Architectures and Alternatives for Broadband Access Networks
ADSL-based Access Network• DSL is asymmetric technology:
• 6.144 Mbps (downstream)
• 640 Kbps (upstream)
• Always-On broadband access
• PSTN is completely replaced by an IP network to offer integrated voice and data services.
• DSLAMs, installed in CO, could efficiently aggregate several hundreds of DSL connections:• Statistical multiplexing
• DSL does not require the deployment of a new network: it runs on the existing PSTN infrastructure.
Limited by the length of the line
ADSL-based Access Network
ADMAccess Server
MDF
FDF
ATU-C
ATU-C
ATU-C
Cooper pair
Cooper pair
Fiber cable ATU-C
ATU-C
ATU-C
DSLAM
DSLAMs MT
AA
DSL
M
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Res
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Gat
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Eth
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PO
TS
Hom
ePN
A
Central office
HomePNA T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p Ethernet
Cooper pair
Metro ring
Cable-based Access Network• Asymmetric technology:
• 30 Mbps (downstream)
• 1.3-5.1 Mbps (upstream)
• Always-On broadband access
• Cable TV network is upgraded to allow new services such as data over cable.
• Cable modems are installed at the customer premise; cable modem termination systems (CMTS) at the head-end.
• Data over cable relies on data channels shared by multiple users using statistical multiplexing.
• Requires power supplies to run deployed active elements, such as amplifiers and nodes.
Cable-based Access Network
ADM
MT
AA
DSL
M
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Res
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Gat
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Eth
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PO
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Hom
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A
Distribution Hub
HomePNA T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p EthernetMetro ring
TxRc
TxRc
RF source
CMTS
CMTS
Drop To TV Set Top Box
Fiber backbone
Tap DropAmplifier
Optical node
Router
Distribution(coaxial cable)
ADSL- vs. Cable- based access network
• HFC network provides relatively higher transmission bandwidth than DSL.
• Recent studies showed that HFC solution is relatively cheaper than DSL: due to the increased cost of DSLAMs. • However, DSLAM port costs are subject to significant cost reductions.
• DSL coverage area is limited.
ADSL- vs. Cable- based access network• HFC architecture has a great degree of equipment sharing:
• Pros:• Statistical multiplexing.
• Possibility to accommodate more users.
• Cons:• Security issues.
• Cost of providing service will increase with traffic growth and new bandwidth intensive applications.
• Efficiency drops substantially during peak hours.
• HFC architecture deploys active elements in the distribution network (DN): i.e., requires power supplies throughout the DN.
OLT
ONU
ONU
ONU
ONU
Fiber To The x in Access Systems
Internet
Telephone
Interactive Video
Optical fiber
Passive Optical splitter
FTTH
FTTB
FTTC
FTTCab
Service modes
DSL
Optical Access
Basic Architecture of PON
EPON Downstream
EPON Upstream
B-PON architectures• Deploying FTTH is expensive and its deployment is very limited.
• FFTB is a cost effective solution, its deployment is vital.
• FFTC and FTTCab are architectures that provide broadband services to customers where fiber optics is not feasible to be deployed:– Service is carried over a DSL access network that connects customers to CO
(where the ONU resides) through DSL connections.
• Thus, B-PON either alone or in conjunction with DSL provides a basis for the implementation of a full service access network (FSAN).
Principles of B-PON
20-25 km
Optical splitter/coupler
OLT
ONU
1.31m
1.55m
• One downstream/upstream channel.
• MAC arbitration mechanism is required to avoid collision between simultaneous transmissions in the upstream direction.
• Fixed-TDM is one possible solution.
• Dynamic Bandwidth Allocation is more suited for bursty traffic.
• Channel speed is 1Gbps.
• Data rate per ONU depends on the splitting ratio of the splitter (1:16, 64)
ATM-PON
• APON systems are based upon ATM as the bearer protocol.
• Downstream transmission is a continuous ATM stream at a bit rate of 155.52 Mb/s or 622.08 Mb/s with dedicated Physical Layer OAM (PLOAM) cells inserted into the data stream .
• Upstream transmission is in the form of bursts of ATM cells, with a 3 byte physical overhead appended to each 53 byte cell in order to allow for burst transmission and reception.
• APON provides a very rich and exhaustive set of OAM features, including BER monitoring, alarms and defects, auto-discovery and automatic ranging, churning as a security mechanism for downstream traffic encryption etc.
ATM-PON
ATM Cell 1
ATM Cell 27
ATM Cell 28
ATM Cell 54
PLOAM1
PLOAM2
ATM Cell 1
ATM Cell 2
ATM Cell 3
ATM Cell 53
Upstream frame format
Downstream frame format
3 bytes overhead per cell (guard time, preamble, delimiter)
Tframe = 56 cells of 53 bytes
Tframe = 53 cells per frame
PLOAM: Physical Layer Operation And Maintenance.
53 upstream grants
Ethernet-PON
• Ethernet for subscriber access networks combines a minimal set of extensions to the IEEE 802.3 Media Access Control (MAC) and MAC Control sub-layers with a family of Physical (PHY) Layers.
• MPCP (Multi-Point Control Protocol) is defined as a function within the MAC control sub-layer. MPCP uses messages, state machines, and timers, to control access to a P2MP topology. Each ONU in the P2MP topology contains an instance of the MPCP protocol, which communicates with an instance of MPCP in the OLT.
• A P2P Emulation Sub-layer makes an underlying P2MP network appear as a collection of point to point links to the higher protocol layers (at and above the MAC Client). It achieves this by pre-pending a Logical Link Identification (LLID) to the beginning of each packet, replacing two octets of the preamble.
• EPON uses variable Ethernet variable frames for transmission.
APON vs. EPON
APON EPON
Standard Body ITU-T/FSAN IEEE
Speed 155/622 Mbps 1Gbps
Protocol overhead for IP services
Large Small
Scalability Low High (up to 10Gbps)
Service Integration Good Good
Players ILECs CLECs
APON vs. EPON
(B-PON + DSL) architecture • A new network architecture that allows more bandwidth, quick
provisioning, guaranteed QoS in a cost effective manner is required.
• PON technology offers mechanism to enable sufficient network bandwidth for the delivery of new services and applications.
• PON is a distribution architecture that provides a unified broadband transport system of converged services from homes and buildings, through FTT-H,-B, or through FTT-C, -Cab by using xDSL transmission technology for residential areas.
Central Office
(Exchange)
Feeder Network
Street Cabinet
Distribution Network
Overhead Feed
Underground Feed
Customer
Typical Access Network Construction
Overview of DSL Architecture
• DSL is a subscriber access network.
• CPE (Customer Premise Equipment) is connected by ADSL to a DSL Access Multiplexer (DSLAM) located in the CO of the network service provider.
• DSLAM aggregates traffic from different customers and sends it over high speed links towards the core of the network (possibly over B-PON) access systems..
• DSL supports the delivery of converged data, video, and voice traffic.
Overview of DSL Architecture
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
Splitter
DSL modem
DSLAM
Broadband Network
Telephone switch
(PSTN)
Customer PremisesCentral Office (CO)
Combined Signal0 to 1 MHz
Voice Band0 to 4 kHz
ADSL Band40 kHz – 1 MHz
Splitter
PSTN
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
DSLAM
Broadband Network
Telephone switch
Customer PremisesCentral Office (CO)
IAD
GW
Data Traffic
Voice Traffic (Packetized)
Both voice and data use the high
frequency band(40 kHz-1MHz)
Overview of DSL Architecture
ATM-based access network
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
user
user user
DSLAM
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
user
ADSL
ATM
PON
ONU
ONU
ONU
xDSL
NT
NT
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p T o p o l o g y
F i l e E d i t L o c a t e V i e w H e l p M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
NT NT NT
ADM
ATM/xDSL
SONET
ADM
ADM ADM
OLT
PSTNLE
ATM switch
ATU-R
VoIP
ATMISP
ISP
Internet
BAS
BAS: Broadband access ServerLE: Local ExchangeATU-R: ATM Interface
ATM-based access network
• ATM in the access complicated the management of the access network, especially when on resource reservation is required.
• VCs creation/termination requires a VB5.2 signaling protocol that is extremely complex.
• IP traffic is expected to be the dominant traffic, thus IP/ATM solution will add more overhead.
B-PON + DSL Architecture
Combined Signal(Data and analog POTS)
T o p o l o g y F i l e E d i t L o c a t e V i e w H e l p
M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
Splitter
DSL modem
DSLAM
Customer Premises Central Office(CO)
Data and packetized voice
D500 POTS Card
PON access
Metro/backbone
PSTN
Internet
OADM
GWOLT
ONU
Copper pair
DSL and ONU can be co-located or geographically separated
Hybrid xDSL-PON Architecture
• A hybrid xDSL-PON architecture is an effective migration to a FSAN delivering converged data, video and voice.
• This architecture will provide high bandwidth access to customers without requiring to install a network that runs in parallel with the PSTN.
• DSLAMs are installed in COs to aggregate traffic from multiple high speed connections. Increased service coverage area.
• DSLAMs in turn could be co-located with ONUs or connected to ONUs.
• Develop QoS functional model that includes functions and features required to support stringent SLA.
• Define how QoS mechanisms can be applied at various points in the network to achieve appropriate performance characteristics.
• Leverage IP QoS mechanisms (e.g. IP Diffserv) to deliver QoS.
• Integration of DSL signaling with MPCP signaling to achieve a cost effective signaling transparent to the protocol framing structure.
Evolution to IP-based Access Networks
B-PON + DSL Architecture (additional slide) T o p o l o g y
F i l e E d i t L o c a t e V i e w H e l p M o u n t 4 3 1 7 4 3 7 1 9 5 0 7 9 % / N e t w o r k
T r a f f i c H e l p
Splittr
DSL modem
DSLAM
D500 POTS Card
E-PON access
OLT
ONU
Metro ring
CPE
DSL broadband access
DSL broadband accessDSL broadband access