized into a ring, with each station quarying the address of the station to its “left” and “right”. When the logical ring is initialized, the highest numbered station may send the first frame. After it is done, it passes permission to its immediate neighbor by sending the neighbor a special control frame called a token. The token propagates around the logical ring, with only the token holder permitted to transmit frames. Since only one station at a time holds the token, collisions do not occur.

An important point to realize is that the physical order in which the stations are connected to the cable is not important. Since the cable is inherently a broadcast medium, each station receives each frame, discarding those not addressed to it. When a station passes the token, it sends a token frame specifically addressed to its logical neighbor in the ring, irrespective of where that station is physically located on the cable. It is also worth noting that when stations are first powered on, they will not be in the ring, so the MAC protocol has provisions for adding stations to, and deleting stations from, the ring.

For the physical layer, the token bus uses the 75 ohm broadband coaxial cable used for cable television. Both single- and dural-cable systems are allowed, with or without head-end. Three different analog modulation schemes are permitted: phase continuous frequency shift keying, phase coherent frequency shift keying, and multilevel duobinary amplitude modulated phase shift keying. Speeds of 1, 5, and 10 Mbps are possible. Furthermore, the modulation schemes not only provide ways to represent 0, 1, and idle on the cable, but also three other symbols used for network control. All in all, the physical layer is totally incompatible with 802.3, and a lot more complicated.

3. Token Ring (802.5)

As mentioned above, the network access mechanism used by Ethernet (CSMA/CD) is not infallible and may result in collisions. Stations may attempt to send data multiple times before a transmission makes it onto the link. This redundancy may create delays of indeterminable length if the traffic is heavy. There is no way to predict either the occurrence of collisions or the delays produced by multiple stations attempting to capture the link at the same time.

Token ring resolves this uncertainty by requiring that stations take turns sending data. Each station may transmit only during its turn and may send only one frame during each turn. The mechanism that coordinates this rotation is called token passing. A token is a simple placeholder frame that is passed from station to station around the ring. A station may send data only when it has possession of the token.

Token ring allows each station to send one frame per turn.

（1）Access Method: Token Passing.

Whenever the network is unoccupied, it circulates a simple three-byte token. This token is passed from NIC to NIC in sequence until it encounters a station with data to send. That station waits for the token to enter its network board. If the token is free, the station may then send a data frame. It keeps the token and sets a bit inside its NIC as a reminder that it has done so, then sends its one data frame.

This data frame proceeds around the ring, being regenerated by each station. Each intermediate station examines the destination address, finds that the frame is addressed to another station, and relays it to its neighbor. The intended recipient recognizes its own address, copies the message, checks for errors, and changes four bits in the last byte of the frame to indicate address recognized and frame copied. The full packet then continues around the ring until it returns to the station that sent it.

The sender receives the frame and recognizes itself in the source address field. It then examines the address-recognized bits. If they are set, it knows the frame was received. The sender then discards the used data frame and releases the token back to the ring.

（2）Priority and Reservation.

Generally, once a token has been released, the next station on the ring with data to send has the right to take charge of the ring. However, in the IEEE 802.5 model another option is possible. The busy token can be reserved by a station waiting to transmit, regardless of that station's location on the ring. Each station has a priority code. As a frame passes by, a station waiting to transmit may reserve the next open token by entering its priority code in the access control (AC) field of the token or data frame (discussed later in this section). A station with a higher priority may remove a lower priority reservation and replace it with its own. Among stations of equal priority, the process is first come, first served. Through this mechanism, the station holding the reservation gets the opportunity to transmit as soon as the token is free, whether or not it comes next physically on the ring.

（3）Time Limits.

To keep traffic moving, token ring imposes a time limit on any station wanting to use the ring. A starting delimiter (the first field of either a token or data frame) must reach each station within a specified interval (usually 10 milliseconds). In other words, each station expects to receive frames within regular time intervals (it receives a frame and expects to receive the next frame within a specified period).

（4）Monitor Stations.

Several problems may occur to disrupt the operation of a token ring network. In one scenario, a station may neglect to retransmit a token or a token may be destroyed by noise, in which case there is no token on the ring and no station may send data. In another scenario, a sending station may neglect to remove its used data frame from the ring or may not release the token once its turn has ended.

To handle these occurrences, one station on the ring is designated as a monitor. The monitor sets a timer each time the token passes. If the token does not reappear in the allotted time, it is presumed to be lost and the monitor generates a new token and introduces it to the ring. The monitor guards against perpetually recirculating data frames by setting a bit in the AC field of each frame. As a frame passes, the monitor checks the status field. If the status bit has been set, it knows that the packet has already been around the ring and should have been discarded. The monitor then destroys the frame and puts a token onto the ring. If the monitor fails, a second station, designated as backup, takes over.

4. FDDI

Fiber distributed data interface (FDDI) is a local area network protocol standardized by ANSI and the ITU-T (ITU-T X.3). It supports data rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. When FDDI was designed, speeds of 100 Mbps required fiber-optic cable. Today, however, comparable speeds are available using copper cable. The copper version of FDDI is known as CDDI.

（1）Access Method: Token Passing.

In a token ring network, a station can send only one frame each time it captures the token. In FDDI access is limited by time. A station may send as many frames as it can within its allotted access period, with the proviso that time-sensitive frames be sent first.

To implement this access mechanism, FDDI differentiates between two types of data frames: synchronous and asynchronous. Synchronous here refers to information that is time sensitive, while asynchronous refers to information that is not. These frames are usually called S-frames and A-frames.

Each station that captures the token is required to send S-frames first. In fact, it must send its S-frames whether or not its time allotment has run out (see below). Any remaining time may then be used to send A-frames. To understand how this mechanism ensures fair and timely link access, it is necessary to understand the FDDI time register and times.

（2）Time Registers.

FDDI defines three time registers to control circulation of the token and distribute link access opportunities among the nodes equitable. Each station has three registers. The registers hold time values that control the operation of the ring. These values are set when the ring is initialized and do not vary in the course of operation. The registers are called synchronous allocation (SA), target token rotation time (TTRT), and absolute maximum time (AMT).

·    Synchronous allocation (SA): The SA register indicates the length of time allowing each station for sending synchronous data. This value is different for each station and is negotiated during initialization of the ring.

·    Target Token Rotation Time (TTRT): The TTRT register indicates the average time required for a token to circulate around the ring exactly once (the elapsed time between a token's arrival at a given station and its next arrival at the same station). The value is the same for all station and is negotiated during the initialization of the ring. Because it is an average, the actual time of any rotation may be greater or less than this value.

·    Absolute Maximum Time (AMT): The AMT register holds a value equal to twice the TTRT. A token may not take longer than this time to make one rotation of the ring. If it does, some station or station are monopolizing the network and the ring must be reinitialized.

（3）Timers.

Each station contains a set of timers that enable it to compare actual timings with the values contained in the registers. Timers can be set and reset, and their values decremented at a rate set by the system clock. The two timers used by FDDI are called the token rotation timer (TRT), and token holding timer (THT).

·    Token Rotation Timer (TRT):The TRT runs continuously and measures the actual time taken by the token to complete a cycle. When the token returns, the station records the time remaining on its TRT into its THT. Then the station resets its TRT based on the TTRT value. As soon as the TRT is set, it begins counting down. The time indicated by the TRT at any given point therefore is the difference between the actual time that has elapsed during the current rotation and the expected or allowed time (TTRT time). When the token completes a rotation and returns to the station, the time indicated by the TRT is equal to the amount of time remaining for that rotation (the difference between the TTRT and the actual elapsed time). That remaining time is then available to the station to send its frames.

·    Token Holding Timer (THT): The THT begins running as soon as the token is received. Its function is to show how much time remains for sending asynchronous frames once the synchronous frames have been sent. Each time the station receives the token, the TRT value is copied into the THT. At that point, the THT starts its own countdown. Any waiting synchronous frames must be sent as soon as the token is received. The THT shows how much time (if any) remains for sending asynchronous frames. The station may send only as many A-frames as it has THT credit for. As long as the THT is positive, the station can send asynchronous data. Once the value of this timer reaches or falls below zero, however, the station must release the token. We may think of the THT as the station's bank account. S-frames are bills that must be paid immediately—even if the station has to go into debt to do so. A-frames are expenditures that can be put off for a while; the station needs to make them but can wait until its bank account can cover the expense.

5. Comparison

Table 3-2 compares the features of the three LANs discussed above. Ethernet is good for low-level loads but collapses as the load increases due to collisions and retransmissions. Token ring and FDDI perform poorly at low-level loads but always guarantee some maximum time between transmission of two adjacent frames.

Table 3-2   LAN comparison

KEYWORDS

NOTES

（1）Ethernet（以太网）。以竞争方式使用网络的IEEE 802.3标准。以太网使用总线或星形拓扑结构，依靠带冲突检测的载波侦听多路复用（Carrier SenseMultiple Access with CollisionDetection）的网络接入方式来调节通信线路上的数据流量。网络节点之间通过同轴电缆、光纤电缆或双绞线实现互连。以太网的标准基带传输速率为10 Mbps。

（2）CDMA/CD（带冲突检测的载波侦听多路访问）。适合总线型网络，即以太网。

（3）Timer（计时器）。又称定时器。按一定的时间间隔来改变其内容用以度量时间的一种寄存器。

EXERCISES

1. Fill in the following blanks.

（1）802.3 defines two categories:        and        .The word base specifies a       

signal (in this case, Manchester encoding).The word broad specifies an        signal (in this case, PSK encoding).

（2）The notation 100BASE-T means that it operates at       Mbps, uses signaling, and can support segments of up to         meters.

（3）Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other. Such overlaps, which turn the signals into unusable noise, are called         .

（4）When the logical ring is initialized, the highest numbered station may send the first frame. After it is done, it passes permission to its immediate neighbor by sending the neighbor a special control frame called a         .

2. Single choice.

（1）In CSMA/CD, the number of collisions is      that in MA.

A．greater than

B．less than

C．equal to

D．twice

（2）10BASE-2 and 10BASE-5 have different       .

A．signal and types

B．fields on the 802.3 frame

C．maximum segment lengths

D．maximum data rates

（3）Another term for CSMA/CD and the IEEE 802.3 standard is        .

A．Ethernet

B．token ring

C．FDDI

D．token bus

（4）FDDI is an acronym for         .

A．fast data delivery interface

B．fiber distributed data interface

C．fiber distributed digital interface

D．fast distributed data interface

（5）In FDDI, if a token arrives later than the time specified in the      register, the ring must be reinitialized.

A．synchronous allocation (SA)

B．target token rotation time (TTRT)

C．absolute maximum time (AMT)

D．token rotation time (TRT)

3. List the six different physical standards the 802.3 standard has defined.

READING MATERIALS

VoWLAN

VoWLAN (Voice over WLAN) is a method of sending voice information in digital form over a wireless broadband network. Essentially, VoWLAN is VoIP delivered through wireless technology. The technology is sometimes called “VoWi-Fi” or “Wi-Fi VoIP” because it uses the IEEE 802.11 set of specifications (informally known collectively as Wi-Fi) for transporting data over wireless local area networks and the Internet.

VoWLAN requires a voice-enabled wireless device, most commonly a PDA or a “Wi-Fi handset”, which looks and operates like a cell phone, but sends voice as discrete data packets rather than an analog voice stream. Callers can also use a software-based phone, known as a soft-

phone that resides on devices including laptop and desktop computers. The VoWLAN system routes calls from the phone to a WLAN access point and then to a VoIP gateway or IP PBX. The callis sent to its appropriate destination within the private network or out onto the Internet or PSTN.

The VoWLAN system allows all regular functions and messaging applications available on wired phones to be available on the VoWLAN devices. Like VoIP, VoWLAN also contributes to cost efficiency. Because calls can be routed over the data network internally or over the Internet externally, mobile telephony costs can be eliminated or decreased significantly.

Major barriers to VoWLAN include inconsistent voice performance and the need for quality of service (QoS); slow and unreliable encryption and authentication; and the proprietary nature of current products. The IEEE and the Wi-Fi Alliance have been developing standards improv- ements to mitigate VoWLAN limitations. The recent 802.11i standard implemented intelligent security to speed up authentication while Roaming. Other upcoming standards enhancements include:

·    The 802.11e standard will define QoS measures that will prioritize voice traffic using two methods, Wireless MultiMedia (WMM) and WMM Scheduled Access.

·    802.11r will specify a fast-roaming protocol to minimize packet loss when users move from one access point to another within a WLAN.

·    802.11k will include the Radio Resource Management protocol to enable a handset to pre-discover the conditions of the WLAN network and choose the best available paths.

VoWLAN is most frequently found in the retail, manufacturing and health care industries, where WLAN was adopted early and use has progressed quickly. In the long term, VoWLAN deployment is a significant step toward interoperability and seamless mobile connectivity between private WLANs and public wireless and cellular networks.

Wireless LAN, WAP, And Bluetooth

1. What is a Wireless LAN

A wireless LAN lets users roam around a building with a laptop (equipped with a wireless LAN card) and stay connected to their network, without being connected to a wire.

A wireless LAN (WLAN) is a local area network (LAN) without wires. WLANs have been around for more than a decade, but are just beginning to gain momentum because of falling costs and improved standards. WLANs transfer data through the air using radio frequencies instead of cables. They can reach a radius of 500 feet indoors and 1 000 feet outdoors, but antennas, trans-

mitters and other access devices can be used to widen that area. WLANs require a wired access point that plugs all the wireless devices into the wired network.

A new standard put out by the Institute of Electrical and Electronics Engineers (IEEE) called 802.11b or Wi-Fi is making WLAN use faster and easier, and the market is growing quickly. The Cahners In-Stat Group predicts that revenue for total enterprise WLAN end-use will reach almost $4.6 billion by 2005.

Many Mac users have been wireless for over a year now with Apple's AirPort product, which is also based on the 802.11 IEEE standard.

（1）Where are WLANs typically used?

WLANs are used on college campuses and in office buildings. They can be set up in houses allowing multiple users to access one Internet connection. Resorts, apartment buildings and airports plan to offer WLAN access (some already do). Often the best uses for WLANs are in places where LANs aren't installed yet, like schools or public institutions that are slow to adopt new technologies.

Starbucks and Microsoft are also getting into the WLAN game. They are teaming up to equip their coffee shops with WLANs, which allow laptop users to surf the Net while sipping coffee.

（2）Are there different kinds of WLANs?

Bluetooth and HomeRF are also WLAN technologies, but Bluetooth works in a smaller area than 802.11b does and HomeRF hasn't become as popular as 802.11b.

（3）What is 802.11b?

It sounds like something from Star Wars, but it is the standard that is behind WLANs current popularity 802.11a, transfers data at speeds of up to 11Mbps (million bits per second) in the 2.4 gigahertz radio band (a license is not required for this band).

The next version, 802.111a, is supposed to transfer data at even higher speeds of up to 54Mbps in the 5 gigahertz band.

（4）Are Wireless LANs secure?

A small research group at the University of California at Berkeley recently put out a report stating that they found flaws in the 802.11 standard (and 802.11b standard). Their report says that they were able to intercept transmissions over the wireless network. These transmissions were encrypted, but the encryption was broken.

（5）Are there other problems?

If too many people or businesses in the same area have WLANs, then the band of air that they transmit signals on can become overcrowded. Problems with signal interference are already occurring and there are fearing that the airwaves will become overloaded.

2. What is WAP

（1）What is WAP?

Have you heard the hype about the wireless Web? Or is the Web without wires a waste of time? Either way, WAP—an acronym for wireless application protocol—is making the wireless Internet a reality, and even if it isn't successful right now, this industry is expecting enormous growth. Market researcher IDC predicts that 1.3 billion wireless Internet users will have WAP-enabled devices by 2004.

（2）Where did WAP come from?

An industry consortium called the WAP Forum promotes WAP. The WAP Forum was founded in 1997 by Ericson, Motorola, Nokia and Phone.com (which has now joined with Software.com to form Openwave Systems Inc.) after Phone developed a server and browser for AT＆T's PocketNet (wireless Web service).

（3）What exactly is WAP?

WAP is a set of protocols used to transfer data to wireless devices. WAP-enabled devices provide wireless users with a limited version of the Web designed to work on the small black and white screens of phones and PDAs.

Websites accessed by WAP phones must be re-written to satisfy the wireless application protocol；in order to do that. Web pages written in HTML must be transferred to the WAP markup language (WML). Internet browsers like Netscape and Internet Explorer read pages in HTML, while a microbrowser on a WAP-enabled device reads pages in WML.

Major websites like Yahoo and Amazon have carefully designed their WAP sites to look and feel much like their wired counterparts, while search engines like Google are transferring pages from HTML to WML, so they can be accessed by wireless users.

（4）What’s the problem with WAP?

WAP faces bandwidth constraints that limit the amount of data that can be transferred to the devices. On the wireless Internet, users are presented with a fraction of the information available on the traditional version.

And while WAP is the leading wireless protocol now, that doesn't mean that it's here to stay. When a better version of the wireless Web becomes available. WAP could very well become obsolete.

One potential competitor is NTT DoCoMo, a Japanese company which has developed a hugely successful wireless data service called i-mode and expects to tap the U. S. market toward the end of 2001.

3. What is Bluetooth

Bluetooth is a technology that connects electronic devices—from camcorders to PDAs to computers—without using wires. Consumers began to see Bluetooth in action when Toshiba starting selling a Bluetooth—enabled PC card over their website in September 2000 for $199. Other vendors plan to follow with devices ranging from PDAs to mobile phones.

A Bluetooth device uses radio signals to send information from one Bluetooth device to another through the air. For example, if you are trying to transfer a PC's address book to a PDA, first the data in an address book is translated into a language that the PDA can understand by a conduit. The data goes through the conduit to the Bluetooth device. The Bluetooth device is made up of a base—band processor, a radio, and an antenna. The base—band processor transfers the data into signals that the radio can understand, and the radio puts out signals in a frequency (2.4 gigahertz) that the antenna transmits through the air to another antenna on another Bluetooth device within 30 feet. The other device receives the data and processes it in the reverse order.

Bluetooth is supported by a Special Interest Group (SIG), which was founded in 1998 and has approximately 2000 members, all of whom have access to Bluetooth specifications (the information needed to make a Bluetooth product). The SIG includes IBM, Intel, Microsoft and Nokia, and works to develop and promote the Bluetooth technology.

But Bluetooth, like many new technologies, may not be an instant hit. There are still plenty of questions about the ability of these devices to speak the same language. So while devices produced by the same company could communicate with each other easily, integration may be difficult when multiple vendors are involved. And while consultants at Forrester Research expect Bluetooth's popularity to grow, the firm said in a brief that many businesses won't buy in. “until user pressure forces them to in 2003”.

INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

A good example of a circuit-switched network is the Integrated Services Digital Network (ISDN), an evolving communications network standard that provides universal end-to-end connectivity over digital lines. ISDN services are still being introduced and are not yet available everywhere. Once ISDN is fully implemented, however, today's disparate transmission and switching services will be accessible internationally via a single set of interface standards.

As a worldwide telecommunications network standard, ISDN will pull together a wide variety of consumer services into a single access package. An ISDN allow users to send data, voice, image, facsimile, and so on over the same wires and to have ready access to digital service.

1. ISDN System Architecture

It is now time to look at the ISDN architecture in detail, particularly the customer's equi-

pment and the interface between the customer and the telephone company or PTT.

In Fig. 3-2 we see the normal configuration for a home or small business. The carrier places a network-terminating device NT1 on the customer's premises and connects it to the ISDN exchange in the carrier's office, several kilometers away, using the twisted pair that was previously used to connect to the telephone. The NT1 box has a connector on it into which a passive bus cable can be inserted. Up to eight ISDN telephones, terminals, alarms, and other devices can be connected to the cable, similar to the way devices are connected to a LAN. From the customer's point of view, the network boundary is the connector on NT1.

Fig. 3-2  Example ISDN system

2. Services

The purpose of the ISDN is to provide fully integrated digital services to users. These  services fall into three categories: bearer services, teleservices, and supplementary services.

（1）Bearer Services.

Bearer services means to transfer information(voice, data, and video) between users without the network manipulating the content of that information. The network does not need to process the information and therefore does not change the content. Bearer services belong to the first three layers of the OSI model and are well defined in the ISDN standard. They can be provided using circuit-switched, packet-switched, frame-relay, or cell-relay networks.

（2）Teleservices.

In teleservices, the network may change or process the contents of the data. These services correspond to layers 4~7 of the OSI model. Teleservices rely on the facilities of the bearer services and are designed to accommodate complex user needs without the user having to be aware of the details of the process. Teleservices include telephony, teletex, telefax, videotex, telex, and teleconferencing. Although the ISDN defines these services by names, they have not yet become standard.

（3）Supplementary Services.

Supplementary services are those services that provide additional functionality to the bearer services and teleservices. Examples of these services are reverse charging, call-waiting, and message handling, all familiar from today's telephone company Service.