Access Networks Based on: Optical Networks, a Practical Perspective (2 nd Edition) Chapter 11, by R.Ramaswami, K.N.Sivarajan
Access Network! The network feeding the metro (and core) networks by gathering data from the end users (us).! The two main contenders are cable companies (e.g., AT&T, Time-Warner) and phone companies (e.g., SBC, Verizon, AT&T, etc.)! This is due to the two existing network infrastructures:! Telephone (POTS) networks (DSL)! Cable TV networks (Cable modems)
Full Service Providers! Since existing cable infrastructure is available companies try to milk their network to provide with data services for high speed Internet access.! This requires upgrading those infrastructures (4kHz limit, unidirectional broadcast)! Services can be switched/broadcast, symmetrical/asymmetrical, unidirectional/bidirectional, and can provide with different bandwidths.
Access Network Architecture! Hubs (inside central office)! Remote Nodes (RN) (inside CO or in the field)! Network Interface Units (NIU) (at subscribers)
Broadcast vs. Switched! In the context of networks broadcast does not refer to whether all users get the same information but rather the topology of the access networks (recall broadcast and select networks).! Broadcast service can be supported on a broadcast or switched network and vice versa.! Broadcast network RNs broadcast data to NIUs (and the later may do the switching) (cable). Cheaper, since NIUs are identical, better for broadcast services.! Switched networks RNs do the switching to individual NIUs (POTS). More security, better for switched services, fault localization, intelligence is in the network.
Dedicated vs. Shared Bandwidth! The feeder network (between RNs and the hub) can share the available bandwidth (usually TDM) among RNs or assign dedicated bandwidth (usually FDM or WDM) to each of them.! Shared bandwidth takes more advantage of statistical multiplexing while making QoS provisioning harder, also, the NIUs must be able to process with the line-speed of the hub.
POTS Access Network! Uses twisted pairs (original BW:4kHz) but by removing explicit bandwidth limiters and using sophisticated modulation, BW can be greatly increased.
Cable (HFC) Access Network! Central office is called head end. Different TV channels are sub-carrier multiplexed over the fibre. An RN can serve 500-2000 homes using coaxial cable (thus hybrid fiber-coax (HFC)). 78 channels can be transmitted (50-550MHz each channel 6MHz). The 4-50MHz window can be used for upstream communications.
ISDN and DSL! The POTS network can be enhanced to carry Internet traffic. ISDN was the first (and not very successful) approach to higher bandwidths (114kbps).! Digital subscriber lines (loops) provides up to a couple Mbps bandwidth over the same structure). Yet bandwidth is inverse proportional to the distance between CO and NIU. Additionally bandwidth limiters have to be removed. Upstream bandwidth is also limited. (Many different standards (ADSL, HDSL, VDSL, etc.). Dedicated bandwidth!
Satellites?! Direct broadcast satellites are geosynchronous limited spatial bandwidth reuse.! Although the bandwidth is greater than that of cable, the number of users is much larger too.! Upstream traffic needs to go through a different access network (e.g., phone network)
Fixed Wireless Access! Seems to be a failed technology (right now).! Multichannel Multipoint Distribution Service (MMDS AT&T has failed), can support 33 users with 6MHz bandwidth, line-of-sight <55km.! Local Multipoint Distribution Service (LMDS), supports 1.3GHz bandwidth <5km line of sight (rain!).! Optical fiberless is emerging (not now) with speeds up to 1Gbps <500m.
Enhanced HFC! Subcarrier Modulated Fiber Coax Bus (SMFCB). Same architecture than that of a HFC system. The bandwidth on the fiber/cable is though increased.! For downlink the 550MHz-1GHz band can be used using 256QAM (7bits/Hz).! Additionally the number of homes/rn can be reduced from 500 to 50 by moving fiber closer to the users.! Typically a POS is used between HE and RN with both 1.5µm and 1.3µm lasers (to enhance capacity between HE and RN)! Most cable structures in the US are able to provide with enhanced HFC.! Users share the available down and upstream bandwidths
Fiber to the Curb (FTTC)
FTTC and other FTTx! Fiber is used from the CO to the ONUs (optical network units). The main difference between HFC and FTTC is that fiber is significantly closer to the subscriber.! Fiber can be as close as in the home (FTTH), where ONUs are NIUs.! Fiber can be as far as in the building or neighborhood (cabinet) (FTTB and FFTCab respectively).! The network from the CO to the ONU is typically a PON (PSON) (sometime with the star located in the CO.
FTTC! Another difference between HFC and FTTC is the fact that analog signals are not easily carried over fiber that is intended for digital communication.
Simplicity! Access networks must be simple (to operate and service) and inexpensive (today s optimization is tomorrow s bottleneck).! Switching should be avoided to reduce costs.! Passive networks are thus a good choice (additionally, no powering is needed). Passive Optical Networks (PONs)! In the ONU, simple (no temperature controlled) components should be used and most the sophistication should reside in the CO.! PONs are also somewhat future proof
Point-to-Point PON! Separate fiber pair for CO to each ONU.! The cost scales with the number of ONUs (bad).! Fiber pairs between CO and ONUs have to be installed (bad).! Provides with high bandwidth dedicated bandwidth (good)
Point-to-Point PON! Instead of a fiber pair, a bi-directional fiber can be used (wavelengths for different directions should be far apart).! Duplexing can be done in the time domain (TDD) or in the wavelength domain (frequency domain FDD).! (If several users are sharing the fiber pairs, then a SONET ring can be created on them, however then the architecture is not a PON anymore. PONs are more cost effective than SONET.)
TPON Telephony-PON! Broadcast architecture., upstream can be TDM, where clocks have to be synchronized (ranging).! The major cost is in the CO that is shared by many users. Transmitters are cheap (LEDs or Fabry-Perot). The number of ONUs is limited by the splitting.! ONUs have to receive at the aggregate bit-rate.
SONET vs. TPON! Upgrading bandwidth for users is easier in TPON (assignment of additional TDM timeslots) (although this is changing now).! A failure of an ONU does not affect the entire network like in SONET, yet SONET has built in protection.! Fiber cuts in TPON require doubling up the fiber plant.! Adding subscribers does not require any disturbance in the service to others with TPON.
ATM based TPON (APON, BPON) Full Service Network! CO-ONU distances of up to 20km (total att:30db)! 622Mbps downstream and 155Mbps upstream.! 16-32 way split (in order to keep the loss budget, more split=>less distance)! Single fiber (FDD with 1550nm (down) and 1300nm (up)). Yet it can operate on fiber pairs in the 1300nm range
WDM PON - WPON! The CO equipment is replaced by WDM transmitters (either an array of laser or tunable lasers).! ONUs run at their designated bit-rate.
WRPON (Wavelength Routed)! If splitter in WPON is replaced by wavelength routing (e.g., AWG), loss can be reduced.! Point-to-point dedicated services can be provided.
PPL - WRPON! Passive Photonics Loop (PPL) 16 channels for downstream in the 1300nm band and 16 channels upstream in the 1550nm band.! Each ONU requires two expensive lasers.! This is not necessarily a necessity.
RITENET - WRPON! Tunable laser at CO.! A transmission consists of two parts: data part (downstream) and return traffic part (upstream, where laser is not modulated in the CO). ONUs have external modulators only.
LARNET - WRPON! ONUs use LED instead of modulator.! CO can have a low cost LED transmitter too (e.g., for low cost broadcast for video).
EPON Ethernet PON
Ethernet for Access! IP is going to be the dominating protocol.! The Full Service Access Network (FSAN) is based on ATM technology which becomes obsolete (and is too expensive too).! Ethernet is a cheap (and well-known) technology (standard) well suited for IP traffic.! QoS provisioning can be achieved by prioritization (802.1p) or VLAN tagging (801.1Q).
EPON! Ethernet can be either shared (CSMA/CD) or switched. EPON is a combination of these two.! Downstream ethernet frames pass through a splitter (not WDM technology) and are received by the corresponding ONU.! Upstream is more tricky, since OANs do not see each other (no CD is available), thus more complicated MAC has to be provided.
EPON Upstream! CSMA/CD can be achieved by jam signal broadcasts from the CO, but due to propagation this would greatly reduce performance. (And QoS could not be provided).! Time sharing is preferred today. It does not only make QoS provisioning easier but also assures fairness to users, provides with predictable service and users do not get used to high upstream bandwidth in sub-peak hours.
EPON Upstream! Timeslots are allocated to each ONU in which they can transmit the accumulated bursts of ethernet traffic.! The scheduling can be controlled either centrally or can be distributed with the central upstream slot assignment being more attractive.
Security in EPONs! Ethernet does not have built in security.! ONUs do receive all packets destined to all other ONUs which makes eavesdropping and impersonation easy.! Encryption is needed to support privacy.! IP level encryption would leave the identities of terminals and traffic patterns yet to be available to the bad guys.! Physical level encryption may be a good pick, yet it would make the physical layer connection aware which has its drawbacks.
IEEE 802.3AH! Ethernet standardization (EPON standardization) for the first mile.! 802.3AH provides (will provide) specifications for EPON over copper, fiber (for both P2P and P2MP) and a common OAM.! Standards are anticipated in 2003 Fall.