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T. M. Rajkumar and Amitava Haldar
A host of technologies are emerging to help organizations support the enterprisewide delivery of new multimedia applications. Some of these technologies work well at the workgroup or local area network level, and others work at the backbone or wide area network level. This chapter reviews many of the technologies in these two groups and evaluates their suitability for distributing multimedia information.
Multimedia applications involving databases, the World Wide Web and desktop videoconferencing challenge IS organizations to implement networking technologies that support a wide variety of information forms. Multimedia applications place different demands on the network than do traditionally supported discrete media (such as text and data). They require support for continuous, delay-sensitive media such as audio and video that need connection-oriented links. Because most networks today support connectionless links, they are poorly suited to support multimedia data.
A host of technologies are emerging to support enterprise multimedia applications. Some of them work well at the LAN or workgroup level, and others work at the WAN or backbone level. This chapter discusses the multimedia networking technologies within these two groups and their suitability for distributing multimedia information.
Because traditional LAN technologies such as Ethernet and Token Ring are designed to process data packets and operate in contentious mode, they cannot meet the demands placed on the LAN by multimedia applications. Several technologies have been proposed to support multimedia on the LAN, including asynchronous Ethernet, FDDI, Fast Ethernet, and ATM.
In contrast to packet-switched technologies, asynchronous transmission guarantees timely delivery of information, avoiding delay and jitter. The premise behind asynchronous Ethernet is that voice (not audio or video) is the critical component in multimedia communications.1 Hence, this technology is most important at locations where videoconferencing is the primary multimedia application.
To ensure that voice receives priority, a special asynchronous ISDN 6.144M-bps line is added to the standard Ethernet technology. This additional bandwidth is sufficient for a multipoint videoconference with six participants, each using 384K bps and additional bandwidth for such ancillary functions as white boarding. In addition, asynchronous Ethernet is easy to add to existing Ethernet networks, because all that is required is an asynchronous Ethernet hub and cards for the computers involved in videoconferencing. On segments where such asynchronous capability is not required, it need not be added.2
Suitability for Multimedia Traffic. Asynchronous Ethernet is a shared-media approach with limited multicasting support for audio only. It is not suited for full-motion video (i.e., the Moving Picture Experts Group or Motion Picture Experts Group standard), but it supports H.261 video (the teleconferencing standard). It provides truly asynchronous support for voice.3 From a business perspective, asynchronous Ethernet is only suitable as a small workgroup solution and where multimedia needs are not great. The technology should be viewed as a transitionary step to ATM technology.
The IEEE 802.3 or 100Base-T is rapidly becoming the technology of choice for providing bandwidth to users because it uses the same CSMA/CD technique as the widely used 10Base-T Ethernet. The high-speed 100Base-T offers 100M-bps performance for a small increase in cost for network adapters and hubs. In addition, it allows existing 10M-bps cards to share the same network, providing for easy transition.
This Fast Ethernet technology has limitations however. It needs a repeater every 250 meters as opposed to the every 2,500 meters required by 10Base-T systems. Because the regular ISA bus architecture is not fast enough to handle the 100M bps of bandwidth 100Base-T provides, the technology needs PCs with PCI buses.
100Base-T supports three different signaling schemes that require specific types of repeaters. It may require four wires as opposed to the standard UTP wiring existing in some organizations. The availability of mitigating technologies minimizes these problems.
100Base-Tx also has a new duplexing feature, allowing it to boost network speed to 200M bps. In general, the network adapter must always be listening on the receiving pair of wires to check for a collision. When Ethernet switching is used, a dedicated 100 Mbps is available to the adapter, avoiding the need to check for collisions. In this way, the UTP wiring can be used and 200M-bps bandwidth is available. This option is typically used for the server and not for each desktop.
Suitability for Multimedia Traffic. Ideally, 100Base-T is a workgroup solution, because the 100M-bps bandwidth is shared among the different users. It relies on appropriate use by every user station, and it cannot make any delay guarantees or provide QOS considerations. This high-speed Ethernet technology is useful because it provides backward compatibility with existing 10Base-T networks, making it easy to configure, install, and manage. In addition, a new set of gigabit Ethernet standards expected by mid-1997 should allow groups employing the technology to use increased bandwidth in the near future.
A second alternative high-speed Ethernet solution is the 100VG-AnyLAN option. This technique does not use the standard CSMA/CD access technique, but rather a new DPA technique. DPA enables the system to assign priorities and ensures on-time delivery of multimedia information.
100VG-AnyLAN also depends on more intelligence in the hub. Under DPA, a node wishing to transmit sends a request-to-send to the hub. If the message is a priority message, a special bit stream sent over two pairs of wiring indicates this to the hub. The hub continuously scans and grants the requests based on their priority. The hub also sends a notice to the receiving node that it is about to receive a message. It then routes the packet from the sending node to the receiving node or nodes. Implementing the priority 100VG-AnyLAN thus requires use of all four wires, which can be costly to implement. The primary advantage of 100VG-AnyLAN over 100Base-T4 is that it also supports Token Ring technology.
Suitability for Multimedia Traffic. 100VG-AnyLAN supports multimedia better than Ethernet, because it assigns priority to multimedia traffic and supports multicasting. It is particularly more suited than 100Base-T, because with a small number of workstations (i.e., less than 30) it can support delays of less than 10 microseconds.4
Despite these advantages, 100VG-AnyLAN does not provide QOS guarantees or support for existing (i.e., Ethernet) infrastructure. From a business perspective, 100VG-AnyLAN is useful for organizations that have invested heavily in Token Ring systems. For these groups of organizations, 100VG-AnyLAN provides an efficient way of supporting multimedia at the workgroup level.
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