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Advanced regional Telecom Training Centre, Ranchi
Introduction
• GPON - Gigabit Passive Optical Network.
• The only active network elements are OLT (Optical Line Termination) and ONT / ONU (Optical Network Termination / Unit)
• PON utilizes Tree network topology
• Offers data rates of up to 2.5 Gbps downstream and 1.2 Gbps upstream.
• Provides extended reach (compared to xDSL)
Advanced regional Telecom Training Centre, Ranchi
Overall GPON data transmission approach
GE/10GE
OLT
ONU
ONT
MDU
ONT
NT xDSL
NT
xDSL
Upstream (ONTs OLT)
λ1 = 1310 nm
Downstream (OLT ONTs)
λ2 = 1490 nm
Fiber feeder
Central office
Infrastructure – Fiber Optic
• Passive Optical splitter
• NT – Network Termination
• OLT – Optical Line Termination
• ONT – Optical Network Termination
• ONU – Optical Network Unit
• DSL – Digital Subscriber Line
Fiber link
As far as one fiber feeder is used between OLT and ONUs for data transmission WDM is implemented to
decouple Uplink and Downlink data paths.
Advanced regional Telecom Training Centre, Ranchi
Downstream data transmission (TDM)
• OLT broadcasts data to every ONU using TDM approach
• TDM – continuous data stream divided into frames
• Optionally FEC coding and AES encryption are applied to the
user data
• Every ONT receives each DS frame and picks up only that data
addressed to it by the OLT
• FEC decoding and /or AES decryption algorithm applied if required
• DS signal is distributed by a passive splitter to all
ONUs connected to it via separate fiber link
User #1
User #3
User #2
2 1 31
21
3
1
2 1 31
21
3
1
3
2
1 1
ONT 1
ONT 2
ONT 3
OLT
Passive Optical
Splitter 1: N
(Distribution point)
To broadcast data from the OLT to all ONTs TDM (Time Division
Multiplexing) is used
Advanced regional Telecom Training Centre, Ranchi
Upstream data transmission (TDMA)
• OLT collects data from all ONUs and filters it
with the use of ONU-IDs
• FEC is applied to data if required
• ONU receives data from the user ports and combines them into
bursts
• For synchronization purposes each ONU transmits its data in a strict
accordance with the Bandwidth Map generated by OLT
• Using DBA mechanism OLT can rearrange US BW to provide more
resources to those ONU tightly loaded with traffic.
• For synchronous transmission within TDMA stream each ONU
introduces equalization delay.
User #1
User #3
User #2
2
1 1ONU 1
ONU 2
ONU 3
OLT
Splitter
33 3
2
1
1
3
3
3
3 1 1 3 2 3
BW Allocation Map
To provide multiple access to a single fiber link for all connected ONUs Time
Division Multiple Access (TDMA) architecture is implemented for the
Upstream channel
Advanced regional Telecom Training Centre, Ranchi
Summary on T-CONTS
T-CONTs or Transmission Containers are used in GPON for provisioning of: traffic separation and bandwidth allocation on the level of CoS.
The main purpose of T-CONT is bundling of GEM traffic for management of US bandwidth allocation.
Every T-CONT may contain one GEM-port or bundle of several GEM-ports
T-CONTs are used only in US direction (from the ONTs to the OLT)
Every ONT in the GPON is allowed to use up to 8 T-CONTs. Every T-CONT may be assigned to a one of four different T-CONT types.
Every type of T-CONT is allocated US bandwidth differently.
T-CONT is identified by 12-bits of Alloc_ID (up to 4095 T-CONTs per GPON)
Advanced regional Telecom Training Centre, Ranchi
T-CONTs and Bandwidth allocation
Static T-CONT Dynamic T-CONT
Advanced regional Telecom Training Centre, Ranchi
User’s data encapsulation
1. User data represented by different types of traffic (Ethernet data, VoIP, TDM, OMCI) is adapted to the format suitable for transmission over GPON
2. This task is accomplished by GTC Adaptation sublayer of GPON stack over GEM (GPON Encapsulation Method which uses GFP-like procedure).
3. GEM frame contains encapsulated user data and overhead
4. User frame may be partitioned prior to encapsulation into GEM-frame
5. If there are no user data to transmit within an allocated resource then idle frames are inserted
PL#1
PL#1H#1
001
PL#2
PL#2H#2
001
PL#3
PL#3AH#3A
000PL#3B
H#3B
001
PL#4
PL#4H#4
001Idle
PLI
12b
Port ID
12b
PTI
3b
HEC
13b
GEM frame
GEM payload partition of GTC Frame #M
User Data frames
GEM payload partition
GEM Header
Advanced regional Telecom Training Centre, Ranchi
GEM-ports. User data encapsulation
AGW
To the ODN
Burst is identified by ONT_ID (64 per GPON)
T-CONT is identified by Alloc_ID (1024 per GPON)
GEM-port is identified by GEM_port_ID (4095 per GPON)
Lea
sed
Lin
esP
OT
S /
Vo
IPD
ata
(rt)
Dat
a (n
rt)
Dat
a (b
est
effo
rt)
ONT
Management
T_CONT # 1
Type 1
(Fixed BW)
T_CONT # 2
Type 1
(Fixed BW)
T_CONT # 3
Type 1
(Fixed BW)
T_CONT # 4
Type 2
(Assured BW)
T_CONT # 5
Type 3
(Assured + Non Assured BW)
T_CONT # 6
Type 4
(Best Effort BW)
Burst
GEM-port-ID1
GEM-port-ID2
GEM-port-ID3
GEM-port-ID4
GEM-port-ID5
GEM-port-IDn1
GEM-port-IDn1+1
GEM-port-IDn2
GEM-port-IDn2+1
GEM-port-IDn3
Host μC
TDM 2
TDM 1
Advanced regional Telecom Training Centre, Ranchi
Summary on GEM-ports
1. From the point of view of GPON GEM-port defines logical bi-directional point-to-point connection with the following exceptions:
DASAN GPON uses the GEM-port ID 0xFFF(4095) for Multicast and Broadcast.
2. GEM-port typically bundles payload per user per CoS.
3. GEM-ports are used for encryption and performance monitoring
4. GEM-port is identified by 12-bits of GEM-port-ID (up to 4095 GEM-ports per GPON)
5. The allocation of GEM-ports per T-CONT and per physical ONU ports is dictated by OLT
Advanced regional Telecom Training Centre, Ranchi
Dynamic Bandwidth Allocation
• Upstream bandwidth (US BW) is granted to ONUs by OLT with the aid of US BW Map broadcasted to all PON participants
• BW assignment may be static or dynamic
• Using DBA mechanism dynamic BW assignment is possible. DBA improves overall BW utilization
• Additional BW is granted by the OLT from the currently unused bandwidth pool
• To know how much BW to grant for a certain ONU (T-CONT) OLT have to collect information on BW utilization
• BW utilization may be assessed using the following modes:
• Non Status Reporting DBA – OLT observes traffic patterns of ONUs (ONUs don’t report)
• Status Reporting DBA – OLT receives information on queue fill level from ONUs (ONUs report to OLT)
Advanced regional Telecom Training Centre, Ranchi
GPON maximum reach
ONU 2 Splitter
ONU N
ONU 1
OLT
Outermost ONU
Nearest ONU
60 km
Logical Reach
Maximum distance between OLT
and outermost ONU supported by GTC
20 km
Physical Reach
Typical maximum distance considering infrastructure losses
a
b
a – b ≤ 20 km20 km
Maximum differential
distance
User Side Outdoor Environment Central Office
GPON Optical module specification
CLASS of Laser B+ Specification
Advanced regional Telecom Training Centre, Ranchi
GPON reach influencing factors
Class of Laser Type Optical Budget GPON Release
A Fabry-Perot 5 – 20 dB
B Fabry-Perot 10 – 25 dB
B+ Fabry-Perot 13 – 28 dB Current version
C Distributed
Feedback Laser
15 – 30 dB
1) Class of Laser used
4) Attenuation of the optical link- Depends on the fiber length (0.3 dB per 1 km)- Number of splices along the fiber channel- Number of connectors (0.3 dB per connector)
5) Attenuation of the passive optical splitter- Depends on the splitting ratio 1: 64 – 19.5 dB 1:32 – 17.8 d
1:16 – 14.4 dB 1:8 – 11 dB 1:4 – 7.6dB 1:2 – 4.2dB
3) Forward Error Correction algorithm- Enabled / Disabled
2) Receiver susceptibility
Optical Budget
Total losses
Optical Budget
Total losses>
OLT
Splitter (1:32)
ONT NONT 2
ONT 1
------------28 dB
Splitter – 17.8 dBFiber - 0.3dB/km x 20km = 6 dBConnectors – 0.3dB x 4 = 1.2 dB
20 km
-------------------------------------------25 dB
Connectors
Advanced regional Telecom Training Centre, Ranchi
OLT
ONT #1
PON ONT #2
ONT #3
FEC enabled for ONT#1
FEC enabled for ONT#3
FEC disabled for ONT#2
• Forward Error Correction (FEC) is the mechanism to improve transmission quality of physical layer.
• Reed Solomon code (RS) FEC is used by GPON. Up to 3 dB budget improvement (7% overhead)
• Only payload part of frames is coded
• OLT and ONUs negotiate the status of FEC using flags in the overhead fields for each ONU independently
Forward Error Correction (FEC)Forward Error Correction (FEC)
Advanced regional Telecom Training Centre, Ranchi
Forward Error Correction (FEC)
• For FEC purposes Reed Solomon code RS (255,239).
• Codeword length is 255 B = 239 B data block + 16 B parity bytes
PCBd Ident FECi / 1b
DownstreamIn DS direction OLT informs all ONTs of the FEC enabled / disabled status by installing FECi bit. FECi is valid for the whole GEM payload.
UpstreamONTs that are capable of FEC support install their Ind bit # 6. Those ONTs not supporting FEC mechanism ignore FEC overhead.
PLOu IND Bit # 6
Advanced regional Telecom Training Centre, Ranchi
OLT
ONT #1
PON
ONT #N
Encryption is enabled for User #1 of ONT#1
User #2
User #1
Encryption is disabled for User #2 of ONT#1
Encrypted connection
• Standardized AES (Advanced Encryption Standard) algorithm is used to protect individual users’ data from trespassing.
• Encryption is applied to the data in the DS direction only
• DS data is encrypted on the level of virtual connection.
• AES Encryption machine operates in counter mode.
• Unpredictable cipher block generation is done with the use of constantly changing counters and random cipher keys generated by each ONU independently
EncryptionEncryption
Advanced regional Telecom Training Centre, Ranchi
Security on GPON Link
The basic concern in PON is that the downstream data is being broadcasted to all ONUs attached to the PON.
AES is used for the encryption algorithm128 (currently supported by Broadlight Chipset), 192, and 256 bit keys
GEM frame payload is encrypted. PLOAM (Physical Layer OAM) for security
Encrypted_Port-ID (downstream) : Enable/Disable EncryptionRequest_Key (downstream) : request ONU to generate new keyEncryption_Key (upstream) : send new key to OLTKey_Switching_Time (downstream) : indicate when to begin using the new
encryption key.
Rekeying time can be configurable ; minimum value is 10 seconds (Broadlight Implementation)
Advanced regional Telecom Training Centre, Ranchi
Security on GPON LinkAuthentication Mechanism in GPON
GPON supports two kinds of authentication mechanism. Serial Number Password
The serial number is mandatory authentication during ONU activation.
After registering the serial number, ONU ID is assigned
Serial_Number_ONU (upstream) : 8 bytes serial number
Password is optionally used for validating the ONU.
Request_password (downstream) Password (upstream) : 10bytes password
Advanced regional Telecom Training Centre, Ranchi
ONU Failure Detection Time
Advanced regional Telecom Training Centre, Ranchi
ONU failure detection time
ONU failure can be detected by LOSi and LOFi. LOSi (i : ONU ID)
No valid optical signal from ONU when it was expected 4 consecutive non-contiguous allocations to that ONU
LOFiWhen 4 consecutive invalid delimiters from ONUi was received.
ONU Failure can be detected within 8 msec. (HW Detection Time)
Advanced regional Telecom Training Centre, Ranchi
OLT GPON port switch over mechanism
Advanced regional Telecom Training Centre, Ranchi
GPON RedundancyType-A Protection in SIU_GPON4R
BL3458
GMAC#4
GMAC#1
GMAC#2
GMAC#3
GPIU
12 x 1GSFU
ONT #1
ONT #2
ONT #3
Active
Stand-by
GPON Port Redundancy within a GPIU
• OLT can detect GPON link down by “LOS” event.
• After receive LOS, switch-over from Active to Standby
• Switch over in the same GMAC
• Fail over time is under 50msec
Advanced regional Telecom Training Centre, Ranchi
GPON Link LoS
• OLT can detect GPON link down by “LOS” event.
• LOS is defined as below in G984.3– LOS : Los of Signal
– The OLT did not receive any expected transmission in the upstream (complete PON failure) for 4 consecutive frames.
• Broadlight supports this as hardware triggering mechanism and GPON line card detect it as soon as it happens.
• It takes 4 consecutive frames (125us * 4) to detect link down.
Advanced regional Telecom Training Centre, Ranchi