GE Security Legend IP, Руководство по установке

Страница: 21 / 82

Chapter 1
Introduction
13
Needed network switch features
Network switches have many associated features, but only a few affect the transmission of digital video and
need to be considered when designing a system. Those few are port speed, switching capacity, multicast
protocol, and IGMP snooping.
• Port speed is the data transmission rate of an input or output channel. Switches provide ports ranging
from Base-T with RJ-45 connectors for CAT-5/5E/6 cabling to fiber optic interconnects with SFP
module connectors for fiber optic cabling. Base-T ports are more common and typically less costly
than fiber optic ports. Copper gigabit Ethernet cable is typically acceptable for most cable runs. Fiber
optic cable, however, may be needed when signal integrity over long high-speed cable runs is an issue.
• Switching capacity is the bandwidth (transmission capacity) of the switch as opposed to the
bandwidth of the network cable. The higher the bandwidth, the faster the transmission speed will be.
Data packets may be lost, however, if network traffic between the switch’s ports reaches the
switch’s bandwidth. It is recommended that each port have a switch capacity of at least
10 Mbps each.
• Multicast protocol is a one-to-many transmission format. With multicast protocols, data is sent to
multiple recipients (that are specified) at the same time. This one-to-many format is similar to
broadcasting, except that in broadcasting the data is sent to everyone. Because multicasting sends data
once from a source through distributed pathways to the recipients, you can send larger volumes of data
using less bandwidth. Unicast systems send data in its entirety, as many times as there are recipients.
• IGMP snooping is a protocol used to manage membership in multicast groups. IGMP stands for
Internet Group Management Protocol. Multicast groups are used to replicate digital video to many
destinations from a single source without requiring retransmission from the source. IGMP snooping is
a special optional feature on most managed switches, as well as many higher-end unmanaged switches
that monitor traffic for IGMP messages and use the contents to send multicast packets to certain ports.
When IGMP is available, it generally must be turned on.
Qualified network switches
Since the network switch is a critical point of failure for a digital video network, we recommend that you use
the highest quality switch you can. High-end switch manufacturers include Cisco, Nortel, and HP. The switch
you use must support multicast protocols and must provide IGMP snooping.
Be aware that you may have to enable the IGMP snooping feature. Many switches are shipped with this feature
turned off by default. Refer to the switch’s documentation. If you do not turn on the IGMP snooping feature
before you use the switch, system errors may occur. Such errors can range from the entire system not working
to severe performance and packet errors as more devices are added to the system.
When evaluating switches, make sure that you verify all of the interdependencies and constraints that can be
expected with varying operating modes. Be aware that not all switch manufacturers publish complete
specifications and that you may have to discuss details with the manufacturer before making a purchase.
GE has qualified several D-link and Nortel switches as compatible with their IP products. These switches need
to have IGMP snooping turned on before they will operate properly. Refer to the switch’s documentation. You