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Broadband LANs

Broadband LANs are multichannel, analog LANs (Figure 9.5) typically based on coaxial cable as the transmission medium, although fiber optic cable is also used on occasion [9-2]. Individual channels offer bandwidth of 1 to 5 Mbps, with 20 to 30 channels typically supported. Aggregate bandwidth is as much as 500 MHz. The various channels are multiplexed onto the carrier through Frequency Division Multiplexing (FDM). Radio modems accomplish the digital-to-analog conversion process, providing the transmitting device access to an analog channel. The modems, which must be tuned and managed carefully, may be fixed-frequency or frequency-agile. Fixed-frequency modems are tuned to a specific frequency channel, while frequency-agile modems have the ability to search for an available channel. Although frequency-agile modems are more expensive to acquire and administer, they offer improved communications and bandwidth utilization. As the LAN is analog in nature, it can easily accommodate voice and video. Some broadband LANs are referred to as 10Broadband36, translated as 10 Mbps, Broadband (multichannel), with 3600 meters maximum separation between devices.


Figure 9.5  Broadband versus Baseband.

The characteristics of Broadband LANs, generally speaking, are not endearing. However, their unique properties do have application. As an example, a chain of theme parks uses Broadband LANs extensively to support paging (audio voice), closed circuit TV (video), and data. Becuase the LAN is analog, it supports audio and TV easily. Further, the application is static, rather than dynamic, as the paging zones and closed-circuit TV channels require fixed amounts of bandwidth, and the frequency channel assignments need be changed infrequently, if ever. Further, the locations of the terminal equipment (paging source and horns, and VCRs and TV monitors) are fixed, or seldom change.

Baseband LANs

Baseband LANs are single channel, supporting a single communication at a time (see Figure 9.5). They are digital in nature, varying the bit state through voltage on/off or light pulse on/off. Total bandwidth of 1 to 100 Mbps is provided over coax, UTP, STP, or fiber optic cable. Distance limitations depend on the medium employed and the specifics of the LAN protocol. Baseband LAN physical topologies include ring, bus, tree, and star.

Baseband LANs are by far the most popular and the most highly standardized. Ethernet, Token Passing, Token Ring and FDDI LANs are all baseband. They are intended only for data—data communications is, after all, the primary reason for the existence of LANs. Recently, however Ethernet, FDDI and other LANs have announced versions to support voice, video, and videoconferencing. While the support of such isochronous traffic offers clear advantages in support of workgroup communications, the LAN data traffic can be affected to a considerable extent.

Media Access Control

LAN access, at the physical and electrical level, is accomplished at the Network Interface Unit (NIU), or Network Interface Card (NIC). A NIU or NIC is at the board level, with the boards typically fitting into an expansion slot of an attached device (workstation or PC). Alternatively, multiple cards may be contained within a multiport device that supports multiple workstations on a pooled basis. Each NIU/NIC has a unique logical address for purposes of identification; the address is hard-coded on a silicon chip.

Media Access Control (MAC)

Media Access Control (MAC) describes the process which is employed to control the basis on which devices can access the shared network. Some level of control is required to ensure the ability of all devices to access the network within a reasonable period of time, thereby resulting in acceptable access and response times. It is also important that some method exist to either detect or avoid data collisions, which are caused by multiple transmissions being placed on the shared medium simultaneously. Media Access Control can be accomplished on either a centralized or decentralized basis, and can be characterized as either deterministic or nondeterministic in nature.

Centralized Control

A centralized controller polls devices to determine when access and transmission by each station is allowed to occur. Stations transmit when requested to do so, or when a station transmission request is acknowledged and granted. This process of polling requires the passing of control packets, adding overhead and reducing the amount of throughput relative to the raw bandwidth available. Additionally, the failure of the central controller will disrupt the entire network; in such an event, the controller is taken off-line and a back-up controller assumes responsibility. Centrally controlled networks generally employ deterministic access control; Token Ring and FDDI networks are centrally controlled.

Decentralized Control

Decentralized control is somewhat anarchistic, as each station assumes responsibility for controlling its access to the shared network. Additionally, each station must assume responsibility for detecting and resolving any data collisions which might occur in the likely event access and transmission overlap with other devices. Decentralized control networks generally use nondeterministic, or contentious, media access control. For instance, Ethernet LAN control is decentralized.


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