i)Classes of BGP Attributes
1.Well Known a)Mandatory - AS_PATH, NEXT_HOP, ORIGIN
b)Discretionary - LOCAL_PREFERENCE, ATOMIC_AGGREGATE
2.Optional c)Transitive - AGGREGATOR, COMMUNITY
d)Non Transitive - MED(metric)
The well-known attributes must be recognised by anything that runs BGP. If it’s mandatory, then its value must be included in every BGP Update message sent: discretionary attributes do not have to always be sent. Well-known attributes, if included, must be passed along to the next BGP peer.
BGP implementations do not have to understand optional attributes, however if it’s a transitive attribute, it should still be passed to the next BGP peer, even if this one doesn’t know what to do with it. If it’s marked as non-transitive, then the router can simply ignore it, and shouldn’t pass it on, although if does understand it, it can act on it if it wants to.
ii)Some notes on BGP route selection at the time of Multihoming
1.If the router is of Cisco make , straight away make use of the Weight attribute to influence outgoing path from a branch office(in case multiple paths-from Same or Different ISP- are available and u want to load share) using route-mpas. WEIGHT is known to only the local router.
2.If not cisco make, use the local-preference attribute to influence outgoing path from a branch office. Also make use of local peference to influence the exit path, in case there are multiple exit points in the AS.(more than one router) since the local preference attribute is propagated throughout the local AS.
3.To influence the interface on which packets are coming in use prepend-as, (When multi homed to different ISP's, since AS_PATH is well known mandatory)
4. Use MED to influence incoming packets path only when multihomed to same ISP(since MED is optional "non-transitive"), and the ISP recognises MED. MEDs are advertised throughout the local AS.
Community Attribute
The community attribute provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set the community attribute. Predefined community attributes are listed here:
•no-export—Do not advertise this route to EBGP peers.
•no-advertise—Do not advertise this route to any peer.
•internet—Advertise this route to the Internet community; all routers in the network belong to it.
Figure 39-7 illustrates the no-export community. AS 1 advertises 172.16.1.0 to AS 2 with the community attribute no-export. AS 2 will propagate the route throughout AS 2 but will not send this route to AS 3 or any other external AS.
Figure 39-7 BGP no-export Community Attribute
In Figure 39-8, AS 1 advertises 172.16.1.0 to AS 2 with the community attribute no-advertise. Router B in AS 2 will not advertise this route to any other router.
Figure 39-8 BGP no-advertise Community Attribute
Figure 39-9 demonstrates the internet community attribute. There are no limitations to the scope of the route advertisement from AS 1.
Figure 39-9 BGP internet Community Attribute
BGP Path Selection
BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order presented, to select a path for a destination:
•If the path specifies a next hop that is inaccessible, drop the update.
•Prefer the path with the largest weight.
•If the weights are the same, prefer the path with the largest local preference.
•If the local preferences are the same, prefer the path that was originated by BGP running on this router.
•If no route was originated, prefer the route that has the shortest AS_path.
•If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete).
•If the origin codes are the same, prefer the path with the lowest MED attribute.
•If the paths have the same MED, prefer the external path over the internal path.
•If the paths are still the same, prefer the path through the closest IGP neighbor.
•Prefer the path with the lowest IP address, as specified by the BGP router ID.
http://www.cisco.com/en/US/docs/internetworking/technology/handbook/bgp.html
refer for BGP and MPLS http://www.netcraftsmen.net/resources/archived-articles/373.html
Saturday, June 26, 2010
Thursday, May 27, 2010
Cisco IOS CAR-Committed Access Rate- / rate-limit to control Download/Upload limit for a user or Uer or VALN using ACL
reference: http://articles.techrepublic.com.com/5100-10878_11-6151305.html
Committed access rate (CAR)—or "rate limiting"—is a method for managing unwanted traffic on your network and making sure it doesn't affect important traffic.
To use CAR, you must enable CEF on your routers.
CAR can be very useful as a basic QoS function.
Essentially, CAR controls the bandwidth of a certain type of traffic, and an access control list (ACL) defines which traffic it controls. Once you've created the ACL, you can set CAR to enforce a bandwidth rate on that traffic in either an INBOUND or OUTBOUND direction, according to the interface on which you applied CAR.
Using CAR requires two simple steps:
1. Create an ACL to define the traffic you want to rate limit.
2. Use the rate-limit command, referencing the ACL on your interface closest to the source of the traffic, referencing the proper direction, and referencing the proper bandwidth amounts.
For example, let's say you have a production application at a remote location across a 128-Kb WAN connection. The application works fine until, one day, a new employee gets a PC, a Web browser, and Internet access.
When in use, that single Web browser slows down the entire remote production network by downloading Web pages and documents over the 128-Kb connection. So how can you prevent the Web browsing from taking over the WAN connection and slowing down the production application?
To return to our example, let's say you have a headquarters location that provides the Internet access for this single PC across the WAN. Even if browsing the Web is a necessary business function, it's negatively affecting the performance of the production application on the remote network.
Let's look at how you can use to control this Web traffic. First, define the traffic you want to rate limit on the headquarters' router. Here's an example:
HQ-Router(config)# access-list 120 permit tcp any eq www host
10.200.200.200
In this example, the remote PC has an IP address of 10.200.200.200. So, we're saying that the source server could be any Web server serving Web pages on port 80.
Next, use the rate-limit command on the interface. Here's an example:
HQ-Router(config)# interface Serial0/0
HQ-Router(config-if)# rate-limit output access-group 120 50000
10000 20000 conform-action transmit exceed-action drop
This applies the rate limit to the interface, referencing ACL 120. We applied it in the outbound direction because we applied it on the headquarters router (not the remote router). That's because we want to prevent unwanted Web traffic from going across the WAN to the remote site—we don't want to wait until the traffic arrives there before slowing it down.
50000, 10000, 20000 represents the normal bits per second (bps) for this traffic (i.e., 50000 bps or about 50 Kb), the normal burst size for the traffic (i.e., 10000 or about 10 Kb), and the maximum burst size for the traffic (i.e., 20000 or about 20 Kb). The traffic must conform to these numbers in order for the router to transmit it (as specified by conform-action transmit). If the traffic exceeds those bandwidth settings, the router will drop it (as specified by exceed-action drop).
Configuring these settings on the headquarters' router on the Serial0/0 interface (i.e., the interface that goes to the remote location) limits the extraneous Web traffic to consuming less than 50 Kb of the 128-Kb circuit used for the production application.
While you can use CAR in a variety of situations, keep in mind that CAR only limits what you tell it to limit with the ACL. In addition, the CAR bandwidth settings you reference limit all traffic referenced in the ACL.
Committed access rate (CAR)—or "rate limiting"—is a method for managing unwanted traffic on your network and making sure it doesn't affect important traffic.
To use CAR, you must enable CEF on your routers.
CAR can be very useful as a basic QoS function.
Essentially, CAR controls the bandwidth of a certain type of traffic, and an access control list (ACL) defines which traffic it controls. Once you've created the ACL, you can set CAR to enforce a bandwidth rate on that traffic in either an INBOUND or OUTBOUND direction, according to the interface on which you applied CAR.
Using CAR requires two simple steps:
1. Create an ACL to define the traffic you want to rate limit.
2. Use the rate-limit command, referencing the ACL on your interface closest to the source of the traffic, referencing the proper direction, and referencing the proper bandwidth amounts.
For example, let's say you have a production application at a remote location across a 128-Kb WAN connection. The application works fine until, one day, a new employee gets a PC, a Web browser, and Internet access.
When in use, that single Web browser slows down the entire remote production network by downloading Web pages and documents over the 128-Kb connection. So how can you prevent the Web browsing from taking over the WAN connection and slowing down the production application?
To return to our example, let's say you have a headquarters location that provides the Internet access for this single PC across the WAN. Even if browsing the Web is a necessary business function, it's negatively affecting the performance of the production application on the remote network.
Let's look at how you can use to control this Web traffic. First, define the traffic you want to rate limit on the headquarters' router. Here's an example:
HQ-Router(config)# access-list 120 permit tcp any eq www host
10.200.200.200
In this example, the remote PC has an IP address of 10.200.200.200. So, we're saying that the source server could be any Web server serving Web pages on port 80.
Next, use the rate-limit command on the interface. Here's an example:
HQ-Router(config)# interface Serial0/0
HQ-Router(config-if)# rate-limit output access-group 120 50000
10000 20000 conform-action transmit exceed-action drop
This applies the rate limit to the interface, referencing ACL 120. We applied it in the outbound direction because we applied it on the headquarters router (not the remote router). That's because we want to prevent unwanted Web traffic from going across the WAN to the remote site—we don't want to wait until the traffic arrives there before slowing it down.
50000, 10000, 20000 represents the normal bits per second (bps) for this traffic (i.e., 50000 bps or about 50 Kb), the normal burst size for the traffic (i.e., 10000 or about 10 Kb), and the maximum burst size for the traffic (i.e., 20000 or about 20 Kb). The traffic must conform to these numbers in order for the router to transmit it (as specified by conform-action transmit). If the traffic exceeds those bandwidth settings, the router will drop it (as specified by exceed-action drop).
Configuring these settings on the headquarters' router on the Serial0/0 interface (i.e., the interface that goes to the remote location) limits the extraneous Web traffic to consuming less than 50 Kb of the 128-Kb circuit used for the production application.
While you can use CAR in a variety of situations, keep in mind that CAR only limits what you tell it to limit with the ACL. In addition, the CAR bandwidth settings you reference limit all traffic referenced in the ACL.
Tuesday, May 25, 2010
BGP Synchronization and Split Horizon
BGP Synchronization rule:
Applicable to the following situations
1.When the AS is a transit AS,
2.When atleast one of the router in the AS is not running BGP
3.All the routers are running bgp but, they are not fully meshed or there is no RR.
When a BGP router learns about a network from an eBGP peer it advertises to other iBGP peers only after verifying that the network information is already available in the IGP.
BGP Split Horizon Rule:An iBGP router never advertises a route learned from its neighbour to any other iBGP peer. ( Conventional split horizon says that a route/network learned through an interface will never be advertised back on that interface)
One doubt: Suppose a Service Provider(SP) network. If all the internal routers are capable of running BGP(ibgp) and if a RR is there what is the need for running an IGP in the network?? Ans: For each of the RR clients to reach the RR, the IGP is required. Also the route to each networks between the routers also are propogated by the IGP.
Applicable to the following situations
1.When the AS is a transit AS,
2.When atleast one of the router in the AS is not running BGP
3.All the routers are running bgp but, they are not fully meshed or there is no RR.
When a BGP router learns about a network from an eBGP peer it advertises to other iBGP peers only after verifying that the network information is already available in the IGP.
BGP Split Horizon Rule:An iBGP router never advertises a route learned from its neighbour to any other iBGP peer. ( Conventional split horizon says that a route/network learned through an interface will never be advertised back on that interface)
One doubt: Suppose a Service Provider(SP) network. If all the internal routers are capable of running BGP(ibgp) and if a RR is there what is the need for running an IGP in the network?? Ans: For each of the RR clients to reach the RR, the IGP is required. Also the route to each networks between the routers also are propogated by the IGP.
Monday, April 12, 2010
Elections in Cisco Switches and Routers
Hi,
I will describe the election processes taking place in routers and switches.
1. Routers: IGMP Designated Querier: Highest IP
2. Switch/Router: HSRP/VRRP/GLBP: Active/Master/AVG :Highest Priority :IP address with HSRP enabled, second is Standby/backup/standby AVF
I will describe the election processes taking place in routers and switches.
1. Routers: IGMP Designated Querier: Highest IP
2. Switch/Router: HSRP/VRRP/GLBP: Active/Master/AVG :Highest Priority :IP address with HSRP enabled, second is Standby/backup/standby AVF
Saturday, December 20, 2008
Linux networking
Let me give the basics of linux networking basics here.
1.To assign an IP address and default gateway to eth0 interface,
#ifconfig eth0 10.37.111.3 netmask 255.255.0.0
#route add default gw 10.37.111.1
#ifconfig ----> shows the current ip and gateway details
1.To assign an IP address and default gateway to eth0 interface,
#ifconfig eth0 10.37.111.3 netmask 255.255.0.0
#route add default gw 10.37.111.1
#ifconfig ----> shows the current ip and gateway details
Thursday, February 15, 2007
SDH-Synchronous Digital Hierarchy
SDH: Synchronous Digital Hierarchy.
SDH is an international optical interface standard to transmit different types of signals on OFC. It accepts various native TDM signal formats and multiplexes-adds- or demultiplexes-drops- these signals without conversion. It provides improved network management- in band- and direct access to tributaries.
Plesiochronous Digital Hierarchy-PDH-includes all ITU TDM hierarchies including T-carrier and E-carrier systems. Signals are considered to be plesiochronous if their rates are nominally same, with any variations constrained to specified limits. PDH is defined by ITU G.702 and includes NADH-North American Digital Hierarchy-, EDH-European Digital Hierarchy- and JDH-Japanese Digital Hierarchy. All these systems share the basic rate of 64kbps that correspond to a PCM voice channel.
Synchronization
1.PRS / Master clock / Stratum1 GPS or Atomic clock.
2.Line timing. Derived from incoming SDH signal from a high-speed interface.
3.Loop timing. Similar to Line timing but incoming SDH is taken from a CPE or TM-Terminal Mux- than an ADM.
4.Stratum3. Free running internal oscillator.
Although synchronous networks display accurate timing, some variations occur between different network devices or between networks. This difference is known as phase variations. Short-term phase variations above 10 Hz are called Jitter and long tem phase variations below 10 Hz are called Wander. In digital networks, jitter and wander are handled by buffers found in the interfaces within different network devices. One example is a slip buffer, used to handle frequency difference between read and write operations.
Here bit stuffing is used and it is called controlled slip. The timing accuracy requirements increase as the stratum hierarchy increases as shown below.
Stratum clock Hierarchy
Mini. Free run Wander Bit slips controlled frame
Accuracy (0.12 hsec Increment) Slips
Stratum 1 +/- 1*10^-11 3.3hr 18hrs 20.6 wks
Stratum2 +/- 1.6*10^-8 7.5 sec 41sec 2.17hrs
Stratum3 +/- 4.6*10^-6 26ms 140ms 27sec
Stratum4 +/- 32*10^-6 4ms 20ms 3.9sec
SDH Layers
SDH has four optical interface layers as follows.
1.Path layer
2.Multiplex Section –MS- layer
3.Regenerator Section –RS- layer
4.Photonic layer.
Path layer multiplexes or demultiplexes the lower level VC-n payloads. POH is added or stripped at this layer and the alarm information contained in this layer represents the end-to-end status. Lower order multiplexers, subscriber loop access systems are examples of Path Terminating Equipments.
Signal at STM-N level is transported between NE’s in this layer. MSOH is added or stripped at this layer. The MSOH is used for communication between major nodes and for monitoring errors. High order mux, Optical Line Terminator are examples of MSTE-MS Terminating Equipments.
This layer is there between a TM and regenerator or between two regenerators.
RSOH is added or stripped at this layer. It is used for long haul transport.
Deals with transport of bits across physical fiber medium i.e. EàO and OàE conversion.
SDH Multiplexing
SDH Multiplexing follows a rigid hierarchy as follows.
1.Low level PDH signal:: Mapped by adding Justification bits à Container ( C11 for T1, C12 for E1, C2 for DS2, C3 for E3 and DS3, C4 for E4)
2.:: Mapped by adding POH à Virtual Circuits (VC11,VC12,VC2,VC3,VC4)
3.:: Aligned with TUPointers à Tributary Units ( TU11,TU12 and TU2)
4.::Multiplexed à TUG –Tributary Unit Group-( TUG2, TUG3)
5.:: Multiplexed à Higher Order VC’s.( VC3,VC4)
6:: Aligned with fixed byte stuffing and AUPointersà Administrative Unit (AU3,AU4)
7.:: Muxed à AUG(AUG1,AUG4,AUG16,AUG64,AUG256)
8.:: Muxed with MSOH and RSOHà STM-N
Higher level of the Synchronous hierarchy is formed by byte interleaving of the payload from a number N of STM-1 signals and then adding TOH of size N times that of an STM-1 and filling it with new management data and pointer values as appropriate. Before transmission the STM-N signal is scrambled to randomize the bit sequence for better transmission performance. A few bytes of overhead are left unscrambled to simplify subsequent de-multiplexing.
SDH is an international optical interface standard to transmit different types of signals on OFC. It accepts various native TDM signal formats and multiplexes-adds- or demultiplexes-drops- these signals without conversion. It provides improved network management- in band- and direct access to tributaries.
Plesiochronous Digital Hierarchy-PDH-includes all ITU TDM hierarchies including T-carrier and E-carrier systems. Signals are considered to be plesiochronous if their rates are nominally same, with any variations constrained to specified limits. PDH is defined by ITU G.702 and includes NADH-North American Digital Hierarchy-, EDH-European Digital Hierarchy- and JDH-Japanese Digital Hierarchy. All these systems share the basic rate of 64kbps that correspond to a PCM voice channel.
Synchronization
1.PRS / Master clock / Stratum1 GPS or Atomic clock.
2.Line timing. Derived from incoming SDH signal from a high-speed interface.
3.Loop timing. Similar to Line timing but incoming SDH is taken from a CPE or TM-Terminal Mux- than an ADM.
4.Stratum3. Free running internal oscillator.
Although synchronous networks display accurate timing, some variations occur between different network devices or between networks. This difference is known as phase variations. Short-term phase variations above 10 Hz are called Jitter and long tem phase variations below 10 Hz are called Wander. In digital networks, jitter and wander are handled by buffers found in the interfaces within different network devices. One example is a slip buffer, used to handle frequency difference between read and write operations.
Here bit stuffing is used and it is called controlled slip. The timing accuracy requirements increase as the stratum hierarchy increases as shown below.
Stratum clock Hierarchy
Mini. Free run Wander Bit slips controlled frame
Accuracy (0.12 hsec Increment) Slips
Stratum 1 +/- 1*10^-11 3.3hr 18hrs 20.6 wks
Stratum2 +/- 1.6*10^-8 7.5 sec 41sec 2.17hrs
Stratum3 +/- 4.6*10^-6 26ms 140ms 27sec
Stratum4 +/- 32*10^-6 4ms 20ms 3.9sec
SDH Layers
SDH has four optical interface layers as follows.
1.Path layer
2.Multiplex Section –MS- layer
3.Regenerator Section –RS- layer
4.Photonic layer.
Path layer multiplexes or demultiplexes the lower level VC-n payloads. POH is added or stripped at this layer and the alarm information contained in this layer represents the end-to-end status. Lower order multiplexers, subscriber loop access systems are examples of Path Terminating Equipments.
Signal at STM-N level is transported between NE’s in this layer. MSOH is added or stripped at this layer. The MSOH is used for communication between major nodes and for monitoring errors. High order mux, Optical Line Terminator are examples of MSTE-MS Terminating Equipments.
This layer is there between a TM and regenerator or between two regenerators.
RSOH is added or stripped at this layer. It is used for long haul transport.
Deals with transport of bits across physical fiber medium i.e. EàO and OàE conversion.
SDH Multiplexing
SDH Multiplexing follows a rigid hierarchy as follows.
1.Low level PDH signal:: Mapped by adding Justification bits à Container ( C11 for T1, C12 for E1, C2 for DS2, C3 for E3 and DS3, C4 for E4)
2.:: Mapped by adding POH à Virtual Circuits (VC11,VC12,VC2,VC3,VC4)
3.:: Aligned with TUPointers à Tributary Units ( TU11,TU12 and TU2)
4.::Multiplexed à TUG –Tributary Unit Group-( TUG2, TUG3)
5.:: Multiplexed à Higher Order VC’s.( VC3,VC4)
6:: Aligned with fixed byte stuffing and AUPointersà Administrative Unit (AU3,AU4)
7.:: Muxed à AUG(AUG1,AUG4,AUG16,AUG64,AUG256)
8.:: Muxed with MSOH and RSOHà STM-N
Higher level of the Synchronous hierarchy is formed by byte interleaving of the payload from a number N of STM-1 signals and then adding TOH of size N times that of an STM-1 and filling it with new management data and pointer values as appropriate. Before transmission the STM-N signal is scrambled to randomize the bit sequence for better transmission performance. A few bytes of overhead are left unscrambled to simplify subsequent de-multiplexing.
Friday, February 02, 2007
Telecom
Hi,
Let me put some notes on TDM, WDM and later SDH, here in this blog
Time Division Multiplexing.
To share a medium by many signals multiplexing techniques like TDM, FDM, CDMA, and WDM are used.
TDM along with digital encoding techniques such as Pulse Code Modulation-PCM- forms the base of telecommunication systems.
Analog voice signal-4000Hz- is sampled at a frequency of 8000 Hz and quantized (non linear) -to reduce non-linear quantization error, i.e. percentage error more at lower amplitude- to get a 64Kbit PCM signal (DS0) .
PCM encoding is done as follows.
The analog signal level 0-4096 mV is divided into 8 (000-111) segments-in A-law - .
Voltage (mV) Segments (ABC)
0-31 0 0 0
32-63 0 0 1
64-127 0 1 0
128-255 0 1 1
256-511 1 0 0
512-1023 1 0 1
1024-2047 1 1 0
2048-4095 1 1 1
Quantization level is determined by dividing the range by 16, to get WXYZ
E.g. If voltage is 102 mV ABC= 010, WXYZ= 1001 (since 64/16 = 4, then 102 comes in the 9th level)
So the corresponding PCM signal is P A B C W X Y Z = 1 0 1 0 1 0 0 1
P=1 for +ve and 0 for -Ve
Decoding of PCM
E.g. 1 0 1 0 1 0 0 1
It will be expanded to 13 bits.
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13
Determine the segment value ‘n’ of the incoming signal… Here n=2.
Write W in D10-n position followed by X Y Z….. Here put W at D8 X at D9,Y at D10 and Z at D11.
W X Y Z
3. Always put 1 after Z…….Here put 1 at D12.
1 0 0 1 1
4. Always put φ before W(φ=0 for segment 0 and 1 for other segments)..Here D7=1.
1 1 0 0 1 1
5. Where ever blank put 0 …
0 0 0 0 0 0 1 1 0 0 1 1 0
6. Convert the binary to decimal……..Here 102mV.
There could be slight variation due to quantization error.
CAS-Channel Associated Signaling (in-band) and CCS-Common Channel Signaling (out of band) are two signaling schemes used in TDM. CAS forms are Ear and Mouth, Loop start, and ground start signaling. ISDN uses CCS.
E-Carrier
The basic unit of the E-Carrier system is 64kbps DS0, which is multiplexed to from transmission formats with higher speeds.
A E1 frame is formed by 32 time slots. TS0-TS31.
A E1 multi frame is formed by 16 frames. F0-F15. And 2 Sub Multi Frames -SMF- of 8 frames each.
The TS0 of even numbered frames is FAW-Frame Alignment Word- and TS0 of odd numbered frames is Alarm and Supervisory bits
TS16 is for signaling information (CAS).TS16 of F0 is MFAW-Multi FAW- of F1 is CAS of 1st and 15+1st channels. TS 16 of F15, first 4 bits are CAS of 15th channel and next 4 bits that of 30th.
CRC of 4 bits is calculated for a SMF and sent in the first bit position of 4 odd frames in the next SMF.
E1 Errors and Alarms.
SES-Severely Errored Seconds- Error rate of 10^-3
UAS-Un Available Seconds- Unavailable time begins at the onset of 10 consecutive SES. It also begins at a LOS-Loss Of Signal or Loss Of Frame.
Bit Errors-Are bits that are in error. Bit errors are not counted during unavailable time.
ISDN-Integrated Services Digital Network
ISDN allows voice and data to be transmitted simultaneously using end-to-end digital connectivity.
All devices that are ISDN ready (e.g. ISDN phone or feature phone) are designated TE-1 –Terminal Equipment 1- All other equipments that are not ISDN capable but have RS-232 or RJ-11 interface , like analog phone or modem, are called TE-2.
The U interface from exchange is terminated in NT-1 at the customer premises to give the customer a S/T interface.
Wavelength Division Multiplexing
Is a process of multiplexing different wavelengths (frequencies c=f*λ) on a single fiber. It can be considered as a form of FDM coupled with TDM.
Frequency is standardized rather than wavelength since it is independent of the transmission medium.
Wavelength is measured in nm and frequency in GHz.
High band width of OFC can be understood from the fact that visible light have a wavelength of 400-750 nm. So freq. is (c= f* λ) 400THz-750 THz (Tera = 10^12). So BW= 350 THz. So if 1 Hz can carry two bits of information, the light have an information carrying capacity of 700 T Bits approximately.
The optical frequency bands used with various WDM systems are as follows:
O-band (Original band) : A range from 1260nm to 1360nm.
E-band (Extended) : 1360 - 1460
S-band (Short wavelength) 1460 - 1530
C-band (Conventional) 1530 - 1565
L-band (Long Wavelength) 1565 - 1625
U-band (Ultra long) 1625 - 1675 nm
Channel Spacing
The minimum wavelength separation between two different channels multiplexed on a fiber is known as channel spacing. Channel spacing ensures that neighboring channels do not overlap. It is a function of the precision of the LASER. (Light Amplification by Stimulated Emission of Radiation, generates EM Signal in IR region i.e. λ > 750 nm).The more precise the tuning, the lower the channel spacing required, but the LASER will be costlier.
In theory, a fiber should be able to carry 150 wavelengths with 100GHz (0.8nm) spacing between wavelengths, 300 wavelengths with 50 GHz (0.4nm) spacing, 600 with 25GHz spacing and 1200 λs with 12.5 GHz (0.1nm) spacing. Assuming that each wavelength operates at 40 Gbps, it provides a theoretical maximum of 6 Tbps with 100 GHz spacing and 48 Tbps with 12.5 GHz spacing.
ITU-T has published a wavelength grid for interoperable standards, for DWDM, anchored to 193.1 THz. Or 1552.52 nm.
CWDM. Coarse Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 2.5 Gbps (STM-16) and can multiplex up to 18 λs providing a maximum of 45Gbps over a single fiber.
CWDM systems are characterized by a channel spacing of 20nm or 2500GHz. CWDM grid is made up of 18 wavelengths defined within the range of 1270 nm to 1610 nm.
DWDM: Dense Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 10Gbps (STM-64) and can multiplex up to 240 λs. i.e. 2.4 Tbps on a single fiber. It uses EDFA (Erbium Doped Fiber Amplifier)
ITU-T DWDM grid channel spacing is 50GHz or 100GHz to be operated in the C or L band window.(Where as legacy fiber is optimized for O-band).
Let me put some notes on TDM, WDM and later SDH, here in this blog
Time Division Multiplexing.
To share a medium by many signals multiplexing techniques like TDM, FDM, CDMA, and WDM are used.
TDM along with digital encoding techniques such as Pulse Code Modulation-PCM- forms the base of telecommunication systems.
Analog voice signal-4000Hz- is sampled at a frequency of 8000 Hz and quantized (non linear) -to reduce non-linear quantization error, i.e. percentage error more at lower amplitude- to get a 64Kbit PCM signal (DS0) .
PCM encoding is done as follows.
The analog signal level 0-4096 mV is divided into 8 (000-111) segments-in A-law - .
Voltage (mV) Segments (ABC)
0-31 0 0 0
32-63 0 0 1
64-127 0 1 0
128-255 0 1 1
256-511 1 0 0
512-1023 1 0 1
1024-2047 1 1 0
2048-4095 1 1 1
Quantization level is determined by dividing the range by 16, to get WXYZ
E.g. If voltage is 102 mV ABC= 010, WXYZ= 1001 (since 64/16 = 4, then 102 comes in the 9th level)
So the corresponding PCM signal is P A B C W X Y Z = 1 0 1 0 1 0 0 1
P=1 for +ve and 0 for -Ve
Decoding of PCM
E.g. 1 0 1 0 1 0 0 1
It will be expanded to 13 bits.
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13
Determine the segment value ‘n’ of the incoming signal… Here n=2.
Write W in D10-n position followed by X Y Z….. Here put W at D8 X at D9,Y at D10 and Z at D11.
W X Y Z
3. Always put 1 after Z…….Here put 1 at D12.
1 0 0 1 1
4. Always put φ before W(φ=0 for segment 0 and 1 for other segments)..Here D7=1.
1 1 0 0 1 1
5. Where ever blank put 0 …
0 0 0 0 0 0 1 1 0 0 1 1 0
6. Convert the binary to decimal……..Here 102mV.
There could be slight variation due to quantization error.
CAS-Channel Associated Signaling (in-band) and CCS-Common Channel Signaling (out of band) are two signaling schemes used in TDM. CAS forms are Ear and Mouth, Loop start, and ground start signaling. ISDN uses CCS.
E-Carrier
The basic unit of the E-Carrier system is 64kbps DS0, which is multiplexed to from transmission formats with higher speeds.
A E1 frame is formed by 32 time slots. TS0-TS31.
A E1 multi frame is formed by 16 frames. F0-F15. And 2 Sub Multi Frames -SMF- of 8 frames each.
The TS0 of even numbered frames is FAW-Frame Alignment Word- and TS0 of odd numbered frames is Alarm and Supervisory bits
TS16 is for signaling information (CAS).TS16 of F0 is MFAW-Multi FAW- of F1 is CAS of 1st and 15+1st channels. TS 16 of F15, first 4 bits are CAS of 15th channel and next 4 bits that of 30th.
CRC of 4 bits is calculated for a SMF and sent in the first bit position of 4 odd frames in the next SMF.
E1 Errors and Alarms.
SES-Severely Errored Seconds- Error rate of 10^-3
UAS-Un Available Seconds- Unavailable time begins at the onset of 10 consecutive SES. It also begins at a LOS-Loss Of Signal or Loss Of Frame.
Bit Errors-Are bits that are in error. Bit errors are not counted during unavailable time.
ISDN-Integrated Services Digital Network
ISDN allows voice and data to be transmitted simultaneously using end-to-end digital connectivity.
All devices that are ISDN ready (e.g. ISDN phone or feature phone) are designated TE-1 –Terminal Equipment 1- All other equipments that are not ISDN capable but have RS-232 or RJ-11 interface , like analog phone or modem, are called TE-2.
The U interface from exchange is terminated in NT-1 at the customer premises to give the customer a S/T interface.
Wavelength Division Multiplexing
Is a process of multiplexing different wavelengths (frequencies c=f*λ) on a single fiber. It can be considered as a form of FDM coupled with TDM.
Frequency is standardized rather than wavelength since it is independent of the transmission medium.
Wavelength is measured in nm and frequency in GHz.
High band width of OFC can be understood from the fact that visible light have a wavelength of 400-750 nm. So freq. is (c= f* λ) 400THz-750 THz (Tera = 10^12). So BW= 350 THz. So if 1 Hz can carry two bits of information, the light have an information carrying capacity of 700 T Bits approximately.
The optical frequency bands used with various WDM systems are as follows:
O-band (Original band) : A range from 1260nm to 1360nm.
E-band (Extended) : 1360 - 1460
S-band (Short wavelength) 1460 - 1530
C-band (Conventional) 1530 - 1565
L-band (Long Wavelength) 1565 - 1625
U-band (Ultra long) 1625 - 1675 nm
Channel Spacing
The minimum wavelength separation between two different channels multiplexed on a fiber is known as channel spacing. Channel spacing ensures that neighboring channels do not overlap. It is a function of the precision of the LASER. (Light Amplification by Stimulated Emission of Radiation, generates EM Signal in IR region i.e. λ > 750 nm).The more precise the tuning, the lower the channel spacing required, but the LASER will be costlier.
In theory, a fiber should be able to carry 150 wavelengths with 100GHz (0.8nm) spacing between wavelengths, 300 wavelengths with 50 GHz (0.4nm) spacing, 600 with 25GHz spacing and 1200 λs with 12.5 GHz (0.1nm) spacing. Assuming that each wavelength operates at 40 Gbps, it provides a theoretical maximum of 6 Tbps with 100 GHz spacing and 48 Tbps with 12.5 GHz spacing.
ITU-T has published a wavelength grid for interoperable standards, for DWDM, anchored to 193.1 THz. Or 1552.52 nm.
CWDM. Coarse Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 2.5 Gbps (STM-16) and can multiplex up to 18 λs providing a maximum of 45Gbps over a single fiber.
CWDM systems are characterized by a channel spacing of 20nm or 2500GHz. CWDM grid is made up of 18 wavelengths defined within the range of 1270 nm to 1610 nm.
DWDM: Dense Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 10Gbps (STM-64) and can multiplex up to 240 λs. i.e. 2.4 Tbps on a single fiber. It uses EDFA (Erbium Doped Fiber Amplifier)
ITU-T DWDM grid channel spacing is 50GHz or 100GHz to be operated in the C or L band window.(Where as legacy fiber is optimized for O-band).
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