ViewsLetter(SM) on Provisioning

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ViewsLetter on Provisioning        29 Feb 2004            #36
A fortnightly look at provisioning automation--chips to business layer.

        --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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 Updated: 17 July 2004 2003

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