Chapter 2
Protocol Architecture, TCP/IP, and OSI
Wireless Communication Technology (WCT)
Today
A simple example – 3 Layers Structure
TCP/IP – 5 Layers Structure *
Standard OSI – 7 Layers Structure
Problem 2.7 and Homework Assignments
The Need For Protocol Architecture
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When computers, terminals, and/or other data processing devices exchange data, the procedures involved can be quite complex. Consider, for example, the transfer of a file between two computers. There must be a data path between the two computers, either directly or via a communication network. But more is needed. Typical tasks to be performed:
1. The source system must either activate the direct data communication path or inform the communication network of the identity of the desired destination system.
2. The source system must ascertain that the destination system is prepared to receive data.
3. The file transfer application on the source system must ascertain that the file management program on the destination system is prepared to accept and store the file for this particular user.
4. If the file formats used on the two systems are different, one or the other system must perform a format translation function.
To transfer data several tasks must be performed:
1.) the source must activate communications path or inform network of destination
2.) the source must make sure that destination is prepared to receive data
3.) the file transfer application on source must confirm file management program at destination is prepared to accept and store file
4.) a format translation function may need to be performed if the formats on systems are different
Functions of Protocol Architecture-Layers
breaks logic into subtask modules which are implemented separately
modules are arranged in a vertical stack
each layer in the stack performs a subset of functions
relies on next lower layer for primitive functions
changes in one layer should not require changes in other layers
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It is clear that there must be a high degree of cooperation between the two computer systems. Instead of implementing the logic for this as a single module, the task is broken up into subtasks, each of which is implemented separately. In a protocol architecture, the modules are arranged in a vertical stack. Each layer in the stack performs a related subset of the functions required to communicate with another system. It relies on the next lower layer to perform more primitive functions and to conceal the details of those functions. It provides services to the next higher layer. Ideally, layers should be defined so that changes in one layer do not require changes in other layers.
Of course, it takes two to communicate, so the same set of layered functions must exist in two systems. Communication is achieved by having the corresponding, or peer, layers in two systems communicate. The peer layers communicate by means of formatted blocks of data that obey a set of rules or conventions known as a protocol.
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A Simple Protocol
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In very general terms, distributed data communications can be said to involve three agents: applications, computers, and networks. Examples of applications include file transfer and electronic mail. These applications execute on computers that typically support multiple simultaneous applications. Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another. Thus, the transfer of data from one application to another involves first getting the data to the computer in which the application resides and then getting it to the intended application within the computer.
agents involved:
examples of applications include file transfer and electronic mail
these execute on computers that support multiple simultaneous applications
applications
computers
networks
Communication Layers
communication tasks are organized into three relatively independent layers:
Network access layer
concerned with the exchange of data
Transport layer
provides reliable data transfer
Application layer
Contains logic to support applications
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With these concepts in mind, it appears natural to organize the communication task into three relatively independent layers: network access layer, transport layer, and application layer.
Network Access Layer
covers the exchange of data between an end system and the network that it is attached to
concerned with issues like :
destination address provision
invoking specific services like priority
access to & routing data across a network for two end systems attached to the same network
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The network access layer is concerned with the exchange of data between a computer and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching, local area networks (LANs), and others. For example, IEEE 802 is a standard that specifies the access to a LAN; this standard is described in Part Three. It makes sense to put those functions having to do with network access into a separate layer. By doing this, the remainder of the communications software, above the network access layer, need not be concerned about the specifics of the network to be used. The same higher-layer software should function properly regardless of the particular network to which the computer is attached.
Transport Layer
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Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. As we shall see, the mechanisms for providing reliability are essentially independent of the nature of the applications. Thus, it makes sense to collect those mechanisms in a common layer shared by all applications; this is referred to as the transport layer.
Protocol Architecture and Networks
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Stallings DCC9e Figure 2.1 indicates that modules at the same level (peers) on different computers communicate with each other by means of a protocol. An application entity (e.g., a file transfer application) in one computer communicates with an application in another computer via an application-level protocol (e.g., the File Transfer Protocol). The interchange is not direct (indicated by the dashed line) but is mediated by a transport protocol that handles many of the details of transferring data between two computers. The transport protocol is also not direct, but relies on a network-level protocol to achieve network access and to route data through the network to the destination system. At each level, the cooperating peer entities focus on what they need to communicate to each other.
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Protocols in a Simplified Architecture
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Let us trace a simple operation. Suppose that an application, associated with port 1 at computer A, wishes to send a message to another application, associated with port 2 at computer B. The application at A hands the message over to its transport layer with instructions to send it to port 2 on computer B. The transport layer hands the message over to the network access layer, which instructs the network to send the message to computer B. Note that the network need not be told the identity of the destination port. All that it needs to know is that the data are intended for computer B.
To control this operation, control information, as well as user data, must be transmitted, as suggested in Stallings DCC9e Figure 2.2. Let us say that the sending application generates a block of data and passes this to the transport layer. The transport layer may break this block into two smaller pieces for convenience, as discussed subsequently. To each of these pieces the transport layer appends a transport header, containing protocol control information. The addition of control information to data is referred to as encapsulation.
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Addressing
Two levels of addressing are needed:
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For successful communication, every entity in the overall system must have a unique address. In our three-layer model, two levels of addressing are needed. Each computer on the network has a unique network address; this allows the network to deliver data to the proper computer. Each application on a computer has an address that is unique within that computer; this allows the transport layer to support multiple applications at each computer. These latter addresses are known as service access points (SAPs), or ports, connoting the fact that each application is individually accessing the services of the transport layer.
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each computer on the network has a unique network address
each application has an address that is unique with that computer (SAPs)
Protocol Data Unit (PDU)
the combination of data and control information is a protocol data unit (PDU)
typically control information is contained in a PDU header
control information is used by the peer transport protocol at computer B
headers may include:
source port, destination port, sequence number, and error-detection code
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The combination of data from the next higher layer and control information is known as a protocol data unit (PDU). Typically, the control information is contained in a PDU header; some data link layer PDUs include a trailer as well. In this case, the PDU is referred to as a transport PDU. Transport PDUs are typically called segments. The header in each segment contains control information to be used by the peer transport protocol at computer B. Examples of items that may be stored in this header include the following:
Source port: This indicates that application that sent the data.
Destination port: When the destination transport layer receives the segment, it must know to which application the data are to be delivered.
Sequence number: Because the transport protocol is sending a sequence of segments, it numbers them sequentially so that if they arrive out of order, the destination transport entity may reorder them.
Error-detection code: The sending transport entity may include a code that is a function of the contents of the segment. The receiving transport protocol performs the same calculation and compares the result with the incoming code. A discrepancy results if there has been some error in transmission. In that case, the receiver can discard the segment and take corrective action. This code is also referred to as a checksum or frame check sequence.
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Network Access Protocol
after receiving segment from transport layer, the network access protocol must request transmission over the network
the network access protocol creates a network access PDU (packet) with control information
header includes:
source computer address
destination computer address
facilities requests
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The next step is for the transport layer to hand each segment over to the network layer, with instructions to transmit it to the destination computer. To satisfy this request, the network access protocol must present the data to the network with a request for transmission. As before, this operation requires the use of control information. In this case, the network access protocol appends a network access header to the data it receives from the transport layer, creating a network access PDU, typically called a packet. Examples of the items that may be stored in the header include the following:
Source computer address: Indicates the source of this packet.
Destination computer address: The network must know to which computer on the network the data are to be delivered.
Facilities requests: The network access protocol might want the network to make use of certain facilities, such as priority.
Note that the transport header is not “visible” at the network access layer; the network access layer is not concerned with the contents of the transport segment.
The network accepts the network packet from A and delivers it to B. The network access module in B receives the packet, strips off the packet header, and transfers the enclosed transport segment to B’s transport layer module. The transport layer examines the segment header and, on the basis of the port field in the header, delivers the enclosed record to the appropriate application, in this case the file transfer module in B.
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TCP/IP
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The TCP/IP Layers
In general terms, computer communications can be said to involve three agents: applications, computers, and networks. Examples of applications include file transfer and electronic mail. The applications that we are concerned with here are distributed applications that involve the exchange of data between two computer systems. These applications, and others, execute on computers that can often support multiple simultaneous applications. Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another. Thus, the transfer of data from one application to another involves first getting the data to the computer in which the application resides and then getting the data to the intended application within the computer. With these concepts in mind, we can organize the communication task into five relatively independent layers (Stallings DCC9e Figure 2.3):
Physical layer
Network access layer
Internet layer
Host-to-host, or transport layer
Application layer
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Physical Layer
covers the physical interface between computer and network
concerned with issues like:
characteristics of transmission medium
nature of the signals
data rates
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The physical layer covers the physical interface between a data transmission device (e.g., workstation, computer) and a transmission medium or network. This layer is concerned with specifying the characteristics of the transmission medium, the nature of the signals, the data rate, and related matters.
Network Access Layer
covers the exchange of data between an end system and the network that it is attached to
concerned with issues like :
destination address provision
invoking specific services like priority
access to & routing data across a network for two end systems attached to the same network
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The network access layer is concerned with the exchange of data between an end system (server, workstation, etc.) and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching (e.g., ATM), LANs (e.g., Ethernet or WiFi), and others. Thus it makes sense to separate those functions having to do with network access into a separate layer. By doing this, the remainder of the communications software, above the network access layer, need not be concerned about the specifics of the network to be used. The same higher-layer software should function properly regardless of the particular network to which the computer is attached.
Internet Layer
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The network access layer is concerned with access to and routing data across a network for two end systems attached to the same network. In those cases where two devices are attached to different networks, procedures are needed to allow data to traverse multiple interconnected networks. This is the function of the internet layer. The Internet Protocol (IP) is used at this layer to provide the routing function across multiple networks. This protocol is implemented not only in the end systems but also in routers. A router is a processor that connects two networks and whose primary function is to relay data from one network to the other on its route from the source to the destination end system.
Host-to-Host (Transport) Layer
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Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. As we shall see, the mechanisms for providing reliability are essentially independent of the nature of the applications. Thus, it makes sense to collect those mechanisms in a common layer shared by all applications; this is referred to as the host-to-host layer, or transport layer. The Transmission Control Protocol (TCP) is the most commonly used protocol to provide this functionality.
Operation of TCP/IP
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Operation of TCP and IP
Stallings DCC9e Figure 2.4 indicates how these protocols are configured for communications. To make clear that the total communications facility may consist of multiple networks, the constituent networks are usually referred to as subnetworks. Some sort of network access protocol, such as the Ethernet or WiFi logic, is used to connect a computer to a subnetwork. This protocol enables the host to send data across the subnetwork to another host or, if the target host is on another subnetwork, to a router that will forward the data. IP is implemented in all of the end systems and the routers. It acts as a relay to move a block of data from one host, through one or more routers, to another host. TCP is implemented only in the end systems; it keeps track of the blocks of data to assure that all are delivered reliably to the appropriate application.
TCP/IP Address Requirements
Two levels of addressing are needed:
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For successful communication, every entity in the overall system must have a unique address. Actually, two levels of addressing are needed. Each host on a subnetwork must have a unique global internet address; this allows the data to be delivered to the proper host. Each process with a host must have an address that is unique within the host; this allows the host-to-host protocol (TCP) to deliver data to the proper process. These latter addresses are known as ports.
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each host on a subnetwork must have a unique global internet address
each process with a host must have an address (known as a port) that is unique within the host
Operation of TCP/IP
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Let us trace a simple operation. Suppose that a process, associated with port 3 at host A, wishes to send a message to another process, associated with port 2 at host B. The process at A hands the message down to TCP with instructions to send it to host B, port 2. TCP hands the message down to IP with instructions to send it to host B. Note that IP need not be told the identity of the destination port. All it needs to know is that the data are intended for host B. Next, IP hands the message down to the network access layer (e.g., Ethernet logic) with instructions to send it to router J (the first hop on the way to B).
To control this operation, control information as well as user data must be transmitted, as suggested in Stallings DCC9e Figure 2.5. Let us say that the sending process generates a block of data and passes this to TCP. TCP may break this block into smaller pieces to make it more manageable. To each of these pieces, TCP appends control information known as the TCP header, forming a TCP segment. The control information is to be used by the peer TCP protocol entity at host B. Examples of items in this header include:
Destination port: When the TCP entity at B receives the segment, it must know to whom the data are to be delivered.
Sequence number: TCP numbers the segments that it sends to a particular destination port sequentially, so that if they arrive out of order, the TCP entity at B can reorder them.
Checksum: The sending TCP includes a code that is a function of the contents of the remainder of the segment. The receiving TCP performs the same calculation and compares the result with the incoming code. A discrepancy results if there has been some error in transmission.
Next, TCP hands each segment over to IP, with instructions to transmit it to B. These segments must be transmitted across one or more subnetworks and relayed through one or more intermediate routers. This operation, too, requires the use of control information. Thus IP appends a header of control information to each segment to form an IP datagram. An example of an item stored in the IP header is the destination host address (in this example, B).
Finally, each IP datagram is presented to the network access layer for transmission across the first subnetwork in its journey to the destination. The network access layer appends its own header, creating a packet, or frame. The packet is transmitted across the subnetwork to router J. The packet header contains the information that the subnetwork needs to transfer the data across the subnetwork. Examples of items that may be contained in this header include:
Destination subnetwork address: The subnetwork must know to which attached device the packet is to be delivered.
Facilities requests: The network access protocol might request the use of certain subnetwork facilities, such as priority.
At router J, the packet header is stripped off and the IP header examined. On the basis of the destination address information in the IP header, the IP module in the router directs the datagram out across subnetwork 2 to B. To do this, the datagram is again augmented with a network access header.
When the data are received at B, the reverse process occurs. At each layer, the corresponding header is removed, and the remainder is passed on to the next higher layer, until the original user data are delivered to the destination process.
Transmission Control Protocol (TCP)
TCP is the transport layer protocol for most applications
TCP provides a reliable connection for transfer of data between applications
A TCP segment is the basic protocol unit
TCP tracks segments between entities for duration of each connection
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TCP and UDP
For most applications running as part of the TCP/IP protocol architecture, the transport layer protocol is TCP. TCP provides a reliable connection for the transfer of data between applications. A connection is simply a temporary logical association between two entities in different systems. A logical connection refers to a given pair of port values. For the duration of the connection each entity keeps track of TCP segments coming and going to the other entity, in order to regulate the flow of segments and to recover from lost or damaged segments.
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Stallings DCC9e Figure 2.6a shows the header format for TCP, which is a minimum of 20 octets, or 160 bits. The Source Port and Destination Port fields identify the applications at the source and destination systems that are using this connection. The Sequence Number, Acknowledgment Number, and Window fields provide flow control and error control. The checksum is a 16-bit frame check sequence used to detect errors in the TCP segment. Chapter 20 provides more details.
User Datagram Protocol (UDP) what is PDU then?
alternative to TCP
does not guarantee delivery, preservation of sequence, or protection against duplication(simpler)
adds port addressing capability to IP
used with Simple Network Management Protocol (SNMP)
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In addition to TCP, there is one other transport-level protocol that is in common use as part of the TCP/IP protocol suite: the User Datagram Protocol (UDP). UDP does not guarantee delivery, preservation of sequence, or protection against duplication. UDP enables a procedure to send messages to other procedures with a minimum of protocol mechanism. Some transaction-oriented applications make use of UDP; one example is SNMP (Simple Network Management Protocol), the standard network management protocol for TCP/IP networks
UDP Header
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Because it is connectionless, UDP has very little to do. Essentially, it adds a port addressing capability to IP. This is best seen by examining the UDP header, shown in Stallings DCC9e Figure 2.6b. UDP also includes a checksum to verify that no error occurs in the data; the use of the checksum is optional.
IPv6
Provides enhancements over existing IP
Designed to accommodate higher speeds and the mix of graphic and video data(multimedia)
Driving force was the need for more addresses due to growth of the Internet
IPv6 includes 128-bit source and destination address fields
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IP and IPv6
For decades, the keystone of the TCP/IP protocol architecture has been IPv4, generally referred to simply as IP. Stallings DCC9e Figure 2.7a shows the IP header format, which is a minimum of 20 octets, or 160 bits. The header, together with the segment from the transport layer, forms an IP-level PDU referred to as an IP datagram or an IP packet. The header includes 32-bit source and destination addresses. The Header Checksum field is used to detect errors in the header to avoid misdelivery. The Protocol field indicates which higher-layer protocol is using IP. The ID, Flags, and Fragment Offset fields are used in the fragmentation and reassembly process. Chapter 18 provides more detail.
In 1995, the Internet Engineering Task Force (IETF), which develops protocol standards for the Internet, issued a specification for a next-generation IP, known then as IPng. This specification was turned into a standard in 1996 known as IPv6. IPv6 provides a number of functional enhancements over the existing IP, designed to accommodate the higher speeds of today’s networks and the mix of data streams, including graphic and video, that are becoming more prevalent. But the driving force behind the development of the new protocol was the need for more addresses. IPv4 uses a 32-bit address to specify a source or destination. With the explosive growth of the Internet and of private networks attached to the Internet, this address length became insufficient to accommodate all systems needing addresses
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IPv6 Header
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As Stallings DCC9e Figure 2.7b shows, IPv6 includes 128-bit source and destination address fields.
Ultimately, all installations using TCP/IP are expected to migrate from the current IP to IPv6, but this process will take many years, if not decades.
TCP/IP Protocols
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Protocol Interfaces
Each layer in the TCP/IP protocol suite interacts with its immediate adjacent layers. At the source, the application layer makes use of the services of the end-to-end layer and provides data down to that layer. A similar relationship exists at the interface between the transport and internet layers and at the interface of the internet and network access layers. At the destination, each layer delivers data up to the next higher layer.
This use of each individual layer is not required by the architecture. As Stallings DCC9e Figure 2.8 suggests, it is possible to develop applications that directly invoke the services of any one of the layers. Most applications require a reliable end-to-end protocol and thus make use of TCP. Some special-purpose applications do not need the services of TCP. Some of these applications, such as the Simple Network Management Protocol (SNMP), use an alternative end-to-end protocol known as the User Datagram Protocol (UDP); others may make use of IP directly. Applications that do not involve internetworking and that do not need TCP have been developed to invoke the network access layer directly.
Standardized Protocol Architectures
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A protocol architecture, such as the TCP/IP architecture or OSI, provides a framework for standardization. Within the model, one or more protocol standards can be developed at each layer. The model defines in general terms the functions to be performed at that layer and facilitates the standards-making process in two ways:
Because the functions of each layer are well defined, standards can be developed independently and simultaneously for each layer. This speeds up the standards-making process.
Because the boundaries between layers are well defined, changes in standards in one layer need not affect already existing software in another layer. This makes it easier to introduce new standards.
Stallings DCC9e Figure 2.9 illustrates the use of a protocol architecture as such a framework. The overall communications function is decomposed into a number distinct layers. That is, the overall function is broken up into a number of modules, making the interfaces between modules as simple as possible. In addition, the design principle of information hiding is used: Lower layers are concerned with greater levels of detail; upper layers are independent of these details. Each layer provides services to the next higher layer and implements a protocol to the peer layer in other systems.
OSI Standardization
framework for standardization was motivator
lower layers are concerned with greater levels of details
each layer provides services to the next higher layer
three key elements:
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In addition, the design principle of information hiding is used: Lower layers are concerned with greater levels of detail; upper layers are independent of these details. Each layer provides services to the next higher layer and implements a protocol to the peer layer in other systems.
Three elements are key:
Protocol specification: Two entities at the same layer in different systems cooperate and interact by means of a protocol. Because two different open systems are involved, the protocol must be specified precisely. This includes the format of the protocol data units exchanged, the semantics of all fields, and the allowable sequence of PDUs.
Service definition: In addition to the protocol or protocols that operate at a given layer, standards are needed for the services that each layer provides to the next higher layer. Typically, the definition of services is equivalent to a functional description that defines what services are provided, but not how the services are to be provided.
Addressing: Each layer provides services to entities at the next higher layer. These entities are referenced by means of a port, or service access point (SAP). Thus, a network service access point (NSAP) indicates a transport entity that is a user of the network service.
The need to provide a precise protocol specification for open systems is self-evident. The other two items listed warrant further comment. With respect to service definitions, the motivation for providing only a functional definition is as follows. First, the interaction between two adjacent layers takes place within the confines of a single open system and is not the concern of any other open system. Thus, as long as peer layers in different systems provide the same services to their next higher layers, the details of how the services are provided may differ from one system to another without loss of interoperability. Second, it will usually be the case that adjacent layers are implemented on the same processor. In that case, we would like to leave the system programmer free to exploit the hardware and operating system to provide an interface that is as efficient as possible.
With respect to addressing, the use of an address mechanism at each layer, implemented as a service access point, allows each layer to multiplex multiple users from the next higher layer. Multiplexing may not occur at each layer, but the model allows for that possibility.
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Protocol specification
Service definition
Addressing
Multimedia Terminology
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With the increasing availability of broadband access to the Internet has come an increased interest in Web-based and Internet-based multimedia applications. The terms multimedia and multimedia applications are used rather loosely in the literature and in commercial publications, and no single definition of the term multimedia has been agreed (e.g., [JAIN94], [GRIM91], [PURC98], [PACK99]). For our purposes, the definitions in Stallings DCC9e Table 2.2 provide a starting point.
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Multimedia Terminology
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Media Types
Typically, the term multimedia refers to four distinct types of media: text, audio, graphics, and video.
From a communications perspective, the term text is self-explanatory, referring to information that can be entered via a keyboard and is directly readable and printable. Text messaging, instant messaging, and text (non-html) e-mail are common examples, as are chat rooms and message boards. However, the term often is used in the broader sense of data that can be stored in files and databases and that does not fit into the other three categories. For example, an organization’s database my contain files of numerical data, in which the data are stored in a more compact form than printable characters.
The term audio generally encompasses two different ranges of sound. Voice, or speech, refers to sounds that are produced by the human speech mechanism. Generally, a modest bandwidth (under 4 kHz) is required to transmit voice. Telephony and related applications (e.g., voice mail, audio teleconferencing, telemarketing) are the most common traditional applications of voice communications technology. A broader frequency spectrum is needed to support music applications, including the download of music files.
The image service supports the communication of individual pictures, charts, or drawings. Image-based applications include facsimile, computer-aided design (CAD), publishing, and medical imaging. Images can be represented in a vector graphics format, such as is used in drawing programs and PDF files. In a raster graphics format, an image is represented as a two-dimensional array of spots, called pixels. The compressed JPG format is derived from a raster graphics format.
The video service carries sequences of pictures in time. In essence, video makes use of a sequence of raster-scan images.
Domains of Multimedia Systems and Example Applications
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The Internet, until recently, has been dominated by information retrieval applications, e-mail, and file transfer, plus Web interfaces that emphasized text and images. Increasingly, the Internet is being used for multimedia applications that involve massive amounts of data for visualization and support of real-time interactivity. Streaming audio and video are perhaps the best known of such applications. An example of an interactive application is a virtual training environment involving distributed simulations and real-time user interaction [VIN98]. Some other examples are shown in Stallings DCC9e Table 2.3.
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Summary
needs and key elements for protocol architecture
TCP/IP protocol architecture
OSI Model & protocol architecture standardization
traditional versus multimedia application needs
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Stallings DCC9e Chapter 2 summary.
P.2.7 page 56
Q: A TCP segment consisting of 1500 bits of data and 160 bits of header is sent to the IP layer, which appends another 160 bits of header. This then transmitted through two networks, each of which uses a 24-bit packet header. The destination network has a maximum packet size of 800 bits. How many bits, including headers, are delivered to the network layer protocol at the destination?
HW #1
Review Questions (page 55)
2.4, 2.6, 2.8
How many layers does a router have? What are they?
Which layer is TCP at? Then how about IP?
2.7 (review)
| Domain | Example Application |
| Information management | Hypermedia, multimedia-capable databases, content-based retrieval |
| Entertainment | Computer games, digital video, audio (MP3) |
| Telecommunication | Videoconferencing, shared workspaces, virtual communities |
| Information publishing/delivery | Online training, electronic books, streaming media |