FAQ for T1

Latency versus Bandwidth... What is it?

One of the most commonly misunderstood concepts in networking is speed and capacity. Most people believe that capacity and speed are the same thing. For example, it's common to hear "How fast is your connection", invariably, the answer will be "640K, 1.5M" or something similar. These answers are actually referring to the bandwidth or capacity of the service, not speed.

Speed (latency) and capacity (bandwidth) are two very separate things. The combination of latency and bandwidth give users the perception of how quickly a webpage loads or a file is transferred. It doesn't help that broadband providers keep referring to "get high speed access" when they probably should be saying "get high capacity access". Notice the term "Broadband", it refers to how wide the pipe is, not how fast.

The most common example to compare latency and bandwidth is: Imagine water running through a pipe. The pressure is latency, the width of the pipe is bandwidth. If you have a wide pipe but low pressure, you can move more water through the pipe but at a slower rate. If you have a narrow pipe but high pressure, you can move less water but at a faster rate.

Another example that is sometimes given: Imagine people in an aircraft. In this example, people are the data packets, the size of the aircraft is the bandwidth, and the speed of the aircraft is the latency. A 747 can carry about 400 people but a 707 can carry only 200 people. Both fly at about 500 knots. If both leave New York at the same time, they will arrive in Los Angeles at the same time. Notice that although, the 747 has more capacity (or bandwidth) it is the same speed (latency) as the 707.

Latency is normally expressed in milliseconds. One of the most common methods to measure it is use the utility ping. A small packet of data, typically 32 bytes, is sent to a host and the time is measured. Normally, the RTT (round-trip time it takes for the packet to leave the source host, travel to the destination host and return back to the source host) is measured.

Bandwidth is normally expressed in bits per second. It's the amount of data that can be transferred during a second. Solving bandwidth is easier than solving latency. To solve bandwidth, more pipes are added. For example, in early analog modems it was possible to increase bandwidth by bonding two or more modems. In fact, ISDN achieves 128K of bandwidth by bonding two 64K channels using a data link protocol called multilink-ppp.

The following are typical latencies, to the first hop, by popular circuit types . Please remember however that latency on the Internet is also effected by routing (number of hops) that an ISP may perform (i.e., if your data packet has to travel farther, latencies increase).

Ethernet: 0.3ms
Analog Modem: 100-200ms
ISDN: 15-30ms
DSL/Cable: 10-20ms
Satellite: >100ms
DS1/T1 ; 2-5ms

Bandwidth and latency are connected. If the bandwidth is saturated then congestion occurs and latency is increased. However, if the bandwidth of a circuit is not at peak, the latency will not decrease. Bandwidth can always be increased but latency cannot be decreased. Latency is the function of the electrical characteristics of the circuit. So, no matter how un-congested the analog modem line is, the latency (speed) will not be reduced (increased).

What is the difference between frame relay and point-to-point?

Frame relay (FR) is a shared connection. Point-to-point (P2P) is a private, non-shared connection.

For P2P, you install a direct private line T1 circuit between the customer's location and the closest ISP's POP (Point of Presence, access point), which is itself connected to the Internet backbone via a P2P connection. The data pipe is always a full T1. In order to obtain a fractional T1, the ISP throttles the bandwidth (to say 384K) at the router port. Increasing bandwidth is a simple router re-configuration. This is the better an more expensive method because the ISP is paying the Telco for a full T1 pipe that is private (i.e. not shared by anyone else). The cost is in direct relation to the distance between the customer's location and the ISP's P2P connection to the Internet backbone. This is important when comparing ISPs, as some connect their POPs to each other and to their NOC (Network Operations Center) using a Frame Relay network. In this scenario you may have a P2P connection to the ISP POP, but to the Internet backbone you have a FR connection!

For FR, a circuit is ordered between the customer's location and the FR "cloud" (usually a large Telco CO (Central Office) close to the customer) and a separate circuit between the ISP and the FR "cloud" (usually another large Telco CO close to the ISP's home office). Within the "cloud", traffic is shared and managed by the Telco. FR is cheaper because it is shared and because it is not distance-sensitive, is of lesser quality and offers lower standards. In order to insure quality, FR circuits come with Committed Information Rates (CIR). The CIR is the guaranteed bandwidth. You may be able to go over it, but you are not guaranteed anything over it. However, you are always guaranteed at least the CIR. The most common FR T1 circuit is a FR T1 (up to 1.5Mbs) which comes with a CIR of 512Kbps.

A P2TP circuit doesn't have a CIR. What you order is what you get. A fractional P2P is obtained by throttling the bandwidth at the router port. In essence you have (and pay for) a full T1 (1.5 Mbps) data pipe that is throttled at the router port to only allow ,say 384 Kbps of data at all times. Increasing you bandwidth is a simple re-configuration of the router port speed.

FR works best for companies that have an average bandwidth of less than 200 or 300 Kbps with only occasional bursts over 1.0Mbps. Companies that require average bandwidth in excess of 512Kbps (most serious web servers and definitely e-commerce servers) should order a full P2P T1 circuit.

PTP offers the high speed, consistent performance and the best control for e-commerce retailers, web hosting companies and medium to large companies with high bandwidth requirements. With P2P services, you have a private, exclusive connection from your company to the Internet. FR allows you to share a line "in the cloud" with other FR customers in order to lower your costs.

P2P is an extremely reliable and tested method of data transport. P2P circuits are what coined the term T-1, and up until 1996 most data transport was carried on P2P circuits. This type of circuit can be envisioned like throwing a cable from one location to another and using it to connect the two locations.

Because P2P circuits require provisioning of the wire from start to end, there are additional mileage costs. The further apart the two points of connection are the more expensive the "local loop" will be.

P2P T1 circuits have a latency of 5-8 milliseconds. FR T1 circuits have a latency of 10-20 milliseconds..