Superfast Broadband - Evidence - Parliament

NICC Ethernet Working Group – written evidence
Both GEA and ALA define multicast support. However they differ in how they implement it
in the customer premises and this has a significant impact on the support of IPoE for GEA
customers.
ALA for the 1:1 VLAN model defines a specific multicast VLAN at the customer premises. If
a customer wishes to use the network operator’s multicast services (e.g. linear TV) then
they send the channel join requests with a VLAN tag corresponding to the multicast VLAN
id.
For normal broadband traffic they must use another VLAN tag corresponding to the point
to point ALA User Connection that they are using for broadband. The ALA tagging at the
customer premises allows this traffic to be untagged and to place this untagged traffic into a
default VLAN for the customer premises if so required. This is defined in ND1030 as the Stagged
UNI.
Because traffic for the multicast AUC (upstream only IGMP is significant) has a separate
VLAN the ALA service determines whether an IGMP channel join request is addressed to
the multicast service by inspecting the VLAN tag. IGMP sent to the multicast AUC will result
in channels being added or dropped by the access network. IGMP traffic injected into the
broadband AUC will not be used to trigger a channel change because it is not within the
multicast AUC (it has the wrong VLAN tag).
GEA uses a single untagged VLAN in the home for all traffic, broadband or multicast; this
violates ALA. (for the 1:1 VLAN model). When this traffic enters the BT network at the
NTE then the NTE tags it based on the ingress port but classifies the traffic as being for the
multicast service simply by determining if it is IGMP. This means that if the customer sends
an IGMP join into the network then the fact it is IGMP results in the NTE forwarding it into
the multicast VLAN and this will result in a channel being added or dropped.
However services other than linear TV may also use IGMP, for example I might have a
broadband service that also uses IGMP to join and leave channels. These channels are
located back in the ISP domain and are nothing to do with the BT GEA service but it is
unable to determine this and will interpret all IGMP traffic as being for the BT multicast
service.
To get round this GEA requires that the customer uses IPoE encapsulation for the BT
multicast service (standard), but they MUST use PPPoE for their broadband service, thus
hiding any IGMP for broadband within a PPP wrapper and rendering it invisible to the NTE.
Since ALA/GEA are layer 2 Ethernet transport services this mandating of the PPP protocol
for broadband is incongruous because a layer 2 transport service is determining the higher
layer protocol for the consumer of the service.
ALA would allow IPoE encapsulation to be used for broadband and for the multicast service,
if the ISP wished to use PPP for their broadband then that would be entirely their choice.
Mandating PPP requires that the ISP must have a relatively specialised routing platform to
terminate it.
Note: GEA does seem to support IPoE for broadband but not (at the moment) if you also
take the multicast service.
516