Labels, or virtual circuit identifiers, are used by the switches to forward traffic along dedicated multiplexed channels. This technology is widely used in backbone networks, where routing is simple, but speed is of the essence. These interlinked switches are often represented as a ‘network cloud’ at the IP layer, concealing the details of transport that doesn’t entirely fit the IP model.

10.4 Alternative network transport technologies

10.4.1Medium sharing

There are two main strategies for sharing media.

•Deterministic sharing: every host is given a predictable chance to send data at a basic minimum rate, e.g. token rings. This concept is easy to predict and sell, and so has often been adopted in commercial technologies.

•Non-deterministic sharing: any host has only a finite chance of being able to send a message – there is no minimum rate, e.g. Ethernet. This method uses the assumption that most LANs are only lightly loaded, so that the probability of transmission is usually high.

These are reflected in the prevalent technologies and each has its usage. Being somewhat simplistic, one might say that well below capacity non-deterministic sharing is the most efficient way of sharing available resources, but as we approach saturation it fails badly and a deterministic approach is required. This is simply because it is easy to share when everyone has plenty; but when competition for resources gets tough, some kind of enforcement of sharing is required.

10.4.2Token rings

There are several kinds of token rings, including the now defunct Fiber Distributed Data Interface (FDDI). Token ring LANs are widely used in IBM networks. The basic idea is that all hosts are arranged in a ring and that packets circulate unidirectionally around the ring. In order to avoid collisions, a control packet (called the token) is circulated around the ring and a host can only begin transmitting if it has received the token. The host can then transmit for a maximum amount of time and it must then pass on the token to the next host. As data are transmitted, all hosts look to see if the packets were intended for them, i.e. anyone can receive all of the time.

10.4.3Ethernet

Ethernet technology was developed by Xerox, Intel and DEC in 1976, at the Palo Alto Research Center (PARC) [103]. In the Ethernet bus approach, every host is connected to a common cable or bus. Ethernet naturally supports broadcasting, since all hosts share the same channel, but it also means that packets can collide and must contend for transmission over each cable segment. This is called

CSMA/CD, or Carrier Sense Multiple Access/Collision Detect. A collision occurs when two hosts attempt to send signals simultaneously.

The naming convention for Ethernet is:

•10Base-T = 10 Mbps, over two twisted-pair cables.

•100Base-T2 = 100 Mbps, over two twisted-pair cables.

•100Base-T4 = 100 Mbps, over four-twisted pair cables.

•1000Base-LX/FX/SX = 1000 Mbps, long wavelength over optical fiber cable.

The latter is nicknamed Gigabit Ethernet.

Ethernet collisions occur when two hosts try to send data at the same time. To give all hosts an equal opportunity to use the shared line, there is a Maximum Transmission Unit (MTU) or Ethernet frame size of 1500 bytes, which limits the time that a host can use the line.

Before sending a frame, the interface checks to see if its receiver sees any existing transmission, if so it waits until the coast is clear. As the speed (bit rate) of Ethernet transmission increases and LANs grow in size, the finiteness of the speed of light becomes noticeable and it is increasingly likely for two hosts to start sending at the same time, at different parts of a cable, before detecting each others’ signal. When this occurs, there is a ‘collision’ and a burst of noise is transmitted to inform all hosts of a collision: both hosts must stop and retry after a short wait.

•An interface can only detect a collision while it is transmitting itself, so we must be careful to prevent the sending of packets that are too short. (If incoming power is greater than outgoing power, there must be a collision.)

•Each bit is encoded by a single wavelenth λ of electromagnetic waves in Ethernet’s Manchester bit-encoding. If packets are long enough then they must fill up the physical size of the cable, because the combined wavelength N λ is larger than the cable size. Thus hosts will not be able to avoid seeing one another’s transmissions at some time during the transmission. We should therefore use a minimum packet size to be certain of detecting all collisions.

Suppose we have a cable segment of length L. The worst case scenario is when two hosts A and B at opposite ends of the cable start transmitting, just as the signal arrives from A to B. In order to detect a collision, a signal must then have travelled a distance of L to the collision point, and then the noise burst must travel back the same distance to be detected by A, before the message finished transmitting. Thus:

N λ ≥ 2L

or in terms of bit-rate (frequency) f ,

N ccopper ≥ 2f L,

where ccopper is the speed of light in copper or fiber, which are both of the order 2 × 108 meters per second. The left-hand side is the distance occupied

by N bits, or wavelengths of the bit signal, and the right-hand side is the distance of the round-trip. There are two variables to fix N or L. The Ethernet standard chooses to fix the minimum size of frames to be 512 bits (64 bytes), giving a maximum length of about 5000 meters for 10Mbs Ethernet; this is usually halved to 2500 meters to allow a wide margin for error. Any small data payloads are padded out with zeros. At gigabit speeds, even larger MTUs are required to extend the length limits of the cables; even so, they cannot be very long.

Other limits on the size of Ethernet segments come from the physical properties of cables, i.e. signal attenuation. The Ethernet standard allows for a maximum of four repeaters (amplifiers), so the average length of each segment above can be divided by the number of repeaters needed to get the signal across this maximum length.

10.4.4Digital Subscriber Line (DSL)

DSL includes variations such as ADSL, RADSL, SDSL, HDSL, VDSL, and is a family of multiplexing transmission methods that uses the existing telecommunications infrastructure. Asymmetric DSL (ADSL) is asymmetric in that it has a higher download capacity than upload capacity. This is an arbitrary decision, derived from observed usage. SDSL is a symmetric version of this. RADSL is a Rate Adaptive DSL that adds some prioritization to traffic types.

DSL technologies employ improved transceiver technology to transmit data more efficiently over copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone exchange or cable television operator to the end user. They are not routable. DSL can transmit at rates limited only by the physical properties of the cable, whereas telephone modem channels are restricted by the bandwidth of filters designed to give a clear voice signal.

10.4.5Integrated Services Digital Network (ISDN)

This is an international communications standard for sending voice, video and data over digital telephone lines or normal telephone wires. ISDN allows multiple devices to share a single line and supports data transfer rates of 64 Kbps and has never really taken off. ISDN was more widely deployed in Europe, but has quickly lost ground to cable and telephone DSL solutions.

10.4.6Fiber: SONET/SDH

After the break up of AT&T, the newly formed Bellcore began to work on the Synchronous Optical Network (SONET). Later the International Telecommunications Union (ITU) joined the effort and renamed the standards slightly, calling the standard not SONET but the Synchronous Digital Hierarchy (SDH). In SDH-speak STM-n is a SONET optical carrier OC-3n or Synchronous Transport Signal STS-3n for the electrical (copper) version.