10 — EPON Network Architecture
10.3 EPON implementation of ISAM
ISAM provides the core processing, switching, and control functions. The ISAM shelves with their NT card and the EPON LT card comprise the conceptual OLT system from an EPON network point of view.
• In the upstream direction, ISAM interacts with the Ethernet switch and voice gateway using the NT cards.
• In the downstream direction, ISAM distributes network traffic to the subscribers via the LT cards that terminate to the ONUs.
The Alcatel-Lucent ONU products are edge devices that use EPON technology to extend a fiber optic cable from an OLT shelf to a subscriber residence, including single-family residences, multi-dwelling units, such as apartment buildings, small office or medium business offices or home office applications. See chapter “ISAM Support for the EPON ONU” for more information about the EPON ONU.
EPON physical layer
Ethernet for subscriber access networks combine a minimal set of extensions to the IEEE 802.3 Media Access Control (MAC) and MAC control sublayers with a family of physical layers.
The physical layers include optical fiber and voice-grade copper cable Physical Medium Dependent (PMDs) sublayers not only for point-to-point (P2P) connections in subscriber access networks, but also for a point-to-multi-point (P2MP) network topology with passive optical splitters.
To support P2MP topology, IEEE 802.3ah extensions to the MAC control sublayers and reconciliation sublayers as well as to optical fiber PMDs. In addition, a
mechanism for network operations, administration, and maintenance is included to
10G-10G ONU 10G-1G
ONU 1G-1G 10 Gb/s, 1270nm ONU
1 Gb/s, 1310nm 1 Gb/s, 1310nm
Upstream Downstream 10 Gb/s, 1577 nm
1 Gb/s, 1490 nm
RF Video, 1555 nm OLT
10 — EPON Network Architecture
10-10 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 where the Data Link Layer (layer 2) consists of:
• LLC (Logical Link Control sublayer)
• MAC (Media Access Control sublayer)
• RS (Reconciliation Sublayer). This sublayer processes PHY Local/Remote Fault messages and handles DDR conversion
and the PHY Layer (layer 1) consists of:
• PCS (Physical Coding Sublayer). This sublayer performs auto-negotiation and coding such as 8b/10b encoding for EPON and 64b/66b encoding for 10G EPON.
• PMA (Physical Medium Attachment Sublayer). This sublayer performs PMA framing, octet synchronization/detection, and x7 + x6 + 1
scrambling/de-scrambling.
• PMD (Physical Medium Dependent sublayer). This sublayer consists of a transceiver for the physical medium.
Figure 10-4 shows the reference model for the P2MP topology over EPON.
Figure 10-4 Reference model for P2MP topology over EPON
Figure 10-5 shows the reference model for the P2MP topology over 10G/10G EPON.
21767
= GIGABIT MEDIA INDEPENDENT INTERFACE GMII
= MEDIUM DEPENDENT INTERFACE MDI
= OPERATIONS, ADMINISTRATION, AND MAINTENANCE OAM
= OPTICAL LINE TERMINAL OLT
10 — EPON Network Architecture Figure 10-5 Architecture model for P2MP topology over 10G/10G EPON
Figure 10-6 shows the reference model for the P2MP topology over 10G/1G EPON
POS (Clause 76) FEC (Clause 76) PMA (Clause 76) PR-type PMD (clause 75)
XGMII
XGMII = 10 GIGABIT MEDIA INDEPENDANT INTERFACE MDI = MEDIA INDEPENDANT INTERFACE
OAM = OPERATIONS, ADMINISTRATION & MAINTENANCE OLT = OPTICAL TERMINAL
ONU = OPTICAL NETWORK UNIT
PCS = PHYSICAL CODING SUBLAYER PHY = PHYSICAL LAYER DEVICE PMA = PHYSICAL MEDIUM ATTACHMENT PMD = PHYSICAL MEDIUM DEPENDENT MDII
10 — EPON Network Architecture
10-12 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 Figure 10-6 Architecture model for P2MP topology over 10G/1G EPON
Physical medium dependent (PMD)
PMD Sublayer for EPON
The EPON PMD sublayer consists of the 1000Base-PX20+ transceiver for the physical medium, and is compliant with the IEEE 802.3-2005 1000Base-PX20 and CCSA EPON regulation amendment for 1000Base-PX20+ sublayer requirement.
In addition to 1000Base-PX20 PMD, the 1000Base-PX20+ also provides P2MP 1000BASE connection over PON up to 20 km with a typical split ratio of 1:32, or 10 km with 1:64 split ratio. The objective of 1000Base-PX20+ is to provides P2MP 1000Base service over a single-mode fiber with BER better than or equal to 10-12 at PHY service interface.
Table10-1 defines the 1000BASE-PX20 PMD.
POS (Clause 76) FEC (Clause 76) PMA (Clause 76) PR-type PMD (clause 75)
XGMII
XGMII = 10 GIGABIT MEDIA INDEPENDANT INTERFACE GMII = GIGABIT MEDIA INDEPENDANT INTERFACE
MDI = MEDIA INDEPENDANT INTERFACE
OAM = OPERATIONS, ADMINISTRATION & MAINTENANCE OLT = OPTICAL TERMINAL
ONU = OPTICAL NETWORK UNIT PCS = PHYSICAL CODING LAYER PHY = PHYSICAL LAYER DEVICE PMA = PHYSICAL MEDIUM ATTACHMENT PMD = PHYSICAL MEDIUM DEPENDENT
GMII
10 — EPON Network Architecture Table 10-1 1000BASE-PX20+ PMD definition
A 1000Base-PX20+ link uses a 1000Base-PX20+-U PDM at one end and a 1000Base-PX20+-D PMD at the other. However a 1000Base-PX20+-D PMD is interoperable with a 1000Base-PX20-U PMD to support upgrade possibilities of 1:32 to 1:64 split ratio. A 1000Base-PX20+ link does allow 1000Base-PX20+-D PMD to interoperate with 1000Base-PX10-U PMD to increase the maximum access range from 10 km to 20 km.
PMD sublayer for 10G EPON
A 1000Base-PX30/1000Base-PRX30 link is used to support upgrade possibilities of 1:32 to 1:128 split ratio. A 1000Base-PX30/1000Base-PRX30 link allows
interoperability to increase the maximum access range from 10 km to 20 km.
Table 10-2 describes the illustrative channel insertion loss and penalties for PR10, PR20 and PR30 (symmetric-rate, 10 Gb/s power budget classes).
Table 10-2 Channel insertion loss and penalties for PR10, PR20, and PR30
Description 1000Base-PX20+-U 1000Base-PX20+_
D
Unit
Fiber type B1.1, B1.3 SMF —
Number of fiber 1 —
Nominal transmit wavelength 1310 1490 nm
Direction Upstream Downstream —
Minimum range .5 m to 20 km —
Optical power budget 30 29.5 dB
Maximum channel insertion loss 28 dB
Minimum channel insertion loss 10 dB
Description PR10 PR20 PR30
US DS US DS US DS
Fiber type IEC 60793-2 B1.1, B1.3 SMF
ITU-T G.652, G.657 SMF
Measurement wavelength for fiber 1270 nm 1577 nm 1270 nm 1577 nm 1270 nm 1577 nm
Nominal distance 10 km 20 km 20 km
Available power budget 23 dB 21.5 dB 27 dB 25.5 dB 32 dB 30.5 dB
10 — EPON Network Architecture
10-14 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 Table 10-3 describes the illustrative channel insertion loss and penalties for PRX10, PRX20, and PRX30 (asymmetric-rate, 10 Gb/s downstream, 1 Gb/s upstream power budget classes).
Table 10-3 Channel insertion loss and penalties for PRX10, PRX20, and PRX30
Physical medium attachment, physical coding, and reconciliation sublayers
The PMA, PCS, and RS sub-layers in ISAM are compliant with Clause 65 of IEEE 802.3-2005 for EPON and IEEE802.3av for 10G EPON.
• The PMA sublayer performs PMA framing, octet synchronization/detection, and x7 + x6 + 1 scrambling/de-scrambling.
• The PCS sublayer helps to define the physical layer specifications, such as speed and duplex modes, for networking protocols like Fast Ethernet, Gigabit Ethernet and 10-GE. It performs auto-negotiation and coding such as 8b/10b encoding.
• The RS sublayer processes PHY local/remote fault messages and handles DDR conversion.
The transmit function of the extended RS replaces some of the octets of the preamble, as transmitted by the MAC.
Multipoint MAC control protocol
The EPON network allows only one ONU to transmit in the upstream direction at a time. The MPCP located at the OLT times the transmissions of multiple ONUs.
MPCP reports traffic congestions, which optimizes the allocation of bandwidth across the PON.
The MPCP comprises the following processing functions:
• discovery processing
This function manages the discovery process of an ONU and compensates for round trip time.
Description PRX10 PRX20 PRX30
US DS US DS US DS
Fiber type IEC 60793-2 B1.1, B1.3 SMF
ITU-T G.652, G.657 SMF
Measurement wavelength for fiber 1310 nm 1577 nm 1310 nm 1577 nm 1310 nm 1577 nm
Nominal distance 10 km 20 km 20 km
Available power budget 23 dB 21.5 dB 26 dB 25.5 dB 30.4 dB 30.5 dB
Channel insertion loss (max) 20 dB 24 dB 29 dB
Channel insertion loss (min) 5 dB 10 dB 15 dB
Allocation for penalties 3 dB 2.5 dB 2 dB 1.5 dB 1.4 dB 1.5 dB
Optical return loss of ODN (min) 20 dB
10 — EPON Network Architecture
• report processing
This function manages the generation and collection of report messages through which bandwidth requests are sent upstream from the ONU to the OLT.
• gate processing
This function manages the generation and collection of gate messages, through which multiplexing of multiple transmitters is achieved.
In the discovery phase, each registered ONU is designated a unique LLID. The OLT supports 64 LLIDs for each PON interface over EPON. For 10G EPON, the OLT supports 128 LLIDs for each PON interface.
Dynamic bandwidth allocation
Dynamic bandwidth allocation (DBA) is the process by which an ONU requests upstream bandwidth and whereby the OLT re-assigns bandwidth accordingly to improve upstream bandwidth utilization and to guarantee service equality.
To process the bandwidth requests from the ONUs, ISAM monitors the queue status of all LLIDs and reassigns upstream bandwidth accordingly.
ISAM supports the following bandwidth types:
• fixed bandwidth. The upstream bandwidth is reserved for the ONU so that the OLT always sends a constant grant to the ONU even without an upstream request.
This bandwidth is not included in DBA scheduling.
• assured bandwidth. The OLT sends the corresponding grant according to the MPCP report message from the ONU. If the upstream request is less than the grant assigned by the OLT, the surplus bandwidth can be used by other ONUs through DBA.
• best-effort bandwidth. The ONU can use the surplus bandwidth on the PON if there are no bandwidth requests from services that have higher priority.
ISAM EPON supports a minimum DBA bandwidth granularity of 64 kb/s and ±5%
DBA precision.
Forward error correction
Forward error correction (FEC) corrects transmission errors on the PON by inserting redundant information into the bit stream to recover errors.
The ISAM transport layer between the ONU and the OLT uses a block-based FEC to transmit the data in an encoded format. The encoding introduces redundancy that allows the decoder to detect and correct the transmission errors. For example, for input BER of 10-4, the BER at the FEC decoder's output may drop to 10-15. By using the FEC technique, data transmission with low error rates can be achieved, and
10 — EPON Network Architecture
10-16 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 ISAM uses Reed-Solomon FEC to correct transmission errors on the PON.
Reed-Solomon FEC is a block-based code that inserts bits at the end of a data block of a specific size to create a code word. Using the extra bits, the FEC decoder processes the data stream, discovers errors, corrects error, and recovers the original data. Reed-Solomon code is specified in CMTT recommendation CCIR 723.
When a system uses a block-based FEC, the original data is preserved. Therefore, by ignoring the parity bits, the original data can be processed, even if the other side does not support FEC. However, block-based FEC error correction is not efficient for very high BER levels (for example, for 10-3 BER, a decoding error will be generated).
In ISAM over EPON, FEC can be enabled or disabled in the downstream direction on a per-PON basis, or in the upstream direction on a per-ONU basis.
For 10G EPON, FEC can be enabled forcibly in both the upstream and downstream directions of 10/10 Gb/s data rate per PON, and in the downstream direction of 10/1 Gb/s data rate per PON. The FEC function of the P-OLT is configurable for upstream of 10/1 Gb/s data rate per PON.
OTDR functionality
Optical Time Domain Reflectometry (OTDR) is used to detect faults and degradations on optical links. OTDR-capable EPON SFPs for 1G only can be deployed in the ISAM. The configuration of the OTDR function in the ISAM and the analysis of the OTDR measurements are done by the 5530 Network Analyzer Fiber.
The PON port on the LT card can support an SFP with an integrated OTDR function.
The OTDR function provides a means to continuously monitor an optical path to measure the length of the fiber, and the physical location of any fiber breaks or degradations.
The operator can enable, on a per-PON basis, a background process on the LT card to collect the OTDR data from the SFP. Raw measurements are collected every 225 s and up to a maximum of two-hours worth of data per enabled PON. After the two-hour period expires, the background process overwrites the raw measurements from the first hour.
Using an EMS or SNMP interface, an operator can retrieve the following OTDR data for analysis and troubleshooting purposes:
• raw measurements for current or previous hour
• summed measurements for up to 25 previous hours
• calibration measurements
Security
ISAM supports the following security features:
• Triple churning
• Advanced Encryption Standard (AES)
10 — EPON Network Architecture
• Authentication
• Other security features to avoid unlawful attacks and interceptions
Triple churning
The ISAM uses broadcasting mode in the downstream. To ensure security of data from the OLT to the ONU, the ISAM EPON supports the triple churning function that is defined in the China Telcom EPON equipment technical requirement specifications.
In general, the OLT requests a churning key (new_key_request) from the ONU, and the ONU responds with a 3-byte churning key (new_churning_key) for 1G EPON and 9-byte churning key for 10 G EPON that the OLT uses to generate a scramble key. The OLT then uses the scramble key to scramble all frames, including OAM frames, before sending to the ONU.
The triple churning can be enabled or disabled on a per-LLID basis, and each LLID can have its own churning key.
Advanced Encryption Standard (AES)
The ISAM supports AES security features for DPoE links for operation and maintenance. Specifications are compliant with IEEE 802.1 ae and provides protection of all frames from malicious attacks at an EPON link in both the upstream and downstream directions.
The EPON OLT and ONU provide link security for up to 64 ONUs using a 128 bits Galois/Counter Mode Advanced Encryption Standard (GCM-AES) authenticated encryption to provide user data confidentiality, frame data integrity, and data origin authenticity to subscribers at a maximum 2 Gbps for the EPON system using Counter-AES (CTR-AES).
Note — DPoE supports AES in Cipher Feed Back (CFB) mode, and CTC supports triple churning.
10 — EPON Network Architecture
10-18 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 Authentication
ISAM only allows ONUs that pass authentication to access the EPON network.
ISAM supports three ONU authentication modes as specified in the CTC EPON equipment technical requirements specifications R2.1. The PON can be configured for one of the following authentication modes:
• physical ID (MAC-based) authentication. Only ONUs with a physical identifier that passes authentication as per IEEE 802.3-2005 can register at the PON. This authentication is performed during the ONU discovery phase. In EPON
deployments, the physical identifier is the MAC address of the ONU.
• Logical ID authentication. Only ONUs with a logical identifier that passes authentication can register at the PON. In release 4.2.30, the logical ID can either be the ONU ID, a password, or the combination of ONU ID and password. It is recommended that the latter be implemented through extended OAM.
This authentication is performed after the ONU discovery phase. When the authentication fails, the OLT will un-register the ONU even though the ONU has passed the discovery phase.
• mix-based authentication. In this mode, the ISAM authenticates the ONU based on the physical ID or the logical ID. If the physical ID authentication fails, ISAM then checks the logical ID before registering the ONU. This mode is applicable for legacy deployments where the authentication mode migrated from physical to logical.
Other security features
In addition to authentication, ISAM also provides the following security features to avoid unlawful attacks and interceptions:
• filtering
• anti-spoofing
• CPU overloading protection
ONU ID method
ISAM allows for configuration of the ONU ID method that can be used at both the system and ONT levels for planning and security purposes. The ONT level ONU ID method takes higher priority than the system level ONU ID method. The operator can configure whether the OLT will provide the MAC address or the LOGICAL ID as the ONU ID in DHCP option 82 or PPPoE relay tag. DHCP option82 is compliant with the MII standard.
When the MAC address is used for ONU authentication, the service configuration is retained to simplify and improve the process of ONU replacement.
Note — To avoid registration storms due to authentication failures, ISAM requires that an ONU waits at least 60 s before attempting to re-register.
10 — EPON Network Architecture
ONU ranging
Ranging is the process by which the propagation delay between the OLT and the ONU is measured. The OLT performs the round trip delay computation using the timestamp in the MPCP messages from the ONU.
The OLT and the ONU have 32-bit counters that increment every 16 ns. These counters provide a local time stamp. When either device transmits an MPCPDU, it records its counter value into the timestamp field. To set the timestamp value, the time of transmission of the first octet of the MPCPDU frame from the MAC control to the MAC is used as the reference time.
When the ONU receives MPCPDUs, the ONU sets its counter according to the value in the timestamp field in the received MPCPDU. When the OLT receives
MPCPDUs, the OLT uses the received timestamp value to calculate or verify a Round Trip Time (RTT) between the OLT and the ONU. The RTT is equal to the difference between the time value and the value in the timestamp field. The MAC client then uses the calculated RTT for ranging.
ONU discovery and activation
Discovery is the process by which offline or newly-connected ONUs are provided access to the PON. The OLT drives the process. On a periodic basis, the OLT makes available Discovery Time Windows. At these times, the offline ONUs can make themselves known to the OLT.
The OLT broadcasts a discovery gate message to advertise a discovery period. The message includes the starting time and length of the discovery window. After the ONU receives this message, offline ONUs wait until the period begins to send a Register Request message to the OLT.
Discovery windows are the only times when multiple ONUs can simultaneously access the PON. Therefore, transmission overlaps may occur. To minimize
overlapping transmissions, online ONUs temporarily stop transmitting to the PON, and offline ONUs wait a random period of time that is less than the length of the discovery time window before sending their Register Request message.
After the OLT receives a valid Register Request message from an ONU, the following events occur.
1 The OLT registers the ONU, assigns a port identifier (LLID) to the ONU, and relates the MAC address of the ONU to the LLID.
2 The OLT sends the LLID and the required synchronization time to the ONU.
3 The OLT echoes the maximum number of pending grants.
4 The OLT schedules the ONU to the PON.
5 The OLT sends a standard gate message to the ONU.
10 — EPON Network Architecture
10-20 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX R4.6.02 The OLT monitors report messages that are received from online ONUs for transmission requests. In return, the OLT sends gate messages to the ONUs to report their allocated DBA grants. To maintain the watchdog timer at the ONU, the OLT periodically generates DBA grants when empty gate messages are sent.
Operations, administration, and maintenance
The ISAM supports the following OAM functions for DPoE, TK, and CTC service modes:
• standard OAM functions specified in clause 57 of IEEE802.3-2005
• the managed object class, and attribute and action in clause 30 of IEEE802.3-2005
• extended DPoE OAM functions compliant with DPoE Specification OAM V1.0, section 9.3 to support the following:
• ACL
• CCL
• enable/disable port - MAC enable status
• S1 Interface port auto-negotiation
• ONU bridging aging time
• optical monitoring
• OAM frame rate
• UNI statistics counter
• ONU bridge mode
dynamic MAC table, dynamic learning mode, and MAC learning MAX allowed
• dying gasp
• LOS alarm
• C-VLAN TPID
• S-VLAN TPID
• S-VLAN TPID