21-05-2013, 03:37 PM
Backbone networks overview
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Backbone networks defined
The term backbone has a different meaning in each of the two environments in which it is
used.
Within the structured cabling environment, the term backbone refers to the cable used to
connect a main or intermediate cross-connect to a telecommunications closet. It also refers
to the cable used to connect the main cross-connect to the intermediate cross-connect.
Therefore, in this context, the term backbone refers to a physical portion of the structured
cabling system.
In networking environments, a backbone, or more specifically, a backbone network refers to
connection services provided to multiple individual LANs.
Organizational LANs
The first objective of local area networking was to integrate a small number of individual
desktop computers into a single network to permit these devices to communicate with each
other. This is sometimes referred to as workgroup computing.
The second step is to connect individual workgroup LANs so they are able to communicate
with each other. This is referred to as the organizational or branch LAN.
The next step is to connect the organizational LANs to form an enterprise network. An
enterprise network is considered to be a combination of Local Area Networking and Wide
Area Networking—featuring elements of both environments. Its purpose is to provide
communications and services across the organization.
The requirements for connecting two or more LANs based on the same technology differs
from those for connecting LANs based on different technologies.
The focus of this chapter will be on connecting LANs based on the same technology, that is,
connecting Ethernet to Ethernet or Token-ring to Token-ring. This interconnection of LANs
through a backbone network can be done using the same network technology—using an
Ethernet backbone to connect two or more Ethernet LANs. However, the interconnection of
LANs can also be accomplished using a different technology on the backbone—for example,
using an FDDI backbone to connect two or more Ethernet LANs.
Using backbone networks for centralized management
Backbone networks may also be used to implement a LAN environment that provides for
centralized management. In such an environment, all network servers are moved to one
central location. The servers are connected to each other via a high-speed network
backbone. The different LANs are also connected to this same backbone using bridge
technology—defined and discussed later. The network backbone allows the stations on the
LANs to communicate with the servers.
Centralizing the servers in this manner may provide for better management of the network
resources.
Wide-area backbones
Wide-area backbones are often used to provide for connections to systems outside the
facilities owned by an organization. That is, they are used to provide for WAN connections.
Wide-area backbones are different from dedicated leased lines which provide full-time
connections between two fixed points. Dedicated lines are usually justified only when there is
a need for steady, continuous traffic between the two points.
More recent services—such as Switched-56, Frame Relay, Switched Multimegabit Data
Service (SMDS) and ATM—provide ways to link LANs without requiring dedicated leased
lines. These newer services provide for bandwidth-on-demand and pay-for-use services.
Wide-area backbone networks provide connections that are high bandwidth, low-cost and
flexible. The newer technologies are capable of providing high bandwidth when needed. A
customer may contract for a specific amount of bandwidth, but can usually increase this
amount when necessary by paying an additional fee. Costs are lower because only the
services required at a given time are paid for. Dedicated lines may be more cost effective
when there is a great deal of traffic between two fixed points. Wide-area backbones are
considered to be more flexible, since they can provide for connections to be made to many
different locations.
Transmission media
It is recommended that the media to be used as a transmission channel for a local (i.e.,
non wide-area) backbone network follow the specifications made in the ANSI/TIA/EIA-
568-A standard. While a detailed discussion of this topic can be found in Chapter 2, a
summary of recognized cable types and acceptable distances is provided below.
Speed
The speed at which the backbone network operates is a function of the technology being
employed. However, it must be considered that backbone networks are often designed to
meet greater traffic requirements over longer distances than individual LAN environments.
For this reason, the technologies employed on the backbone network often use high-speed
networking.
It is not unusual to have the backbone network operating at 100 Mbps and greater,
regardless of the speed at which the connected LANs operate. With newer highperformance
technologies becoming more readily available, backbone network speeds are
expected to increase, possibly exponentially.
Collapsed backbone design issues
A collapsed backbone permits all bridging and routing functions to be located in a single
device. This device itself becomes the network backbone, reducing or even eliminating the
need for multiple bridges and routers for individual LANs. Instead of running a backbone
cable throughout the premises, a transmission channel is attached to a hub servicing each
LAN. At the other end, this transmission channel is attached to the backbone device.
A typical collapsed backbone will locate this central bridge/router device in a secure
environment in the building. In these cases when a two-level hierarchy is used, these
central devices can in turn be connected to a backbone branch or enterprise hub.
Many such hubs are built using a chassis with a high-speed backplane—the board which
allows the individual hub modules to communicate. These units are meant to be modular
and expandable. Expansion boards are inserted into the hub chassis to access the highspeed
bus of the backplane. Many of these modular backbone hubs accept Ethernet,
Token-ring, FDDI and WAN interface modules. As well, most feature optional diagnostic
and management modules
Types of routers
Originally, routers were used solely to provide LAN-to-WAN connectivity. Today, they also
provide LAN-to-LAN connectivity.
The classification of router types is based primarily on the environment in which the
routers will operate. There are two factors which must be considered when selecting a
router—the number and types of LAN and WAN access interfaces required and the
number and types of Network layer protocols to be supported.