Telecom & Transport Speed Schemes

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North American Digital Signal Hierarchy

In the 1960’s The Bell System / AT&T came up with a  transport system based off 64K (bits) “channels.”  To come up with 64K,  consider the  voice frequency (VF) or voice band frequencies used in  telephony, approximately 300 Hz to 3400 Hz.  You can refer to this as baseband or narrowband.  (Telecom likes to have several names for the same thing.)

A single voice transmission channel is about 4 kHz and is sampled at  8 kHz.  Why?  Because the  Nyquist theorem says so.

(4,000 Hz  is an adequate sweet-spot for human speech, it’s not exact but when reproduced at the other end, you can recognize it’s grandma when she calls.)

So what ya get is:

2 x 4K = 8K samples per second,  each one of those sample is/used 8-bit pulse-code modulation which ends up as 8K x 8 = 64K bits per second – a DS0. (It will be called D-S-O or D-S-Zero interchangeably.)

Robbed bits for signaling

Here’s the catch –  the low order bit is used for signaling purposes.  For voice this created noise that you really can’t hear so Ma Bell didn’t care.  For digital data you can’t fudge it like that, only 7 bits can be used. 8,000 7-bit samples gives you  56 kbps. Today you can get around this using different line codes and bit stuffing.


So the hierarchy using a DS0 of 64K or 64,000 bits per second:

Hierarchy Speed Digital
Carrier DS0’s Notes
1st 1.544 Mbit/s DS1 T-1 24 In ISDN PRI = 23B (user) + 1D (signaling) channels
IntermediateLevel 3.152 Mbit/s DS1C   48 DS1C uses two DS1 signals combined and sent on a 3.152 megabit per second carrier which allows 64 kilobits per second for synchronisation and framing using pulse stuffing.  Never common, you won’t see this in use.
2nd 6.312 Mbit/s DS2 T-2 96 4 x DS1. Never common, you won’t see this in use.
3rd 44.736 Mbit/s DS3 T-3 672 28 x DS1
Intermediate Level 139.264 Mbit/s DS4NA   2016 3 x DS3 Highest designed in ANSI T1.107
4th Level 274.176 Mbit/s DS4 T-4 4032 Replaced with Optical Carrier / OCx
5th Level 400.352 Mbit/s DS5 T-5 5760 Replaced with Optical Carrier / OCx
HOLD UP –  24 x  64,000 bits per second won’t get you 1.544 Mbit/s. What you have is 24 x 64,000 = 1.536 Mbit/s. Bits are lost between frames because a frame separator is needed for every 8 bit sample of the of the 24 channels .  So yes, 24 x 8 = 192 but adding the separator, 193 bits per frame  x 8K samples = 1.544 Mbit/s.
Speed DS0 Carrier
64 Kbps 1
1.544 Mbit/s 24 T-1
2.048 Mbit/s 32
6.312 Mbit/s 96 T-2
7.786 Mbit/s 120
8.448 Mbit/s 128
32.064 Mbit/s 480
34.368 Mbit/s 512
44.736 Mbit/s 672 T-3
97.728 Mbit/s 1440
139.264 Mbit/s 2016 DS4NA
139.264 Mbit/s 2048
274.176 Mbit/s 4032 T-4
400.352 Mbit/s 5760 T-5
565.148 Mbit/s 8192

Anything above a T3 is now optical/fiber.

Optical Carrier

SONET (Synchronous Optical Network) in North America or SDH (Synchronous Digital Hierarchy) elsewhere is the modern day optical transmission systems.  It’s nice because everything is a multiple of the OC-1 rate of 51.84 Mbps.

Hierarchy Data Rate SONET SDH OCx
Level Zero 155.52 STS-3 STM-1 OC-3
Level One 622.08 STS-12 STM-4 OC-12
Level Two 2488.32 Mbit/s STS-48 STM-16 OC-48
Level Three 9953.28 Mbit/s STS-192 STM-64 OC-192

Optical Carrier Rates

Optical Carrier Data Rate Payload-SONET (SPE) User Data Rate SONET SDH
OC-1 51.84 Mbit/s 50.112 Mbit/s 49.536 STS-1
OC-3 155.52 Mbit/s 150.336 Mbit/s 148.608 STS-3 STM-1
OC-9 466.56 Mbit/s 451.044 Mbit/s 445.824 STS-9 STM-3
OC-12 622.08 Mbit/s 601.344 Mbit/s 594.824 STS-12 STM-4
OC-18 933.12 Mbit/s 902.088 Mbit/s 891.648 STS-18 STM-6
OC-24 1244.16 Mbit/s 1202.784 Mbit/s 1188.864 STS-24 STM-8
OC-36 1866.24 Mbit/s 1804.176 Mbit/s 1783.296 STS-36 STM-12
OC-48 2488.32 Mbit/s 2.4 Gbps 2377.728 STS-48 STM-16
OC-192 9953.28 Mbit/s 9.6 Gbps 9510.912 STS-192 STM-64
OC-768 40Gbit/s STS-768 STM-256
OC-3072 160Gbit/s STS-3072 STM-1024

Virtual Tributary

To slice up the 51.84 Mbit/s, you can have a sub-STS-1 facilitie.  VT1.5 is  common because it can carry 1.728 Mbit/s (enough room for a DS1/T1 signal.)  There’s a lot of overhead in SONET.

Name Speed
VT-1.5 1.728Mbit/s
VT-2 2.304Mbit/s
VT-3 3.456Mbit/s
VT-6 6.912Mbit/s
STS-1 50.112Mbit/s
STS-3 150.336Mbit/s

Don’t leave your dog in the car.

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You can see how dangerous it is to leave a pet inside a car. The car reaches 117 degrees within 30 minutes even with all four windows opened 1 to 2 inches.

Remember, dogs don’t perspire.

Vistaprint – small cost, small card

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A standard US-sized business card is a 2” x 3.5” rectangle.

If you read the product specs at Vistaprint, what you end up with is 3.43″ x 1.93″  – slightly smaller the what’s customary.


Perhaps it’s because they are not based out of the US. It’s hard to beat the cost but the size difference was noticeable to me, so I don’t think I will use them again.



Chinese business cards:
Size: 3.543” × 2.125” (90mm × 54mm); Aspect Ratio: 1.667
(This applies to China, Hong Kong, Singapore)
Japanese business cards:
Size: 3.54” × 2.165” (90mm × 55mm); Aspect Ratio: 1.636
Korean business cards:
Size: 3.54” × 2.165” (90mm × 55mm); Aspect Ratio: 1.636
USA Business Cards:
Size: 3.5” × 2” (88.9mm × 50.8mm); Aspect Ratio: 1.75

Traffic Shaping

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Policing Versus Shaping

Traffic shaping (also known as “packet shaping”) is a computer network traffic management technique which delays some or all datagrams to bring them into compliance with a desired traffic profile. Traffic shaping is a form of rate limiting.

So… Let’s say you’re a business that processes credit cards.  You have 15 stores and 1 corporate headquarters that has your computer network.  They 15 stores only need a little bandwidth to send the transactions to HQ but the HQ needs more bandwidth to accept the data from all of the 15 stores.  What you have is a mismatch in bandwidth (or CIR, Committed Information Rate.)  The stores would only need a 56K DS0 (56,000 bits per second) circuit but the HQ would need a full T1 (DS1) running at 1.544 megabits per second to handle all the traffic coming from and going to the stores.

So the problem is, the HQ is a fire hose and the stores are a garden hose… You can’t spray a fire-hose into a garden-hose and not expect some water is going splash out.  In data, that water would be drops or lost packets.

Networks are often asymmetrical, that is, the access rate at one site may differ from the access rate at another. In such cases, it may be necessary to configure the faster rate to shape to the access rate of the slower rate.

To limit bandwidth, you can shape or you can police.

Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate, excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.


Policing Versus Shaping


Shaping implies the existence of a queue and of sufficient memory to buffer delayed packets, while policing does not. Queueing is an outbound concept; packets going out an interface get queued and can be shaped. Only policing can be applied to inbound traffic on an interface. Ensure that you have sufficient memory when enabling shaping. In addition, shaping requires a scheduling function for later transmission of any delayed packets. This scheduling function allows you to organize the shaping queue into different queues. Examples of scheduling functions are Class Based Weighted Fair Queuing (CBWFQ) and Low Latency Queuing (LLQ).

Simply stated, both shaping and policing use the token bucket metaphor. A token bucket itself has no discard or priority policy. Let’s look at how the token bucket metaphor works:

  • Tokens are put into the bucket at a certain rate.
  • Each token is permission for the source to send a certain number of bits into the network.
  • To send a packet, the traffic regulator must be able to remove from the bucket a number of tokens equal in representation to the packet size.
  • If not enough tokens are in the bucket to send a packet, the packet either waits until the bucket has enough tokens (in the case of a shaper) or the packet is discarded or marked down (in the case of a policer).
  • The bucket itself has a specified capacity. If the bucket fills to capacity, newly arriving tokens are discarded and are not available to future packets. Thus, at any time, the largest burst a source can send into the network is roughly proportional to the size of the bucket. A token bucket permits burstiness, but bounds it.

Use the police command to specify that a class of traffic should have a maximum rate imposed on it, and if that rate is exceeded, an immediate action must be taken. In other words, with the police command, it is not an option to buffer the packet and later send it out, as is the case for the shape command.

 This example shows the configuration of two traffic-shaped interfaces on a router. Ethernet interface 0 is configured to limit User Datagram Protocol (UDP) traffic to 1 Mbps. Ethernet interface 1 is configured to limit all output to 5 Mbps.

access-list 101 permit udp any any
interface Ethernet0
traffic-shape group 101 1000000 125000 125000
interface Ethernet1
traffic-shape rate 5000000 625000 625000