Introduction to Local Area Networks

Transcription

1 For Summer Training on Computer Networking visit Introduction to Local Area Networks Prepared by : Swapan Purkait Director Nettech Private Limited swapan@nettech.in

2 Introduction A local area network interconnects a variety of data communicating devices within a small geographic area broadcasts data at high data transfer rates very low error rates. Local area networks first appeared in 1970s, its use has now become widespread in both commercial and academic environments.

3 Primary Function of a LAN Information Exchange File serving - A large storage disk drive acts as a central storage repository. Print serving - Providing the authorization to access a particular printer, accept and queue print jobs, provide a user access to the print queue to perform administrative duties. Remote Execution

4 Local Area Networks Technology for connecting multiple computers across short distances ( within a building - up to a few KM) Inexpensive Highly reliable Easy to install and manage

5 Example Local Area Networks

6 Basic Network Topologies Local area networks are interconnected using one of four basic configurations: 1. Bus 2. Tree 3. Star 4. Wireless

7 Bus Topology

8 Interconnections in a LAN

9 Network Interface Card (NIC) Each computer is connected to the network through a special Network Interface Card (NIC) LAN transfers data independently of types of computers attached to it. The interface board shields the LAN from the characteristics of each device (e.g., speed).

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12 Bidirectional Propagation of Signals

13 Tree Topology

14 Star Topology

15 Wireless Topology Not really a specific topology A workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point. Several versions of IEEE standard define the various forms of wireless LAN connections.

16 Medium Access Control Sublayer To better support local area networks, the data link layer of the OSI model was broken into two sublayers: 1. Logical link control sublayer 2. Medium access control sublayer Medium access control sublayer defines the frame layout more closely tied to a specific medium at the physical layer. When people refer to LANs They often refer to its MAC sublayer name, such as 10BaseT.

17 Medium Access Control Protocols How does a workstation get its data onto the LAN medium? A medium access control (MAC) protocol is the software that allows workstations to take turns at transmitting data. Two basic categories: 1. Contention-based protocols 2. Round robin protocols

18 Contention-Based Protocols -CSMA/CD Most common example is carrier sense multiple access with collision detection (CSMA/CD). If no one is transmitting, a workstation can transmit. If someone else is transmitting, the workstation backs off and waits.

19 Two Packets Experiencing Collision

20 Contention-Based Protocols CSMA/CD cont If two workstations transmit at the same time, a collision occurs. When the two workstations hear the collision, they stop transmitting immediately. Each workstation backs off a random amount of time and tries again. Hopefully, both workstations do not try again at the exact same time. CSMA/CD is an example of a nondeterministic protocol.

21 Round Robin Protocol Each workstation takes a turn transmitting the turn is passed around the network from workstation to workstation. Most common example is token ring LAN a software token is passed from workstation to workstation. Token ring is an example of a deterministic protocol. Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated? Hogged? Token ring LANs are losing the battle with CSMA/CD LANs.

22 Wired Ethernet Originally, CSMA/CD was 10 Mbps Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps Then 1000 Mbps (1 Gbps) was introduced 10 Gbps is now being installed in highend applications

23 Small Company/ Engg. College

24 Small Office Home Office (SOHO)

25 Ethernet Most popular CSMA/CD protocol. What if a computer transmits a very long message? It keeps the line busy for very long time, while all other computers in the LAN must wait for the long message to end All messages must be small, to allow other computers to access the line For Ethernet, the maximum size of the payload is 1,500 bytes

26 Ethernet (cont d) What is expected performance? When only one computer needs to transmit: it can immediately access the line. When many computers want access (high traffic): Average waiting time is high. There is high probability of collision. For every collision, Xmission must start agan Conclusion: expected delay depends on the traffic on the LAN!

27 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern followed by one byte with pattern used to synchronize receiver, sender clock rates

28 Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped

29 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission } then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} else {wait until ongoing transmission is over and goto A}

30 Ethernet s CSMA/CD Cont... Exponential Backoff: Goal: adapt retransmission attemtps to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten or more collisions, choose K from {0,1,2,3,4,,1023}

31 Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

32 10BaseT and 100BaseT 10/100 Mbps rate; latter called fast ethernet T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus star topology

33 10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can disconnect jabbering adapter Hub can gather monitoring information, statistics for display to LAN administrators

34 Gbit Ethernet Uses standard Ethernet frame format Allows point-to-point links and shared broadcast channels CSMA/CD is used Uses hubs, Called here Buffered Distributors Full-Duplex at 1 Gbps for point-to-point links

35 Switched Ethernet Point-to-point connections to multi-port hub acting like switch; no collisions. More efficient under high traffic load: break large shared Ethernet into smaller segments. Hub Switch

36 Ethernet name cable max. nodes adv. segm. 10Base5 thick 500m 100 for backbones coax 10Base2 thin coax 200m 30 cheapest 10Base-T TP 100m 1024 easy maintenance 10Base-F fiber 2000m 1024 between buildings

37 Interconnecting LANs Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth Limited length: specifies maximum cable length Large collision domain (can collide with many stations) Limited number of stations: have token passing delays at each station

38 Hubs Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

39 Hubs Cont... Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: simple, inexpensive device Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m per Hub)

40 Hub Limitations Single collision domain results in no increase in max throughput multi-tier throughput same as single segment throughput Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage Cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

41 Bridges Link Layer devices: operate on Ethernet frames, examine frame header and selectively forwarding frame based on its destination Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

42 Bridges Cont... Bridge advantages: Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters

43 Bridges: frame filtering, forwarding Bridges filter packets same-lan -segment frames not forwarded onto other LAN segments Forwarding: how to know which LAN segment on which to forward frame? looks like a routing problem (more shortly!)

44 Backbone Bridge

45 Interconnection Without Backbone Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

46 Bridge Filtering bridges learn which hosts can be reached through which interfaces: maintain filtering tables when frame received, bridge learns location of sender: incoming LAN segment records sender location in filtering table filtering table entry: (Node LAN Address, Bridge Interface, Time Stamp) stale entries in Filtering Table dropped (TTL can be 60 minutes)

47 Bridge Filtering filtering procedure: if destination is on LAN on which frame was received then drop the frame else { lookup filtering table } if entry found for destination then forward the frame on interface indicated; else flood; /* forward on all but the interface on which the frame arrived*/

48 Bridge Learning: example Suppose C sends frame to D and D replies back with frame to C C sends frame, bridge has no info about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

49 Bridge Learning: example D generates reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so selectively forwards frame out via interface 1

50 Bridges Spanning Tree for increased reliability, desirable to have redundant, alternate paths from source to destination with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

51 Switches A switch is a combination of a hub and a bridge. It can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns. When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection. Workstations that connect to a hub are on a shared segment. Workstations that connect to a switch are on a switched segment.

52 Switches A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch. Multiple workstations connected to a switch use dedicated segments. This is a very efficient way to isolate heavy users from the network. A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server.

53 Workstation Connected to a Shared Segment

54 Workstation Connected to a Dedicated Segment

55 Routers The device that connects a LAN to a WAN or a WAN to a WAN. A router accepts an outgoing packet, removes any LAN headers and trailers, encapsulates the necessary WAN headers and trailers. Because a router has to make wide area network routing decisions, the router has to dig down into the network layer of the packet to retrieve the network destination address.

56 Routers Routers are often called layer 3 devices. OSI network layer. Routers often incorporate firewall functions. An example of a router s operation is shown on the next slide.

57 Router Operation

58 Bridges vs. Routers Both store-and-forward devices routers: network layer devices (examine network layer headers) bridges are Link Layer devices Routers maintain routing tables, implement routing algorithms Bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms

59 Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less processing bandwidth - Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

60 Routers + and - Routers vs. Bridges + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide firewall protection against broadcast storms - require IP address configuration (not plug and play) - require higher processing bandwidth bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

61 Summary LAN Topologies Ethernet addresses MAC layer issues CSMA/CD Types of Ethernet Hubs, Bridges, Routers, Switches

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