Sharp R.The poor man's guide to computer networks and their applications.2004

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An ACK PDU to be sent from destination d to source s will be formatted as shown in Figure 4.6(b), where t is the 8-bit sequence 10000010, s and d are represented by 32-bit IP addresses, and c is a 32-bit checksum eld evaluated over all the other elds of the PDU according to the CRC-32 algorithm.

More complicated rules of procedure and a greater variety of PDU formats can be expected to occur in more realistic examples. In such cases, a more formal notation than ordinary prose is often preferred, in order to achieve a concise description with a high degree of precision. Most such notations are based on one of two principles for describing the behaviour of the protocol:

1.In terms of a state machine, which reacts to incoming events and produces outgoing events. Two well-known internationally standardised languages based on this principle are SDL (standardised by the International Telecommunications Union, ITU-T [29])and ESTELLE (standardised by the International Organization for Standardization, ISO [10]).

2.In terms of a set of interacting processes which exchange messages. Process algebraic languages such as CCS [30] and CSP [7], and the language LOTOS (standardised by ISO [9]) are typical examples of notations which have been used with this approach.

5Network Technology

These notes will not deal in any depth with network technology, and if you need to know more you will need to look at some of the more technology-oriented references. However, we shall try to explain what some of the commonly used terms mean, so that you understand what the salesman is talking about when he calls to sell you some network equipment.

Referring back to the introductory section on computer networks, you may be wondering what exactly the communication nodes and the end systems consist of, and how all this ts into the scheme of the OSI Reference Model (or its Internet variant). One typical answer is shown in Figure 5.1. In this example, the communication nodes implement the OSI layers up to and including the Network layer, and are thus responsible for:

Accepting PDUs on an incoming link from another node at the Data Link level.

Routing these PDUs to an outgoing link at the Network level.

Transmitting PDUs on an outgoing link to another node at the Data Link level.

Sending or receiving PDUs at the Data Link level of course requires the node to activate the facilities of the Physical layer in order to deal with the task of signalling on the physical medium.

Internal network

Router + Filter

To network C

To network A

Internet

To network B

Figure 5.2: A combined router and lter on the boundary between an internal network and the Internet.

or subnet from attack by intruders such as hackers. Many modern routers in fact combine the functions of a router and a rewall in the same piece of equipment.

5.2Bridges

From Figure 5.1 you might get the impression that there is always a direct connection (via the Data Link layer) between any two routers in a network. This is not entirely true. Often the subnet (the part of the network) which lies between two routers is, for practical reasons, divided into a series of segments which are joined together by bridges. These implement a junction between two parts of the network in the Data Link layer, as illustrated in Figure 5.3.

If F is busy sending to or receiving from another node, the path cannot be set up, and C must try again later. But if the path can be set up, then C can send the PDU to F at the full data rate allowed by the network technology. The switch enables several pairs of nodes to talk to one another using the full network bandwidth at the same time, thus avoiding much of the contention which arises in the shared medium of a traditional shared Ethernet. Of course, there will still be contention when several senders simultaneously try to reach the same destination, but if this only happens rarely then switch technology allows you to build a network with an bigger overall data carrying capacity than a shared Ethernet operating at the same data rate.

5.3.2Wireless LAN Technology

Wireless LAN technology has become very important in recent years, as it o ers a convenient way to achieve not just distributed computing but mobile computing, where the computing systems are allowed to move physically from one site to another. In a Wireless LAN, signals from transmitting nodes propagate in all directions4, and can be picked up by other nodes which are within a certain range, usually somewhere between 25m (inside a building) and 300m (in the open air). There are three basic styles of Wireless LAN, which are illustrated in Figure 5.7. In the BSS architecture, a central, immobile Access Point (AP) performs coordinating functions, and is used as an intermediate station for all tra c between the set of mobile nodes which are within its range. In the ESS architecture, several Access Points are connected by a wired network, thus permitting mobile nodes to keep in contact, as long as they are within range of at least one Access Point. Tra c intended for the di erent Access Points is typically sent on di erent radio channels in the frequency band in use. In the Ad hoc architecture, there is no Access Point, and all the mobile nodes communicate directly with one another.

Practical computer systems which make use of Wireless LAN technology are nowadays almost all based on the IEEE 802.11 standard5. This prescribes two alternative MAC protocols, of which the most commonly used is closely related to the CSMA/CD protocol described above, and is known as the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol. The CA part of the name refers to a slightly di erent mechanism used to deal with contention: Instead of letting all nodes with a pending transmission send as soon as the channel seems to be unoccupied, and trying to detect collisions, the nodes use a time slot mechanism to choose an instant at which to test whether the channel really is unoccupied. Each node with a pending transmission chooses a slot at random, and the one which chooses the lowest number nds the channel really is unoccupied and thus wins the \race" to be allowed to use the channel. In a wireless network, this is not quite foolproof, as stations may not all be able to hear one another, so the receiving station

4At least in principle, since sometimes the sending or receiving antenna is directional. 5Actually, it is a family of standards, as we shall see later.

(c)

Figure 5.7: Basic architectures of Wireless LANs

(a)Basic Service Set (BSS)

(b)Extended Service Set (ESS)

(c)Ad hoc (IBSS or peer-to-peer)

must send a positive acknowledgment for receipt of a PDU. If the sender receives no ACK, it retransmits the PDU after a new random delay. This is illustrated in Figure 5.8.

The basic 802.11 Wireless LAN standard describes the operation of the Medium Access protocol and three variants of the Physical Layer protocol for:

Operation in the 2.4 GHz Industrial, Scienti c and Medical (ISM) wireless band, using the frequency-hopping spread spectrum (FHSS) technique to achieve a 1 Mbit/s data rate.

Operation in the 2.4 GHz wireless band, using the direct sequence spread spectrum (DSSS) technique to achieve a 1 or 2 Mbit/s data rate.

Operation in the infrared band to achieve a 1 or 2 Mbit/s data rate.

As time has passed, this standard has accumulated a considerable number of additional variants and amendments, indicated by extra letters after the number. Some of the ones you are most likely to meet are:

802.11a Describes the operation of the Physical Layer in the 5 GHz wireless band, to achieve a data rate up to 54 Mbit/s.

802.11b Describes the operation of the Physical Layer in the 2.4 GHz wireless band, to achieve a 1, 2, 5.5 or 11 Mbit/s data rate.

802.11e Enhancements to the Medium Access protocol to provide QoS.

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Position

Figure 5.8: Contention Avoidance in the CSMA/CA protocol

IFS is the Inter-Frame Space, a period of silence which separates one transmission from the rst reservation slot for the next transmission.

802.11g Describes an extension to achieve data rates of 22 or 54 Mbit/s in the 2.4 GHz wireless band.

802.11h Describes extensions to achieve Spectrum and Transmit Power Management, in order to use the 5 GHz band in Europe.

802.11i Describes security enhancements to the Medium Access protocol.

In Europe, important parts of the 5 GHz wireless band are reserved for other purposes, so the 802.11a technology is unlikely to be used (even if 802.11h is followed). Instead, the high data rates o ered by 802.11a are likely to be achieved by using equipment which follows the 802.11g standard.

6Basic Protocols in the Internet

In these notes, most of the examples will be concerned with the kind of protocols used in the Internet. An overview of some of the best known ones can be seen in Figure 6.1. There are no special Internet protocols assigned for use in the Data Link or Physical layers. The protocols in these layers are very technology dependent, and the appropriate choice will depend on the environment in which the network is to operate. For example, if communication takes place within a single building or building complex, it would be natural to base the network on LAN technology, and one of the IEEE 802.x protocols

Transfer of data between arbitrary NETWORK systems in arbitrary networks.

Connectionless-mode: IP

Figure 6.1: Protocols in the Internet layered architecture