A Data Service Unit/Channel Service Unit (DSU/CSU) WAN Interface Card ( WIC) these days is usually a blade on a router. In the past, these were separate. The CSU originated at AT&T as an interface to their non-switched digital data system. The DSU provides an interface to the data terminal equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities. They evolved from standalone hardware, to shelf type systems and are now just a blade or Wan Interfacde Card (WIC) in a router.
The functions of the LEDs
Data is being transmitted to the DTE interface.
Data is being received from the DTE interface.
Internal DSU/CSU is in loopback mode.
One of these alarm conditions is present: no receive signal, loss of frame signal from the remote station, or out of service signal from the remote station. This LED is off during normal operation.
Internal DSU/CSU in the WIC is communicating with another DSU/CSU. This LED is on during normal operation
There are many manufactures, each with their own ideas of abbreviations so TD, TX, or TXD all mean you’re transmitting data. You may not have every LED but in general…
Critical alarm indicator
Diagnostic loopback indicator
DTE sync indicator (Receive signa from telco)
TD, TX, or TXD
RD, RX, or RXD
Clear to send (per flow control)
Carrier loss of signal
Receiver loss of signal
What you could see
Normal – up and passing traffic
Loopback mode detected from telco or configured in CPE
Circuit is experiencing errors, but still passing traffic.
CSU detects a total disconnect. Circuit disconnected/no cable.
Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation
Receiver loss of signal. Possible timing, switch misconfiguration
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:
In ISDN PRI = 23B (user) + 1D (signaling) channels
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.
4 x DS1. Never common, you won’t see this in use.
28 x DS1
3 x DS3 Highest designed in ANSI T1.107
Replaced with Optical Carrier / OCx
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.
Anything above a T3 is now optical/fiber.
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.
Optical Carrier Rates
User Data Rate
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.
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.
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.
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
traffic-shape group 101 1000000 125000 125000
traffic-shape rate 5000000 625000 625000