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INSTALLATIONCONFIGURATIONOPERATION

FURTHER

INFORMATION

INDEX

APPENDIX D - Glossary

Internet Group Management Protocol

Where an ALIF transmitter is required to stream video to

two or more receivers, multicasting is the method used.

Multicasting involves the delivery of identical data to

multiple receivers simultaneously without the need to

maintain individual links. When multicast data packets enter

a subnet, the natural reaction of the switches that bind

all the hosts together within the subnet, is to spread the

multicast data to all of their ports. This is referred to as

Multicast ooding and means that the hosts (or at least

their network interfaces) are required to process plenty of

data that they didn’t request. IGMP offers a partial solution.

The Internet Group Management Protocol (IGMP) is

designed to prevent multicast ooding by allowing Layer

3 switches to check whether host computers within

their care are interested in receiving particular multicast

transmissions. They can then direct multicast data only to

those points that require it and can shut off a multicast

stream if the subnet has no recipients.

There are currently three IGMP versions: 1, 2 and 3, with

each version building upon the capabilities of the previous

one:

• IGMPv1 allows host computers to opt into a multicast

transmission using a Join Group message, it is then

incumbent on the router to discover when they no

longer wish to receive; this is achieved by polling them

(see IGMP Querier below) until they no longer respond.

• IGMPv2 includes the means for hosts to opt out as well

as in, using a Leave Group message.

• IGMPv3 encompasses the abilities of versions 1 and 2

but also adds the ability for hosts to specify particular

sources of multicast data.

AdderLink Innity units make use of IGMPv2 when

performing multicasts to ensure that no unnecessary

congestion is caused.

IGMP Snooping

The IGMP messages are effective but only operate at

layer 2 - intended for routers to determine whether

multicast data should enter a subnet. A relatively recent

development has taken place within the switches that

glue together all of the hosts within each subnet: IGMP

Snooping. IGMP snooping means these layer 2 devices now

have the ability to take a peek at the IGMP messages. As a

result, the switches can then determine exactly which of

their own hosts have requested to receive a multicast –

and only pass on multicast data to those hosts.

IGMP Querier

When IGMP is used, each subnet requires one Layer 3

switch to act as a Querier. In this lead role, the switch

periodically sends out IGMP Query messages and in

response all hosts report which multicast streams they

wish to receive. The Querier device and all snooping Layer

2 switches, then update their lists accordingly (the lists are

also updated when Join Group and Leave Group (IGMPv2)

messages are received).

IGMP Fast-Leave (aka Immediate Leave)

When a device/host no longer wishes to receive a

multicast transmission, it can issue an IGMP Leave Group

message as mentioned above. This causes the switch to

issue an IGMP Group-Specic Query message on the port

(that the Leave Group was received on) to check no other

receivers exist on that connection that wish to remain a

part of the multicast. This process has a cost in terms of

switch processor activity and time.

Where ALIF units are connected directly to the switch

(with no other devices on the same port) then enabling

IGMP Fast-Leave mode means that switches can

immediately remove receivers without going through

a full checking procedure. Where multiple units are

regularly joining and leaving multicasts, this can speed up

performance considerably.

Jumbo frames (Jumbo packets)

Since its commercial introduction in 1980, the Ethernet

standard has been successfully extended and adapted to

keep pace with the ever improving capabilities of computer

systems. The achievable data rates, for instance, have risen

in ten-fold leaps from the original 10Mbit/s to a current

maximum of 100Gbit/s.

While data speeds have increased massively, the standard

dening the number of bytes (known as the Payload)

placed into each data packet has remained resolutely stuck

at its original level of 1500 bytes. This standard was set

during the original speed era (10Mbits/s) and offered the

best compromise at that speed between the time taken to

process each packet and the time required to resend faulty

packets due to transmission errors.

But now networks are much faster and les/data streams

are much larger; so time for a change? Unfortunately, a

wholesale change to the packet size is not straightforward

as it is a fundamental standard and changing it would mean

a loss of backward compatibility with older systems.

Larger payload options have been around for a while,

however, they have often been vendor specic and at

present they remain outside the ofcial standard. There

is, however, increased consensus on an optional ‘Jumbo’

payload size of 9000 bytes and this is fully supported by

the AdderLink Innity (ALIF) units.

Jumbo frames (or Jumbo packets) offer advantages for

ALIF units when transmitting certain high resolution video

signals across a network. This is because the increased data

in each packet reduces the number of packets that need to

be transferred and dealt with - thus reducing latency times.

The main problem is that for jumbo frames to be possible

on a network, all of the devices on the network must

support them.

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