WHEN electronic security installers get
involved in networked security solutions they’re going to find themselves
facing a host of connectivity devices, the most common of which are repeaters,
especially if the network is employing Cat-5 to get around a relatively large
multi-story facility.
Repeaters are essential if a larger
network is going to operate effectively. These devices connect different
sections of the cable plant, receiving the signal and pumping it back up to
full strength to combat attenuation caused by cable impedance. This done, the
signal is then sent on its way. Think of repeaters as existing in the physical
layer of a network where they support network media comprising the cable plant.
Depending on how much you spend on
repeaters, there are 2 different ways to buffer a signal – the first being that
you can re-amplify it. This is an effective technique, especially if there are
not too many repeaters in the system. The trouble with amplification is that it
will also boost signal noise so that over time a re-amplified signal will
degrade. Baseband systems reamplify signals by kicking in power as the signal
passes through the repeater.
The best repeater amplification method
is signal regeneration. This is easier for digital systems because they’re
built around a digital code and signals can be completely recreated and then
re-transmitted fresh – as if they’d just left the originating NIC. Broadband
systems always regenerate signals.
Next come bridges, routers and brouters.
A bridge is a device that links network segments and represents an integral
component of both segments. When you think of network geography consider that a
NIC and the network device it connected together comprised a node, with a
number of local nodes connected to a hub. Once signals travel through the hub,
the next piece of hardware they’ll come to is the bridge. Imagine 2 groups of
nodes, each connecting to their own hub and network cable running from each hub
and into a bridge. The bridge operates on the network’s data link layer.
“When you think of network geography consider that a NIC and the network device it connected together comprised a node, with a number of local nodes connected to a hub. Once signals travel through the hub, the next piece of hardware they’ll come to is a bridge”
In terms of network geography, this
bridge is a component of both network segments – it has an address on both
parts of the network and all the packets of data sent from all the nodes
attached to both hubs will pass through the bridge. They’ll then be passed on
to other bridges responsible for different network segments.
Inside the bridge is a list of addresses
and the bridge uses this list to check whether an incoming packet is destined
for one of its nodes. If so, the packet is passed to the appropriate node NIC
for processing. Bear in mind that bridges aren’t the perfect answer on large
networks, though they work fine in smaller or carefully designed architectures.
The weakness of a bridge is that if the
address of a packet received is not on the network segments that bridge holds
addresses for, it will send out a network broadcast looking for the address of
the homeless packet. In a quiet network such an event is generally not an issue
but if there are many such packets loose in a network then mass signaling can
cause performance drops.
Routers are similar to bridges in some
ways and function by breaking a large network into segments. The beauty of a
router is that it can not only direct traffic to nodes on its own segments – it
has a full list of network addresses and is able to direct traffic to the right
location wherever that might be on the network.
In a large system, maintaining router
tables is virtually a full time job, as every router on the network needs to
have a fully updated version of all addresses on a LAN. These addresses will
include those related to DVRs and security management servers, though in many
cases these systems may be running on a local VPN between a select number of
machines.
A cool thing about routers is that the
larger a network and the more subnets linked together by routers, the more
paths there are allowing packets to get through to their destination should
there be traffic jams anywhere in the network. Routers are also designed to
establish the quickest way for a packet to get through a network.
Routers operate in the network layer of
a system and they have a logical network address allowing them to pass data
packets to NICs located outside their own network segment. An excellent feature
of routers is that they can not only connect to a primary Cat-5 network
segment. These devices are also able to link with other network media such as
coaxial cables if routable protocols, a bridge or a brouter are installed.
Routers pass information between
themselves using either router information protocol – that’s the RIP – or the
open shortest path first protocol (the OSPF). When RIP is employed the
connected routers pass their entire routing table between themselves, while
OSPF updates variations to the table only.
Meanwhile, brouters function at both the
data layer and physical layers of a network. For these devices, operation
depends on the protocol that reaches them across the network. Routable protocol
packets are routed, while non-routable packets are sent on their way using the
physical address of a node is a connected subnet – it works the same way as a
bridge.
Last of all the network connecting
devices is the gateway, which converts signals from completely unrelated
networks and allows communication between them. Such devices exist in the form
of software on a server and might translate communications between a Cat-5
PC-based network and UNIX-based operating system for instance.
