RE: [EFM] RE: [EFM-P2MP] Point-to-Point plus Shared Media
May I draw your attention to the way in which P2P Ethernet Access systems are commonly built today, and how the issue of 'single copy broadcast' is handled in these. I apologize for the large scope & length of this message; hopefully most of it is of some relevance to all of us.
Broadcast content (typically TV programming) is handled as Multicast, since not all end users (e.g. households) are expected to request the same content (same TV channel(s)) at any given time.
Currently, the recognized & wide spread way of realizing this functionality in the access network is by L3 IP multicast, and more specifically IGMP. Personally, I would be more comfortable with doing this based on L2 Ethernet multicast instead, but that's another story.
[Application note: These TV channels are typically IP encapsulated MPEG2, and people have told me this requires -- for high quality playback on 28"-40" screens -- up to 6 Mbps for NTSC, and up to 8 Mbps for PAL & SECAM.]
With Multicast Group Management (join & leave messages captured by nodes, multicast tree pruned & grafted), a P2P access network with [physical and logical] tree topology becomes very efficient.
Consider an example with both broadcast/multicast content and VOD/PPV type unicast content, and a first level aggregation switch with 120 user ports [households]. Use 2*GbE for the uplink to second level aggregation.
Assume there are 100 broadcast channels, and that 20% of these are being watched by 80% of the households. For simplicity of the example, assume that all of the remaining 20% (24 households) at the same time watch their own VOD type content.
. 20 channels of 8 Mbps multicast = 160 Mbps
. 20 channels of 8 Mbps unicast = 160 Mbps
The 2GbE uplink is loaded with 0.32 Gbps, not much. Still, this scheme allows for each and every household to simultaneously watch several of the 20 most popular broadcast channels.
-- Try stressing the network: Add more VOD, say _each_ household also has their own VOD content, meaning another 80 channels = 640 Mbps added. In this case the uplink is loaded to some 50% _downstream_; there is still some 1 Gbps of capacity left for other downstream traffic.
> -----Original Message-----
> From: John Limb [mailto:limb@xxxxxxxxxxxx]
> Sent: Tuesday, November 20, 2001 7:47 PM
> To: 'John Pickens'
> Cc: 'Norman Finn'; firstname.lastname@example.org;
> Subject: [EFM] RE: [EFM-P2MP] Point-to-Point plus Shared Media
> You raise an interesting and important challenge.
> However, if a system only
> supports one view then I think one could make a case that the
> single copy
> broadcast view is more important than the point-to-point
> view, at least for
> the residential market. The reasoning is as follows: let us
> say that we are
> able to do a 64 way split in a year or two. With a
> point-to-point service
> (not emmulation) we would be limited to providing a maximum
> of 1000/64 = 16
> Mb/s of broadcast capacity before we would run out of
> bandwidth. It would be
> even less in practice as we would need to leave some capacity
> open for data
> and administration. 16 Mb/s is not even enough to broadcast
> one HDTV channel
> and no capacity for NVOD. If we ever get to a split of 128
> the situation
> gets worse.
> Am I right in assuming that the worst inefficiency that
> would happen by
> reflecting all upstream traffic on the downstream is 50%?
> This seems like a
> much smaller penalty to make than the drastic limitation that
> would occur
> with not permiting true broadcast.
> Of course we could allocate a separate wavelength for
> video broadcast, but
> that would be a severe restriction on how service could be
> deployed. Even
> then, there are data broadcast services that would very
> rapidly eat into
> downstream channel capacity.
> John Limb
> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org]On Behalf Of John
> Sent: Tuesday, November 20, 2001 12:49 PM
> To: Norman Finn; email@example.com; firstname.lastname@example.org
> Subject: Re: [EFM-P2MP] Point-to-Point plus Shared Media
> Good clarification.
> I would like to study one additional question related to this topic.
> How can an operator offer the benefits (in the EPON link
> segment) of both
> point to point AND point to multipoint to a single endpoint
> beyond the ONU
> (e.g. personal computer concurrently a. viewing a 20Mbps HDTV
> video and b.
> engaging in a 400Kbps point to point instant messenger
> video/audio session)
> and also maintain the link efficiencies gained by point to point.
> It is certainly possible to maintain separate networks to the
> end point -
> separate MAC in ONU, separate 100BT port in the ONU, separate ethernet
> LANs, and separate NICs in the personal computer (even
> better, separate
> personal computers). What is less clear is how to converge
> the networks -
> and configure the networks (PC, LAN, ONU, OLT) so that the
> "right" traffic
> traverses the "right" path (instant messenger traverses point
> to point;
> HDTV traverses shared media).
> It is also possible to limit the options here and say that an
> ONU can be
> either shared only or point to point only. And to say that if
> single-copy-broadcast attribute of the media needs to be
> accessed, that it
> is acceptable to operate in shared mode (up to 50% reduction in link
> capacity if all ONUs require single-copy-broadcast).
> I know there is a contingent within the working group that does not
> consider it a requirement to access the single-copy-broadcast
> attribute of
> the media, so probably we should poll this question at some point.