1. Local area Network overview
Networks and Communication Department
Chapter 15

2. Lecture Contents LAN applications LAN topologies LAN mediums
Interconnecting LANs
Bridges
Routers
17-Sep-18
Networks and Communication Department

3. LAN Applications
17-Sep-18
Networks and Communication Department

4. LAN Applications (1) personal computer LANs back end networks low cost
limited data rate
back end networks
interconnecting large systems (mainframes and large storage devices)
high data rate
high speed interface
distributed access
limited distance
limited number of devices
We turn now to a discussion of local area networks (LANs). The variety of applications for LANs is wide. To provide some insight into the types of requirements that LANs are intended to meet, this section provides a brief discussion of some of the most important general application areas for these networks.
A common LAN configuration is one that supports personal computers. LANs for the support of personal computers and workstations have become nearly universal in organizations of all sizes. Even those sites that still depend heavily on the mainframe have transferred much of the processing load to networks of personal computers. Perhaps the prime example of the way in which personal computers are being used is to implement client/server applications. For personal computer networks, a key requirement is low cost. In particular, the cost of attachment to the network must be significantly less than the cost of the attached device. Thus, for the ordinary personal computer, an attachment cost in the hundreds of dollars is desirable. For more expensive, high-performance workstations, higher attachment costs can be tolerated.
Backend networks are used to interconnect large systems such as mainframes, supercomputers, and mass storage devices. The key requirement here is for bulk data transfer among a limited number of devices in a small area. High reliability is generally also a requirement. Typical characteristics include: High data rates, High-speed interface, Distributed access, Limited distance, and Limited number of devices.

5. LAN Applications (2) storage area networks (SANs)
DEF : separate network to handling storage needs
detaches storage tasks from specific servers
shared storage facility
eg. hard disks, tape libraries, CD arrays
accessed using a high-speed network
eg. Fibre Channel
improved client-server storage access
direct storage to storage communication for backup
A concept related to that of the backend network is the storage area network (SAN). A SAN is a separate network to handle storage needs. The SAN detaches storage tasks from specific servers and creates a shared storage facility across a high-speed network. The collection of networked storage devices can include hard disks, tape libraries, and CD arrays. Most SANs use Fibre Channel, which is described in Chapter 16. In a SAN, no server sits between the storage devices and the network; instead, the storage devices and servers are linked directly to the network. The SAN arrangement improves client-to-storage access efficiency, as well as direct storage-to-storage communications for backup and replication functions.

6. Storage Area Networks
Stallings DCC8e Figure 15.1 contrasts the SAN with the traditional server-based means of supporting shared storage. In a typical large LAN installation, a number of servers and perhaps mainframes each has its own dedicated storage devices. If a client needs access to a particular storage device, it must go through the server that controls that device. In a SAN, no server sits between the storage devices and the network; instead, the storage devices and servers are linked directly to the network.

7. LAN Applications (3) high speed office networks backbone LANs
desktop image processing
high capacity local storage
backbone LANs
interconnect low speed local LANs
reliability
capacity
cost
Traditionally, the office environment has included a variety of devices with low- to medium-speed data transfer requirements. However, applications in today's office environment would overwhelm the limited speeds (up to 10 Mbps) of traditional LAN. Desktop image processors have increased network data flow by an unprecedented amount. Examples of these applications include fax machines, document image processors, and graphics programs on personal computers and workstations. In addition, disk technology and price/performance have evolved so that desktop storage capacities of multiple gigabytes are common. These new demands require LANs with high speed that can support the larger numbers and greater geographic extent of office systems as compared to backend systems.
The increasing use of distributed processing applications and personal computers has led to a need for a flexible strategy for local networking. Support of premises-wide data communications requires a networking service that is capable of spanning the distances involved and that interconnects equipment in a single (perhaps large) building or a cluster of buildings. Although it is possible to develop a single LAN to interconnect all the data processing equipment of a premises, this is probably not a practical alternative in most cases. A more attractive alternative is to employ lower-cost, lower-capacity LANs within buildings or departments and to interconnect these networks with a higher-capacity LAN. This latter network is referred to as a backbone LAN. If confined to a single building or cluster of buildings, a high-capacity LAN can perform the backbone function.

8. LAN Topologies
17-Sep-18
Networks and Communication Department

9. LAN Topologies
In the context of a communication network, the term topology refers to the way in which the end points, or stations, attached to the network are interconnected. The common topologies for LANs are shown in Stallings DCC8e Figure 15.2 and include: bus, tree, ring, and star. The bus is a special case of the tree, with only one trunk and no branches.

10. Bus and Tree used with multipoint medium
transmission propagates throughout medium
heard by all stations
full duplex connection between station and tap
allows for transmission and reception
need to regulate transmission
to avoid collisions and hogging
terminator absorbs frames at end of medium
tree a generalization of bus
headend connected to branching cables
Both bus and tree topologies are characterized by the use of a multipoint medium. For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.
The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend. One or more cables start at the headend, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts. Again, a transmission from any station propagates throughout the medium and can be received by all other stations.

11. Frame Transmission on Bus LAN
Two problems present themselves in these topologies. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended. Second, a mechanism is needed to regulate transmission. To solve these problems, stations transmit data in small blocks, known as frames. Each frame consists of a portion of the data that a station wishes to transmit, plus a frame header that contains control information. Each station on the bus is assigned a unique address, or identifier, and the destination address for a frame is included in its header.
Stallings DCC8e Figure 15.3 illustrates the scheme. In this example, station C wishes to transmit a frame of data to A. The frame header includes A's address. As the frame propagates along the bus, it passes B. B observes the address and ignores the frame. A, on the other hand, sees that the frame is addressed to itself and therefore copies the data from the frame as it goes by. The use of frames also provides the basic tool for solving the second problem, the regulation of access. In particular, the stations take turns sending frames in some cooperative fashion. This involves putting additional control information into the frame header.

12. Ring Topology
a closed loop of repeaters joined by point to point links
receive data on one link & retransmit on another
links unidirectional
stations attach to repeaters
data in frames
circulate past all stations
destination recognizes address and copies frame
frame circulates back to source where it is removed
media access control determines when a station can insert frame
In the ring topology, the network consists of a set of repeaters joined by point-to-point links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received. The links are unidirectional; that is, data are transmitted in one direction only, so that data circulate around the ring in one direction (clockwise or counterclockwise). Each station attaches to the network at a repeater and can transmit data onto the network through the repeater. As with the bus and tree, data are transmitted in frames. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

13. Frame Transmission Ring LAN
Stallings DCC8e Figure 15.4 illustrates how a frame continues to circulate until it returns to the source station, where it is removed .

14. Star Topology each station connects to central node
usually via two point to point links
either central node can broadcast
physical star, logical bus
only one station can transmit at a time
or central node can act as frame switch
In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links. In this case, although the arrangement is physically a star, it is logically a bus: A transmission from any station is received by all other stations, and only one station at a time may successfully transmit. In this case, the central element is referred to as a hub. Another approach is for the central node to act as a frame-switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.

15. Choice of Topology reliability expandability performance
needs considering in context of:
medium
wiring layout
access control
The choice of topology depends on a variety of factors, including reliability, expandability, and performance. This choice is part of the overall task of designing a LAN and thus cannot be made in isolation, independent of the choice of transmission medium, wiring layout, and access control technique.

16. LAN Mediums
17-Sep-18
Networks and Communication Department

17. Media Available Voice grade unshielded twisted pair (UTP)
Cat 3 phone, cheap, low data rates
Shielded twisted pair / baseband coaxial
more expensive, higher data rates
Broadband cable
even more expensive, higher data rate
High performance UTP
Cat 5+, very high data rates, switched star topology
Optical fibre
security, high capacity, small size, high cost

18. Choice of Medium Capacity: to support the expected network traffic
Reliability: to meet requirements for availability
Types of data supported: tailored to the application
Environmental scope: to provide service over the range of environments required
The choice of transmission medium is determined by a number of factors. It is, we shall see, constrained by the topology of the LAN. Other factors come into play, including
• Capacity: to support the expected network traffic
• Reliability: to meet requirements for availability
• Types of data supported: tailored to the application
• Environmental scope: to provide service over the range of environments required
The choice is part of the overall task of designing a local network, which is addressed in Chapter 16. Here we can make a few general observations.

19. Interconnecting LANs
17-Sep-18
Networks and Communication Department

20. Interconnecting LANs
there is a need to expand a single LAN, to provide interconnection to other LANs and to wide area networks.
Two general approaches are used for this purpose: bridges and routers.
The bridge is the simpler of the two devices and provides a means of interconnecting similar LANs.
The router is a more general-purpose device, capable of interconnecting a variety of LANs and WANs.
In virtually all cases, there is a need to expand beyond the confines of a single LAN, to provide interconnection to other LANs and to wide area networks. Two general approaches are used for this purpose: bridges and routers. The bridge is the simpler of the two devices and provides a means of interconnecting similar LANs. Because the devices all use the same protocols, the amount of processing required at the bridge is minimal. More sophisticated bridges are capable of mapping from one MAC format to another. There are several reasons for the use of multiple LANs connected by bridges:
• Reliability: By using bridges, the network can be partitioned into self-contained units, to provide fault isolation
• Performance: A number of smaller LANs will often give improved performance if devices can be clustered so that intranetwork traffic significantly exceeds internetwork traffic.
• Security: The establishment of multiple LANs may improve security of communications, keeping different types of traffic with different security needs on physically separate media.
• Geography: Clearly, two separate LANs are needed to support devices clustered in two geographically distant locations.

21. Wireless LAN
Networks and Communication Department
Chapter 17

22. Lecture Contents Applications Requirements Technologies 17-Sep-18
Networks and Communication Department

23. Overview of Wireless LANs
use wireless transmission medium
issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed
key application areas:
LAN extension
cross-building interconnect
nomadic access
ad hoc networking
A wireless LAN makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used. The reasons for this included high prices, low data rates, occupational safety concerns, and licensing requirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly.
Will now consider four application areas for wireless LANs in turn: LAN extension, cross-building interconnect, nomadic access, and ad hoc networks.

24. Applications
17-Sep-18
Networks and Communication Department

25. Single Cell LAN Extension
Early wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired LANs. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. However, this motivation for wireless LANs was overtaken by events. In a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN. Typically an organization will also have a wired LAN to support servers and some stationary workstations. Therefore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension.
Stallings DCC8e Figure 17.1 indicates a simple wireless LAN configuration that is typical of many environments. There is a backbone wired LAN, such as Ethernet, that supports servers, workstations, and one or more bridges or routers to link with other networks. In addition, there is a control module (CM) that acts as an interface to a wireless LAN. The control module includes either bridge or router functionality to link the wireless LAN to the backbone. It includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems. Note that some of the end systems are standalone devices, such as a workstation or a server. Hubs or other user modules (UMs) that control a number of stations off a wired LAN may also be part of the wireless LAN configuration. This configuration can be referred to as a single-cell wireless LAN; all of the wireless end systems are within range of a single control module.

26. Multi Cell LAN Extension
Stallings DCC8e Figure 17.2 illustrates another common configuration, a multiple-cell wireless LAN. In this case, there are multiple control modules interconnected by a wired LAN. Each control module supports a number of wireless end systems within its transmission range. For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support.

27. Cross-Building Interconnect
connect LANs in nearby buildings
point-to-point wireless link
Not a LAN per se
connect bridges or routers
Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.

28. Nomadic Access link LAN hub & mobile data terminal
laptop or notepad computer
enable employee to transfer data from portable computer to server
also useful in extended environment such as campus or cluster of buildings
users move around with portable computers
may wish access to servers on wired LAN
Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personal portable computer to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable computers and may wish access to the servers on a wired LAN from various locations.

29. Infrastructure Wireless LAN
Stallings DCC8e Figure 17.3 suggests the differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. In the former case, the wireless LAN forms a stationary infrastructure consisting of one or more cells with a control module for each cell. Within a cell, there may be a number of stationary end systems. Nomadic stations can move from one cell to another.

30. Ad Hoc Networking temporary peer-to-peer network
An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting. Stallings DCC8e Figure 17.3 suggests the differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. In contrast to the previous slide, there is no infrastructure for an ad hoc network. Rather, a peer collection of stations within range of each other may dynamically configure themselves into a temporary network.

31. Requirements
17-Sep-18
Networks and Communication Department

32. Wireless LAN Requirements
throughput - efficient use wireless medium
no of nodes - hundreds of nodes across multiple cells
connection to backbone LAN - using control modules
service area to 300 m
low power consumption - for long battery life on mobiles
transmission robustness and security
collocated network operation
license-free operation
handoff/roaming
dynamic configuration - addition, deletion, and relocation of end systems without disruption to users
A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment:
• Throughput: The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.
• Number of nodes: to support hundreds of nodes across multiple cells.
• Connection to backbone LAN: interconnection with stations on a wired backbone LAN through control modules connecting both types of LANs.
• Service area: typical coverage area has a diameter of 100 to 300 m.
• Battery power consumption: Mobile workers use battery-powered workstations that need a long battery life when used with wireless adapters.
• Transmission robustness and security: a wireless LAN may be especially vulnerable to interference and eavesdropping.
• Collocated network operation: likely that two or more wireless LANs operate in same or adjacent areas with possible interference between LANs.
• License-free operation: want wireless LAN products without having to secure a license for the frequency band used by the LAN.
• Handoff/roaming: enable mobile stations to move from one cell to another.
• Dynamic configuration: The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.

33. Technologies
17-Sep-18
Networks and Communication Department

34. Technologies infrared (IR) LANs spread spectrum LANs
individual cell of IR LAN limited to single room
IR light does not penetrate opaque walls
spread spectrum LANs
mostly operate in ISM (industrial, scientific, and medical) bands
no Federal Communications Commission (FCC) licensing is required in USA
narrowband microwave
microwave frequencies but not use spread spectrum
some require FCC licensing
Wireless LANs are generally categorized according to the transmission technique that is used. All current wireless LAN products fall into one of the following categories:
• Infrared (IR) LANs: An individual cell of an IR LAN is limited to a single room, because infrared light does not penetrate opaque walls.
• Spread spectrum LANs: This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (industrial, scientific, and medical) microwave bands so that no Federal Communications Commission (FCC) licensing is required for their use in the United States.

35. Internet Protocols Chapter 18 Networks and Communication Department
17-Sep-18

36. Addressing
The concept of addressing in a communications architecture is a complex one and covers a number of issues, including:
Addressing level
Addressing mode
The concept of addressing in a communications architecture is a complex one and covers a number of issues, including:
• Addressing level
• Addressing mode
During the discussion, we illustrate the concepts using Figure 18.2, which shows a configuration using the TCP/IP architecture. The concepts are essentially the same for the OSI architecture or any other communications architecture.

37. Addressing Level
Refer to the level in architecture where entity is named.
have a unique address for each intermediate (e.g. router) and end system (e.g. workstation or server).
usually a network-level address
e.g. IP address or internet address
e.g. OSI - network service access point (NSAP)
at destination data must routed to some process
e.g. TCP/IP port
e.g. OSI service access point (SAP)
Addressing level refers to the level in the communications architecture at which an entity is named. Typically, a unique address is associated with each end system (e.g., workstation or server) and each intermediate system (e.g., router) in a configuration. Such an address is, in general, a network-level address. In the case of the TCP/IP architecture, this is referred to as an IP address, or simply an internet address. In the case of the OSI architecture, this is referred to as a network service access point (NSAP).

38. Addressing Mode address usually refers to single system
individual or unicast address
It is also possible for an address to refer to more than one entity or port . Such an address identifies multiple simultaneous recipients for data.
An address for multiple recipients may be :
broadcast for all entities within domain
multicast for specific subset of entities
Another addressing concept is that of addressing mode. Most commonly, an address refers to a single system or port; in this case it is referred to as an individual or unicast address. It is also possible for an address to refer to more than one entity or port. Such an address identifies multiple simultaneous recipients for data. For example, a user might wish to send a memo to a number of individuals. The network control center may wish to notify all users that the network is going down. An address for multiple recipients may be broadcast, intended for all entities within a domain, or multicast, intended for a specific subset of entities. Stallings DCC8e Table 18.1 illustrates some possibilities.

39. Internetworking Terms
Communication Network - A facility that provides a data transfer service among devices attached to the network.
Internet - collection of networks interconnected by bridges and/or routers.
Intranet - An internet used by a single organization that provides the key Internet applications.

40. Internetworking Terms
End System (ES) - A device attached to one of the networks of an internet that is used to support end- user applications or services.(e.g.: server)
Intermediate System (IS) - A device used to connect two networks and permit communication between end systems attached to different networks.(e.g: router, bridge(

41. Requirements of Internetworking
Link between networks
Routing and delivery of data between processes on different networks
Provide an accounting service that keeps track of the use of the various networks and routers and maintains status information.
The overall requirements for an internetworking facility are as follows (we refer to Stallings DCC8e Figure 18.2 as an example throughout):
1. Provide a link between networks. At minimum, a physical and link control connection is needed. (Router J has physical links to N1 and N2, and on each link there is a data link protocol.)
2. Provide for the routing and delivery of data between processes on different networks. (Application X on host A exchanges data with application X on host B.)
3. Provide an accounting service that keeps track of the use of the various networks and routers and maintains status information. 4.

42. References Chapter 15. Chapter 17 Chapter 18 17-Sep-18
Networks and Communication Department