CHAPTER 4
UTILITIES & FEATURES
Revised: 6 Sep 11
4-10
EST P/N AA107A
IGMP SNOOPING
Internet Group Management Protocol (IGMP) Snooping allows the ESTeem 195Ea to operate more efficiently in
networks with high Multicast (UDP, etc.) traffic. IGMP Snooping will define the destination for the Multicast traffic and
send the data to the correct wireless Ethernet modem, not the entire network. This limiting of Multicast traffic to
specific destinations greatly increases the overall network efficiency.
The problem with using multicast traffic over a wireless connection is that multicast packets do not require an
Acknowledge on the protocol layer. If the wireless network misses a data packet on the RF network, that multicast
data packet is lost.
Theory of Operation
There are two types of Ethernet packets on a network, unicast and multicast. Unicast is intended for exactly one
recipient (and ignored by all others). Multicast is intended to be received by multiple recipients. Interested parties can
listen for particular multicast packets, but most nodes ignore it. In any case, the network medium is still utilized no
matter how many nodes are listening. Broadcast packets are a special type of multicast traffic which all nodes always
receive. These are particularly useful for global announcements (Hey, I'm Alice!) and queries (Hey, I'm Alice and I'm
looking for Bob!). More relevantly, it is how hosts on a network find out each other's addresses, and are absolutely
crucial to the proper functioning of a network.
As the network grows physically, it encounters some growing pains. The first is more physical, relating to the cabling
limitations. Bridges solve this problem by joining two physical networks together so they appear to be a single large
network. Through use of bridges, a network can scale to hundreds or even thousands of hosts. There is a downside,
however. Well before the physical limitations of bridging hit, you start to run into efficiency problems, as all traffic has to
travel everywhere on the network. The raw carrying capacity of this shared medium, often referred to as bandwidth, is
the second scaling problem.
Smart bridges, or switches, help alleviate the bandwidth problem by only passing traffic across the bridge if the
destination host was on the other side. This greatly increases the capacity and efficiency of the network by allowing two
pairs of hosts communicating simultaneously on each side of the switched bridge. Only when the packet needs to go
to the other side is both mediums utilized simultaneously for the same packet.
Unfortunately, broadcast and multicast traffic by its very nature must always be relayed across the bridge. While the
number of network nodes may grow linearly, the multicast traffic tends to grow exponentially. This isn't generally a
problem for wired networks, as they have a considerable amount of bandwidth to spare, but wireless networks have, at
best, an order of magnitude less bandwidth to begin with. If a wireless network is bridged with a wired network, while
the absolute numbers are the same, a much higher proportion of the wireless network’s available bandwidth is used up
by multicast traffic. This effect is further magnified if multiple wireless repeaters are in use.
The preferred way of dealing with excess multicast traffic is to put the wireless network on a different sub-network
(subnet) and use a router to join it to the wired network. This ensures that only unicast traffic intended for the wireless
network crosses over, as multicast and broadcast traffic stays within its local subnet. Normally this is fine, but there are
times where you need multicast traffic to span subnets. For IP traffic, this can be obviated by using a multicast-capable
router, but for legacy multicast protocols that were not designed to be routed, another solution must be found.
There are two primary types of communication networks used in wireless Ethernet systems; fixed points
communicating to each other through an Ethernet Bridge and mobile devices communicating to these fixed points.
The fixed point Ethernet bridge communication in the 195Ea is through Repeater Peer links, while the mobile
communication is from the Client modes (EtherStation, Station Router and Station Masquerade) to the Access Points
(Fixed Locations) in the network. Each of these networks handles Multicast traffic differently as explained below.
Ethernet Bridge Mode (Repeater Peers)