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ViewsLetter on Provisioning
29 Feb
2004
#36
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A fortnightly look at provisioning automation--chips to business layer.
CAN ETHERNET OVER PASSIVE OPTICAL NETWORKS
BRING PLUG & PLAY TO THE LOCAL LOOP?
--by Vladimir Kaminsky, Contributing Editor
Ethernet is the most popular technology for Local Area Networks in
large part because it usually runs the first time a device plugs
in. Why? All modern Ethernet interfaces auto-configure
among very few choices, share an almost universal packet format, and
support the same protocols.
That same easy provisioning is a goal for the effort to standardize
Ethernet over PONs for the first/last mile between customer premises
and a carrier network. Potentially, ePON is plug-and-play
broadband.
As described in the last issue, PON has a tree or star topology, not
the local bus form of the original Ethernet. As a result, the
CSMA/CD protocol developed for local bus topologies is not suited to
PONs and must be altered in certain parts.
Downstream, PON traffic can take place in broadcast mode, almost
identical to the method on a Bus-style LAN. All stations receive
every packet, but they discard packets that don't have the local
address in the designated field. A different multiple-access
scheme is necessary for the upstream traffic. IEEE chose
statistical multiplexing with adaptable time slots.
Speeds anticipated for EPON are 100 Mbit/s to 1 Gbit/s. Working
in broadcast mode, this bandwidth is necessarily shared among all users
on the PON tree. Shielding users from one another, for privacy,
can rely on the VLAN protocol header (802.1q)--routers and switches
configured for VLANs will restrict packets with a specific VLAN address
to physical or logical ports configured to be part of that VLAN.
The present VLAN standard has a relatively small field for the VLAN
address, allowing maximum of 4096 VLANs per network. Public
carrier networks require larger numbers of VLANs. Under project
number P802.1X/D10, the IEEE 802.1 committee is working on a standard
for 'Port-based Network Access Control' to provide authentication and
authorization of individual users on an Ethernet. ANSI/ASC
T1/E1.4 committee and ETSI are working on the problem too.
Another solution in the form of ATM over PON had an earlier
start. APON standardization began in June 1995 in the FSAN
project (Full Service Access Networks) initially set up by 7
operators. Today FSAN encompasses more than 20 operators
worldwide (including BellSouth, NTT and KPN). The ITU-T (Study
Group 15) continued specifications standardization, resulting in the
G.983.1 recommendation, "Broadband Optical Access systems based on
Passive Optical Networks (PON)," which was published in November
1998. Later came the extensions G.983.2 (06/99) on "ONT
management and control interface specifications for ATM PON" and G983.3
(02/2001) on a new channel division for supplemental services.
There are FSAN-workgroups on Optical Access Networks (OAN), Operations
and Maintenance (OAM), Service Capabilities, and xDSL (several flavors
of DSL are based on ATM transmission).
The G.983 APON standard starts with a single mode fiber (G.652, for
example) over a maximum span of 20 km between OLT and ONU/ONTs.
Both single fiber (the most popular variant) and dual fiber PONs are
possible. In G.983.3 (latest version) downstream video traffic
uses 1550 nm wavelength; interactive traffic (telephony,
Internet) are on 1310 nm upstream and 1490 nm downstream. The
topology of signal splitters is defined with a maximum of 64 ONUs(32 to
48 in practice) per fiber tree.
The speeds are 155 Mbit/s or 622 Mbit/s downstream and 155 Mbit/s
upstream. The ATM cells of the downstream traffic are broadcast
and arrive at (and are recognized by) the proper ONU through VPI/VCI
addressing (virtual path identifier, virtual channel identifier).
That is, the VPI/VCI can offer separation among end users similar to
that of VLANs, but potentially stronger if the carrier manages the
router or switch that terminates the fiber on customer premises.
For upstream traffic, contention exists among those 32-64 ONUs (as they
need to send simultaneously). The OLT manages contention for the
fiber tree via a TDMA protocol (a grant mechanism not unlike SDLC
polling). The OLT at the central office end has a burst-mode
receiver to which the ONUs take turns in transmitting. The
mechanism that grants permission to send upstream produces statistical
multiplexing and concentration of ONU traffic. A distance ranging
protocol adjusts for the differences in distance from the OLT to the
various ONUs. Data payloads are encrypted with a separate key for
each ONU. Dynamic Bandwidth Allocation for interactive channels
is being developed separately.
GPON operates similar to APON, but at higher (1.25 Gbit/s - 2.5 Gbit/s)
line-rates. It also has a number of enhancement techniques,
including Forward Error Correction.
Another form of traffic separation works even better--wavelength
multiplexing. Depending on the level of complexity the carrier
wants to support in the PON, it's possible to filter out wavelengths
from portions of a fiber tree. That way, privacy is ensured
without relying on stations to ignore packets not addressed to them.
Wavelength Division Multiplexing (WDM) in PONs potentially brings huge
bandwidth capacity--up to hundreds of wavelengths per
fiber. Designing WDM over PON is a topic for another time.
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THE COMMERCIAL
-- Visit www.flanagan-consulting.com when you need independent review
and verification of network architecture, RFPs, or proposals. We also
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"Flanagan Consulting" and "ViewsLetter" are
Service Marks of W. A. Flanagan, Inc.
Updated: 17 July 2004 2003
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