NETWORKING AND INTERNETWORKING
SECTION 3.5 CASE STUDIES: ETHERNET, WIFI AND BLUETOOTH 131 bridges operate at the level of Ethernet frames, forwarding them to adjacent Ethernets
SECTION 3.5 CASE STUDIES: ETHERNET, WIFI AND BLUETOOTH 131
132 CHAPTER 3 NETWORKING AND INTERNETWORKING
Ethernet packet layout • The packets (or more correctly, frames) transmitted by stations on the Ethernet have the following layout:
bytes: 7 1 6 6 2 46 < length < 1500 4
Preamble S Destination address
Source address
Length of data
Data for transmission Checksum
Apart from the destination and source addresses already mentioned, frames include a fixed 8-byte prefix, a length field, a data field and a checksum. The prefix is used for hardware timing purposes and consists of a preamble of 7 bytes, each containing the bit pattern 10101010 followed by a single-byte start frame delimiter (S in the diagram) with the pattern 10101011.
Despite the fact that the specification does not allow more than 1024 stations on a single Ethernet, addresses occupy 6 bytes, providing 248 different addresses. This enables every Ethernet hardware interface to be given a unique address by its manufacturer, ensuring that all of the stations in any interconnected set of Ethernets will have unique addresses. The US Institute of Electrical and Electronic Engineers (IEEE) acts as an allocation authority for Ethernet addresses, allocating separate ranges of 48- bit addresses to the manufacturers of Ethernet hardware interfaces. These are referred to as MAC addresses, since they are used by the medium access control layer. In fact, MAC addresses allocated in this fashion have also been adopted as unique addresses for use in other network types in the IEEE 802 family, including 802.11 (WiFi) and 802.15.1 (Bluetooth).
The data field contains all or part (if the message length exceeds 1500 bytes) of the message that is being transmitted. The lower bound of 46 bytes on the data field ensures a minimum packet length of 64 bytes, which is necessary in order to guarantee that collisions will be detected by all stations on the network, as explained below.
The frame check sequence is a checksum generated and inserted by the sender and used to validate packets by the receiver. Packets with incorrect checksums are simply dropped by the data link layer in the receiving station. This is another example of the application of the end-to-end argument: to guarantee the transmission of a message, a transport-layer protocol such as TCP, which acknowledges receipt of each packet and retransmits any unacknowledged packets, must be used. The incidence of data corruption in local networks is so small that the use of this method of recovery when guaranteed delivery is required is entirely satisfactory and it enables a less costly transport protocol such as UDP to be employed when there is no need for delivery guarantees.
Packet collisions • Even in the relatively short time that it takes to transmit packets there is a finite probability that two stations on the network will attempt to transmit messages simultaneously. If a station attempts to transmit a packet without checking whether the medium is in use by other stations, a collision may occur.
The Ethernet has three mechanisms to deal with this possibility. The first is called carrier sensing: the interface hardware in each station listens for the presence of a signal (known as the carrier by analogy with radio broadcasting) in the medium. When a station wishes to transmit a packet, it waits until no signal is present in the medium and then begins to transmit.
SECTION 3.5 CASE STUDIES: ETHERNET, WIFI AND BLUETOOTH 133 Unfortunately, carrier sensing does not prevent all collisions. The possibility of collision remains due to the finite time W for a signal inserted at a point in the medium (travelling at electronic speed: approximately 2 u 108 metres per second) to reach all other points. Consider two stations A and B that are ready to transmit packets at almost the same time. If A begins to transmit first, B can check and find no signal in the medium at any time t < W after A has begun to transmit. B then begins to transmit, interfering with A’s transmission. Both A’s packet and B’s packet will be damaged by the interference.
The technique used to recover from such interference is called collision detection.
Whenever a station is transmitting a packet through its hardware output port, it also listens on its input port and the two signals are compared. If they differ, then a collision has occurred. When this happens the station stops transmitting and produces a jamming signal to ensure that all stations recognize the collision. As we have already noted, a minimum packet length is necessary to ensure that collisions are always detected. If two stations transmit approximately simultaneously from opposite ends of the network, they will not become aware of the collision for 2W seconds (because the first sender must be still transmitting when it receives the second signal). If the packets that they transmit take less than W to be broadcast, the collision will not be noticed, since each sending station would not see the other packet until after it has finished transmitting its own, whereas stations at intermediate points would receive both packets simultaneously, resulting in data corruption.
After the jamming signal, all transmitting and listening stations cancel the current packet. The transmitting stations then have to try to transmit their packets again. A further difficulty now arises. If the stations involved in the collision all attempt to retransmit their packets immediately after the jamming signal, another collision will probably occur. To avoid this, a technique known as back-off is used. Each of the stations involved in a collision chooses to wait a time nW before retransmitting. The value ofn is a random integer chosen separately at each station and bounded by a constant L defined in the network software. If a further collision occurs, the value of L is doubled and the process is repeated if necessary for up to 10 attempts.
Finally, the interface hardware at the receiving station computes the check sequence and compares it with the checksum transmitted in the packet. Using all of these techniques, the stations connected to the Ethernet are able to manage the use of the medium without any centralized control or synchronization.
Ethernet efficiency • The efficiency of an Ethernet is the ratio of the number of packets transmitted successfully as a proportion of the theoretical maximum number that could be transmitted without collisions. It is affected by the value of W, since the interval of 2W seconds after a packet transmission starts is the ‘window of opportunity’ for collisions – no collision can occur later than 2W seconds after a packet starts to be transmitted. It is also affected by the number of stations on the network and their level of activity.
For a 1 km cable, the value of W is less than 5 microseconds and the probability of collisions is small enough to ensure high efficiency. The Ethernet can achieve a channel utilization of between 80 and 95%, although the delays due to contention become noticeable when 50% utilization is exceeded. Because the loading is variable, it is impossible to guarantee the delivery of a given message within any fixed time, since the network might be fully loaded when the message is ready for transmission. But the
134 CHAPTER 3 NETWORKING AND INTERNETWORKING
probability of transferring the message with a given delay is as good as, or better than, that of other network technologies.
Empirical measurements of the performance of an Ethernet at Xerox PARC, reported by Shoch and Hupp [1980], confirm this analysis. In practice, the load on Ethernets used in distributed systems varies quite widely. Many networks are used primarily for asynchronous client-server interactions, and these operate for most of the time with no stations waiting to transmit. Their low level of contention results in a channel utilization close to 1. Networks that support bulk data access for large numbers of users experience more load, and those that carry multimedia streams are liable to be overwhelmed if more than a few streams are transmitted concurrently.
Physical implementations • The description above defines the MAC-layer protocol for all Ethernets. Widespread adoption across a large marketplace has resulted in the availability of very low-cost controller hardware to perform the algorithms required for its implementation, and this is included as a standard part of many desktop and consumer computers.
A wide range of physical Ethernet implementations have been based on it to offer a variety of performance and cost trade-offs and to exploit increased hardware performance. The variations result from the use of different transmission media – coaxial cable, twisted copper wire (similar to telephone wiring) and optical fibre – with differing limits on transmission range, and from the use of higher signalling speeds, resulting in greater system bandwidth and generally shorter transmission ranges. The IEEE has adopted a number of standards for physical-layer implementations, and a naming scheme is used to distinguish them. Names such as 10Base5 and 100BaseT are used. They have the following form:
<R><B><L> Where: R= data rate in Mbps
B= medium signalling type (baseband or broadband) L = maximum segment length in metres/100 or T
(twisted pair cable hierarchy)
We tabulate the bandwidth and maximum range of various currently available standard configurations and cable types in
Figure 3.23 Ethernet ranges and speeds
10Base5 10BaseT 100BaseT 1000BaseT
Data rate 10 Mbps 10 Mbps 100 Mbps 1000 Mbps
Max. segment lengths:
Twisted wire (UTP) 100 m 100 m 100 m 25 m
Coaxial cable (STP) 500 m 500 m 500 m 25 m
Multi-mode fibre 2000 m 2000 m 500 m 500 m
Mono-mode fibre 25000 m 25000 m 20000 m 2000 m
Figure 3.23. Configurations ending with the T designation are implemented with UTP cabling – unshielded twisted wires (telephone wiring) – and this is organized as a hierarchy of hubs with computers as the leaves of the tree. In that case, the segment lengths given in our table are twice the maximum permissible distance from a computer to a hub.
SECTION 3.5 CASE STUDIES: ETHERNET, WIFI AND BLUETOOTH 135 Ethernet for real-time and quality of service critical applications • It is often argued that the Ethernet MAC protocol is inherently unsuitable for real-time or quality of service critical applications because of its lack of a guaranteed delivery delay. But it should be noted that most Ethernet installations are now based on the use of MAC-level switches, as illustrated in Figure 3.10 and described in Section 3.3.7 (rather than hubs or cables with a tap for each connection, as was formerly the case). The use of switches throughout results in a separate segment for each host with no packets transmitted on it other than those addressed to that host. Hence if traffic to the host is from a single source, there is no contention for the medium – efficiency is 100% and latency is constant. The possibility of contention arises only at the switches, and these can be, and often are, designed to handle several packets concurrently. Hence a lightly loaded switched Ethernet installation approximates to 100% efficiency with a constant low latency, and they are therefore often successfully used in these critical application areas.
A further step towards real-time support for Ethernet-style MAC protocols is described in [Rether; Pradhan and Chiueh 1998] and a similar scheme is implemented in an open-source Linux extension [RTnet]. These software approaches address the contention problem by implementing an application-level cooperative protocol to reserve timeslots for the use of the medium. This protocol depends upon the cooperation of all the hosts connected to a segment.
3.5.2 IEEE 802.11 (WiFi) wireless LAN
In this section, we summarize the special characteristics of wireless networking that must be addressed by a wireless LAN technology and explain how IEEE 802.11 addresses them. The IEEE 802.11 standard extends the carrier-sensing multiple access (CSMA) principle employed by Ethernet (IEEE 802.3) technology to suit the characteristics of wireless communication. The 802.11 standard is intended to support communication between computers located within about 150 metres of each other at speeds up to 54 Mbps.
Figure 3.24 illustrates a portion of an intranet including a wireless LAN. Several mobile wireless devices communicate with the rest of the intranet through a base station that is an access point to the wired LAN. A wireless network that connects to the world through an access point to a conventional LAN is known as an infrastructure network.
An alternative configuration for wireless networking is known as an ad hoc network. Ad hoc networks do not include an access point or base station. They are built
‘on the fly’ as a result of the mutual detection of two or more mobile devices with wireless interfaces in the same vicinity. An ad hoc network might occur, for example, when two or more laptop users in a room initiate a connection to any available station.
They might then share files by launching a file server process on one of the machines.
At the physical level, IEEE 802.11 networks use radio frequency signals (in the licence-free 2.4 GHz and 5 GHz bands) or infrared signalling as the transmission medium. The radio version of the standard has received the most commercial attention, and we shall describe that. The IEEE 802.11b standard was the first variant to see widespread use. It operates in the 2.4 GHz band and supports data communication at up to 11 Mbps. It has been installed from 1999 onwards with base stations in many offices, homes and public places, enabling laptop computers and handheld devices to access local networked devices or the Internet. IEEE 802.11g is a more recent enhancement of
136 CHAPTER 3 NETWORKING AND INTERNETWORKING
802.11b that uses the same 2.4 GHz band but a different signalling technique to achieve speeds up to 54 Mbps. Finally, the 802.11a variant works in the 5 GHz band and delivers a more certain 54 Mbps of bandwidth over a somewhat shorter range. All variants use various frequency-selection and frequency-hopping techniques to avoid external interference and mutual interference between independent wireless LANs, which we shall not detail here. We focus instead on the changes to the CSMA/CD mechanism that are needed in the MAC layer for all versions of 802.11 to enable broadcast data transmission to be used with radio transmission.
Figure 3.24 Wireless LAN configuration
LAN Server
Wireless LAN Laptops
Base station/
access point Tablet
Radio obstruction
A B C
D E
Like Ethernet, the 802.11 MAC protocol offers equal opportunities to all stations to use the transmission channel, and any station may transmit directly to any other. A MAC protocol controls the use of the channel by the various stations. As for the Ethernet, the MAC layer also performs the functions of both a data link layer and a network layer, delivering data packets to the hosts on a network.
Several problems arise from the use of radio waves rather than wires as the transmission medium. These problems stem from the fact that the carrier-sensing and collision-detection mechanisms employed in Ethernets are effective only when the strength of signals is approximately the same throughout a network.
We recall that the purpose of carrier sensing is to determine whether the medium is free at all points between the sending and receiving stations, and that of collision detection is to determine whether the medium in the vicinity of the receiver is free from interference during the transmission. Because signal strength is not uniform throughout the space in which wireless LANs operate, carrier detection and collision detection may fail in the following ways:
Hidden stations: Carrier sensing may fail to detect that another station on the network is transmitting. This is illustrated in Figure 3.24. If tablet D is transmitting to the base station E, laptop A may not be able to sense D’s signal because of the radio obstruction shown. A might then start transmitting, causing a collision at E unless steps are taken to prevent this.
SECTION 3.5 CASE STUDIES: ETHERNET, WIFI AND BLUETOOTH 137