Embed Size (px)
Citation preview
Topics:
1. LAN Characteristics.
2. LAN Topologies.
3. Topologies and LLC Protocols.
4. Topologies and MAC Protocols (Star, Bus, Ring, etc)
5. Hubs/Switches (intelligent Hubs).
6. Network Access.
7. Internetworks: (Bridges, Routers and Gateways)
8. Distributed Environments
9. What Network to buy?
1. Why LANS?
• widespread distribution of cheap stand-alone microcomputer systems
• Users need to share resources such as:
• relatively expensive peripherals
• information such as common data files and shared databases.
• A network allows computer systems to communicate directly with each other over a common communications system
• connect terminals & PCs to powerful host computers.
• access to shared resources, e.g. high quality printers, large disks
• transfer information, e.g. programs, data, mail.
2. LAN characteristics:
• transmission medium is shared by all devices
• connected by a common cable
• transmission by one device is received by all others, i.e. a broadcast network
• transmission in the form of packets
• limited distribution of machines; up to 10 km and typically around 1km
• typically restricted to a single site, e.g. an industrial plant
• high data rate; typically 10 times faster than WANs
• sharing of resources, e.g. users accessing common fileservers, printers, plotters, etc.,
• WANs are usually used to transfer information between sites
• single ownership of all elements of the network.
• connection of incompatible equipment to the network, i.e. machines and software from different manufacturers
• Since the communication lines are not owned by the PTT their bandwidth is not limited artificially and the error incidence is lower.
• Thus much higher data rates can be maintained and the protocols can be simpler since efficiency is not as crucial a consideration.
• Further, not using PTT lines automatically restricts the network to a single private site and hence restricts the overall diameter of the network.
3. LAN topologies
• LANs are usually described in terms of their topology (logical layout).
Star, bus and ring network topologies
• Note that the topologies shown above are LOGICAL topologies – physical layout may be different.
• bus networks are usually physically wired as a backbone spine off . which individual cable segments are dropped by repeaters or bridges, e.g. on each floor.
• Often the shorter segments are of lower specification but there are strict specifications on segment lengths, number of stations per segment, number of repeaters, etc.
4. MAC and LLC protocols
• The lower three layers (Physical, Data Link and Network) of the ISO 7 layer model are usually called the subnet deal with the communications aspects of the network.
• Application layers (4, 5, 6 and 7) are looked after by the stations
• Physical Layer is the physical node to node link transfers raw data (bits)
• Data Link Layer manages the node to node data transmission (flow control,•error correction, etc) which transfers data frames
• Network Layer manages the network and the station to node link. Data transfer is in the form of packets.
• The majority of LANs are broadcast networks all stations share a common communication medium all receive the transmitted signal.
• In this case the Data Link Layer is split into two protocols:
• MAC (Medium Access Control protocol)
• determines which device has access to the medium (cable) at any time.
• for example; bus topology.
• LLC (Logical Link Control protocol)
• manages the link, i.e. flow control, error correction (corrupt frames, lost frames, multiple frames), etc.
• Host computers connected to NIUs (Network Interface Units) which handle the communication across the network
In the past NIUs were seperate units, today are usually built onto motherboard
• In broadcast networks the MAC protocol also deals with physical NIU addressing, e.g. MAC address
• LAN network layer manages the host/NIU interface and splitting the host messages up into packets.
• In the case of a WAN addressing and routing is a function of the Network layer.
5. Topologies and MAC protocols
5.1 Star configuration
• Similar in concept to telephones connected to a an exchange.
• Common in the early 1980's when hard disks were very expensive; the machine at the centre of the star acting as a fileserver for the others.
• Today many networks are physically wired as stars using network hubs although their logical structure may be bus or rings.
5.2 Bus configuration e.g. Ethernet, IEEE 802.3
• Machines physically tap into a common cable
• Stations multi dropped off the cable with transmission by one station propagating in both directions along the bus.
• All other stations 'hear' this transmission and the intended destination copies the data content.
• The signals are absorbed by terminators at each end of the segment.
• Most bus networks are based on Ethernet developed by Xerox.
• The medium was usually coaxial cable which has been replaced over the past few years by twisted pair cable.
• The bus is a passive mechanism
• when no device is transmitting the cable is idle (no signal present)
• if any node crashes the remainder of the network remains operational.
• No need to remove messages from the bus; resistors absorb signals.
• Message needs to be acknowledged explicitly by the receiving device, e.g. using a stop and wait protocol.
• Most bus networks are based on the Ethernet specification and use a contention MAC protocol for sharing access to the cable, see next section.
4.2.1 Bus MAC protocols
4.2.1.1 Carrier Sense Multiple Access (CSMA) protocol
• CSMA is an extension of a simple protocol developed at the University of Hawaii in the late 1960s.
• large number of islands which makes connection by cable very difficult
• use radio - all devices used the same frequency
• any device would send its message whenever it felt like it.
• If only one device sent, the signal would get through but if two or more sent at the same time, the signals would collide to produce garbage.
• If the signal was not corrupted the receiver would send an acknowledgement which, assuming no collision, would be received by the transmitter.
• This protocol is called ALLOHA and is very inefficient.
• if traffic is very low, this inefficiency doesn't matter.
• CSMA is a simple extension of this principle which improves the efficiency:
• no point in transmitting if someone else is already doing so
• then, how to improve on ALLOHA?
• Therefore:
• any station wishing to transmit first listens to see if cable is busy.
• If it is idle, the station sends its message otherwise it defers sending.
• How quickly it tries again is called the persistence; it may:
• keep sensing for an idle cable continuously (1-persistent) or
• back off for a random amount of time before trying again (non-persistent) or
• generate a random number 0<r<1 and retry only if r is greater than some number p (p-persistent). Each time it finds the cable busy the random wait increases.
• Sensing that the cable is idle does not guarantee that two stations will not send at the same time causing a collision, (WHY?)
• i.e. two signals corrupt each other.
• The LLC (Logical Link Control) protocol would resend the frame if an acknowledgement of successful receipt has is not been received in a given time, i.e. it will timeout and retransmit the frame.
5.2.1.2 Carrier Sense Multiple Access/ Collision Detect (CSMA/CD) protocol
• There is no point in continuing to send if a collision has occurred
• CSMA/CD - station listens to its own signal and aborts if it hears a collision:
• Transmits a short jamming signal and then waits a random amount of time before trying again.
• Ethernet detects a collision by the interface monitoring the signal level, i.e. if > maximum collision has occurred. •Attenuation is a problem so bus networks are restricted in length.
• Length also limited due to possibility of missing collision due to signal propagation time, see notes.
• Message may still be corrupt (noise on line) so LLC protocol would provide an acknowledgement of receipt of the data.
5.2.2.3 Throughput on CSMA/CD
• The protocol outlined above is called 1 persistent CSMA/CD in which stations wishing to transmit wait for the line to become free then transmit.
• this has the problem that as network traffic increases collisions are more frequent and unpredictable performance degradation occurs when the traffic reaches approximately 30% of the bandwidth capacity.
• In non-persistent CSMA/CD the stations wait a random amount of time before even looking to see if the line is clear then apply random delays if it is busy.
• This results in improved line utilization but at the cost of greater average delays in particular at low line utilization.
• CSMA/CD protocol - offered load vs. net utilization
• The use of intelligent hubs and bridges (see next section) to break up a large network can help to alleviate the problem.
parameter 10BASE5 10BASE2 10BASET 100BASET 10BROAD3
6
Transmission
medium
coaxial cable
(50 ohm)
coaxial cable
(50 ohm)
Unshielded
twisted pair
Unshielded
twisted pair
coaxial
cable
(75 ohm)
Signaling
technique
Baseband
(Manchester)
Baseband
(Manchester)
Baseband
(Manchester)
Baseband
(Manchester)
Broadband
(DPSK)
data rate
(Mbps)
10 10 10 100 10
maximum
segment length
500 185 100 100 1800
network span
(meters)
2500
(4 repeaters)
925
(4 repeaters)
500
(4 repeaters)
500
(4 repeaters)
3600
(1 repeater)
Nodes/segment 100 100 - - 100
cable diameter
(mm)
10
Thick
Ethernet
5
Thin Ethernet
0.4 - 0.6 0.4 - 0.6 0.4 - 1.0
5.2.2.4 IEEE 802.3 CSMA/CD bus standards
10BASE5 Was the original 802.3 standard based on the Ethernet bus using thick high quality (and expensive) co-axial cable. Due to the cable being high-quality it has low attenuation and the maximum segment length is 500 meters which can be extended to 2500 using a maximum of 4 repeaters (otherwise the collision window would become too large).
10BASE2 this is similar to 10BASE5 but uses thinner lower cost coaxial cable (sometimes called thin Ethernet or Cheapernet). Due to the lower quality cable the number of stations and the segment length is reduced (but is usually adequate for small office networks).
10BASET the T stands for unshielded twisted pair cable (cheap telephone cable) and, by sacrificing maximum segment length (100 meters), runs at a data rate of 10 Mbps (in the early 1980's this tended replace 10BASE2 in many networks).
100BASET is the latest addition to 802.3: runs at a data rate of 100 Mbps (this option is used where network load is high, e.g. systems using multimedia).
10BROAD36
is a broadband option providing support for more stations over a wider area but at greater cost.
5.2.2.5 Token passing bus
• Stations are connected physically by a common bus but with the stations logically organized (by the network software) as a ring, e.g. ARCnet.
• A token passing protocol (described below) is then used to control access to the bus.
• The software of token passing busses is very complex (handling such problems as nodes joining and leaving the network).
• They tend to be use for specialized applications where the advantages of both bus and ring are required, e.g. factory automation.
5.3 Ring configuration
• A ring network consists conceptually of a single loop of cable along which traffic flows in one direction.
• Cable passes through each node which repeats the signal so that its strength is continuously maintained.
• hence a ring can cover a larger distance than a bus
• the ring is an active mechanism
• if the cable is broken or a node crashes, the ring would be disabled so implementations contain duplicate cable loops and interface circuits
• Since a message is continuously regenerated by each station it must be explicitly removed.
• convention is that the sender removes its message when it returns round the ring;
• an acknowledgement of receipt can be 'piggybacked' onto the end of the original message by the receiver.
• However, interference on the ring could result in the transmitter not recognizing its message.
• the message would then circulate endlessly
• a special station, the monitor, had responsibility for clearing garbage messages.
5.3.1 Ring MAC Protocols
5.3.1.1 Token passing ring (IEEE 802.5)
• A special token packet (e.g. 11111111) circulates around the ring.
• If a station wants to transmit, it must wait until it receives the token.
• It seizes the token by flipping the last bit and converting it to a connector (11111110) and follows this by inserting its message packet.
• This transmitted bit stream then passes through each node on the ring.
• No other station can send since it hasn't got the token.
• Each station looks at the address in the message to see if it is addressed to it
• if not it ignores it and passes it on.
• If the address is that of the station it copies the packet into its buffers and flips an acknowledgement bit at the end of the packet to indicate receipt.
• The packet is repeated by each node and eventually arrives back at the sender node which:
• takes it off the ring.
• recreates the token and sends it on
• Thus a station may only use the token once before passing it on ensuring fair access to the network.
• Packets are normally kept small to prevent long messages hogging the ring.
•A monitor station is required to removed damaged frames, and recreate a missing token or remove duplicate tokens due to line errors.
5.3.1.2 FDDI Network (ANSI X3T9.5)
• FDDI makes use of the high bandwidth and noise immunity of fiber optic cable to operate at speeds greater than 100Mbit/sec and over distances of hundreds of kilometers.
• FDDI uses a Token passing protocol except that when a station waiting to transmit captures the token it transmits packets of information and then issues a new token which the next station can capture for its messages.
• Thus packets of information are circulating followed by a token.
• This slight change makes better use of the very high network bandwidth.
• If the network is broken the two rings reform to become a single ring.
The FDDI (Fiber Distributed Data Interface) Network
FDDI operating as a single ring when a break occurs in a link between nodes
• Up to 1000 nodes may be connected to an FDDI ring with a maximum spacing of 2km between nodes giving a maximum circumference of 200km.
• The major problem with FDDI is that it is expensive - tends to be used to interconnect buildings on a large site with high speed networks based on 100baseT used within the buildings.
6.1 Hubs (or wiring centers)
• 10Base2 networks using coaxial cables are usually multi-dropped from the cable using coaxial T connectors so that the physical and logical representations look similar, e.g.:
• 10BaseT connections are usually taken to a wiring centre called a hub which organizes the network into a bus. In this case the physical layout looks like a star although the logical layout is still a bus
• Rings often use a similar hub system with the stations connected on 'petals', e.g.:
• This still operates as a logical ring since all traffic follows the path:
•A - B - C - D - E - F - G - H - A - etc.
• Using wire centers makes networks much easier to install and to identify and isolate faulty loops, e.g. a broken cable, bad cable connector or faulty station.
6.2 Intelligent Hubs
• In cases where the load on a CSMA/CD bus network is heavy and many collisions would occur an intelligent (or active) hub many be used.
• In this case the hub has a processor and allocates RAM memory to buffer data going to/from the stations, removing the possibility of collisions, e.g. when the destination line is free the packets are transmitted.
• In addition the fileserver connection may be faster (e.g. 100BaseT) than the workstations (e.g. 10BaseT) to improve throughput.
7. Network Access:
Probabilistic or Deterministic
• Probabilistic access occurs when devices compete for access:
• CSMA/CD protocol - there is only a certain probability that a particular device will be granted access.
• There is no guarantee of access within a specified time and under heavy network loads a device may never get access.
• Deterministic access occurs when the protocol pre-determines when a device will be granted access:
• token passing protocol access is granted to each device in strict rotation.
• Thus a device will always get access (within a time which can be • calculated from network speed, number of stations, etc.) •no matter how busy the network is.
• Effect on applications
• The application area may well determine what type of network and/or protocol may be used.
• For example, in real time applications (such as process control, real-time voice, etc.) need to have guaranteed access within specified time limits.
• When attempting to use Ethernet to transmit real-time voice conversations one has no idea when the data will get through (there may be long breaks in the conversation).
• In a real-time control systems (e.g. car, nuclear power station, etc.) it is very important that critical messages can get through within a specified time.
• Multimedia applications which involve transferring large files over a network can also cause sever problems if there are bottlenecks either on the servers or the network.
• Contention busses (such as Ethernet) are fine for applications where traffic is relatively low and there are no real-time applications running.
• Rings react evenly to heavy traffic but have the problem of maintaining physical ring integrity.
• The token passing bus has the advantages of both (easier to install than rings but with deterministic access)
• but the software is very complex (commonly used in factory automation).
8 Internetworks:
Bridges, Routers and Gateways
• An internetwork is a collection of WANs and/or LANs that are connected together via bridges, routers and gateways.
• A bridge
• is used to interconnect two similar LANs and acts as an address filter
• to break up a large network into logical components.
• contains addressing and routing intelligence and is aware of which stations are on which network.
• only packets destined to stations on the other network are passed across.
• Splitting up a network into a number of semi-independent sections can assist with network management, security and the problems encountered with bus networks using the CSMA/CD protocol.
• A gateway
• is used is used to interconnect dissimilar networks, e.g. bus LAN to ring LAN.
• it must contain protocol conversion software in addition to addressing and routing intelligence.
• A router is used in where more than two networks are to be interconnected.
• In practice a bridge may also act as a router when interconnecting networks of the same type otherwise a sophisticated gateway may be required.
• The routing information is generally provided by static routing tables which are set up by the network manager.
• If an internetwork is configured correctly the majority of traffic on the network is local to the semi-independent sections with the bridges and gateways handling a (relatively) small amount of long distance traffic.
9. Distributed Environments
• Modern networks tend to form a distributed environment:
• The network: communications hardware and software.
• User workstations: terminals, PCs and/or professional workstations
• Servers: powerful computer systems which provide general services, e.g.:
• fileservers: hold general system files, compilers, user files, etc. • print servers • database servers holding centralized databases. • computational server: provides computational power beyond the capacity of workstations• X terminal server
• Servers may be a high powered PC, a specialized workstation, a minicomputer, a mainframe or supercomputer.
• Bridges and gateways:
• in a distributed environment care must be taken not to overload the network and to provide adequate security for sensitive information.
• Splitting a large network up into a number of semi independent networks linked by bridges and gateways can assist with these problems.
9.1 Performance factors in a distributed environment
• Distributed environments can be very complex and the overall performance depends upon many factors.
• Performance of the network or networks
• This is dependent upon the physical network configuration and speed, the communications protocol used, number of bridges and gateways, etc.
• Network configuration
• The number of user workstations, their distribution over the network(s) together with servers, bridges, gateways, etc.
• Modern network have hardware and software tools (which can keep track of network traffic, files used, etc.) to assist with network configuration.
• User workstations
• Computational power, main memory size and disk size - high powered workstations may be slowed down by bottlenecks elsewhere,
• e.g. by a slow networks or overloaded servers.
• Fileservers
• The number and their power in terms of:
• computational power: power to cope with network and disk I/O.
• I/O bandwidth: a good as possible to support disk and network I/O.
• size of main memory: very important - sufficient to buffer information.
• disk speed and size
• network interface: good, possibly with DMA facilities.
• The distribution of software packages and user files around the fileservers is critical:
• complex intensive centralized tasks could well require a dedicated fileserver;
• spreading the end-user files around the fileservers prevents overloading of particular fileservers .
• Also, if a fileserver breaks down some users can still do their work.
9.2 Common problems
• Clearly great care is needed in configuring a distributed environment with a slight error giving the impression of 'clockwork' powered machines.
• Common problems are:
• Network speed too slow for modern workstations together with poor distribution of files.
• too few fileservers for the number of user workstations and/or poor distribution of fileservers across the network.
• too little main memory on fileservers causing bottlenecks in the accessing of centralized file systems.
10. What network to buy?
• There is no easy answer as to which type of network is most suitable for a given application.
• Networks based on baseband bus (e.g. Ethernet) were the earliest and are still widely available and used.
• The mechanism is robust and uses many well proven components.
• e.g. from cable TV and telephone technology
• Opponents would argue that the CSMA/CD contention protocols used are unsuitable for high traffic loads, cannot support voice traffic and severely limit the maximum length of the bus.
• However, intelligent hubs can overcome some of these problems.
• Rings are capable of covering greater distances and of reacting more evenly to heavy traffic but are typically more expensive and have suffered reliability problems in the past.
• Stars have the great advantage that they can often be installed using existing cabling but HAVE the disadvantage of all centralized networks – dependence on a master station or hub.
• Broadband networks can handle enormous traffic flows at very high speeds.
• they are the only effective mechanism at present which can mix data, voice and image traffic; but they are extremely expensive.
