How Fast Is Fast?

One of the cardinal rules of computers: Things keep getting faster. There are a lot more parts to the speed equation than processor speed, although the CPU is certainly part of the equation. This article looks at how fast the computer moves data.

DISCLAIMER: This article looks at theoretical throughput, which doesn’t necessarily equate with the real world. Serial communication has stop bits, parity bits, and headers. Memory often has wait states. And most modems use data compression. The comparisons here are general guidelines, not absolutes.

Serial Ports

The original Macintosh included a “high speed serial bus” also known as an RS-422 serial port. For standard serial communication using a modem, the early Macs could reliably move data at 19.2 Kbps. In an era of 1200 and 2400 bps modems, that was pretty impressive.

Later models pushed the envelope, fully supporting the fastest speed built into Apple’s ROM toolkit – 57.6 Kbps.

But RS-422 isn’t just for modems and printers. It also supports networking. Running LocalTalk pushed data through the serial port at 230.4 Kbps. Up until the early 1990s, that was considered pretty good for networking.

Some Macs, such as the Quadra AV models and most Power Macs, could support serial speeds to 2 Mbps (2048 Kbps).

Last summer [1998], Apple upped the ante by adopting USB 1.1 on the iMac. This new serial bus runs at up to 12 Mbps. That’s faster than regular 10Base-T ethernet and plenty fast for almost everything.

But Apple didn’t rest on their laurels. Instead, they pushed the envelope with FireWire, a standard feature on the Blue and White G3. FireWire supports speeds up to 400 Mbps.

Parallel Ports (incl. Drives)

In 1986, Apple introduced the SCSI port with the Mac Plus. SCSI moved 8 bits of data at a time, which made it far more efficient than serial ports of that era. As implemented on the Plus, SCSI could move data at 2100 Kbps or about 0.25 MBps (capital B = bytes, which are 8 bits).

The original SCSI specification allowed speeds as high as 5 Mbps or 40 Kbps – speeds that no 68K Mac ever achieved without a SCSI accelerator. The Mac II series topped out at 1.4-2.0 MBps, depending on the model, while the Quadras peaked at 3.4 MBps.

With the Power Macs, Apple started to use Fast SCSI (a.k.a. SCSI-2). This allowed throughput to reach 10 Mbps. Wide SCSI doubled that, SCSI-3 (a.k.a. Ultra SCSI) doubled that, and Ultra2 SCSI doubled that yet again.


The earliest computer modems ran at 110 bits per second. This gave way to 300, 1200, and 2400 bps modems. Using multiplexing, manufacturers pushed this to 9600 bps and 14.4 Kbps. Over time, this gave way to 28.8 Kbps modems, and finally so-called 56k modems (which usually top out around 48 Kbps).

Then there’s ISDN, which uses a special digital phone line. Single-channel ISDN usually runs at 56 or 64 Kbps; dual-channel doubles that to 112 or 128 Kbps.

There are several competing forms of DSL (digital subscriber lines). All are faster than ISDN. Some have the potential to hit 3-6 Mbps.

And then there are cable modems, which are a lot like ethernet (see below). Speeds of up to 30 Mbps are claimed, making this an incredibly fast way to download data. The biggest drawback to cable modems is that some systems only provide a download connection – to send data, you still need a slow phone modem.


The first Mac networks used LocalTalk, a 230.4 Kbps protocol. For the most part, that was adequate until the early 1990s, when 10Base-T simplified ethernet.

Ethernet is about 40 times faster than LocalTalk, offering a bandwidth of 10 Mbps. Then came Fast Ethernet (100 Mbps), which is ten times faster. And now they’re starting to push Gigabit Ethernet, which runs at up to 1000 Mbps.

The Internet

Besides the familiar dialup modems, there are high speed internet connections called T1 and T3 lines. This is how most ISPs (internet service providers) connect to the Internet – and they often have multiple connections to increase bandwidth and keep things alive should one connection fail.

A full T1 connection runs at 1.5 Mbps. For businesses needing less speed, most ISPs will sell fractional T1 service at rates as low as 56 Kbps. Beyond that, T3 moves data at 45 Mbps.

The Motherboard

The first Macs had an 8 MHz motherboard with 16-bit memory. That provides a maximum data speed of 16 MBps (128 Mbps).

With the Mac II, Apple adopted a 32-bit data path and a 16 MHz bus. Maximum possible throughput reached 64 MBps.

Today, Apple is shipping computers with 100 MHz 64-bit data paths, providing potential throughput of 800 MBps. (And caches are even faster!) Next year’s 500 MHz G4 will allow full speed communication between multiple CPUs on a 128-bit bus – that could achieve 6,400 MBps.

And the G4 will get faster than that.

The Big Picture

The point of all this is not simply to show how far we’ve come, but to put these numbers in perspective. For instance, if you think ISDN is fast at 128 Kbps, why do you think 230.4 Kbps LocalTalk is slow?

Here’s the throughput hierarchy by maximum throughput (ignoring data compression):




35 Bps
300 Bps

300 bps
2400 bps

Ancient modems
Older modems

4.2 KBps
16 KBps
 28.8 KBps

33.6 Kbps
128 Kbps
 230.4 Kbps

33.6 modem
Dual channel ISDN

0.75 MBps
1.25 MBps
1.5 MBps
3.75 MBps
5 MBps
12.5 MBps
40 MBps
50 MBps
64 MBps
125 MBps
533 MBps
800 MBps

6 Mbps
10 Mbps
12 Mbps
30 Mbps
40 Mbps
100 Mbps
320 Mbps
400 Mbps
512 Mpbs
1.0 Gbps
4.2 Gbps
6.4 Gbps

Cable modem
Fast Ethernet
Ultra SCSI
Gigabit Ethernet
iMac, old G3 RAM (66 MHz bus)
System RAM, Blue G3


8.0 GBps

64 Gbps

CPU-to-CPU, 500 MHz G4

I hope this helps you put the various modem, networking, drive, and memory speeds in perspective. For instance, it’s interesting to ponder the fact that Gigabit Ethernet can move data twice as fast between computers as a Mac II can move bytes on the motherboard. Or that poky old LocalTalk is about twice as fast as ISDN.

There’s a healthy reality check in there!

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