WANs

• Point-to-point, multipoint, mesh networks, clouds

• Hybrid (multiple topologies) often found in WANs

Nodes attach directly to bus

Cables can weave throughout building making it quite long

Often used for small temporary installation

Traditionally Ethernet used a bus topology

Drawbacks:

• A break anywhere in the bus will cause entire network to fail

• Difficult to troubleshoot

Repeaters at each node repeat signals, minimising signal degradation

Same failure risk as buses

• To prevent such failure most implementations are wired as star topology

• Out-loop and in-loop between workstation and wiring hub

Can use twisted pair or coax (usually twisted pair for modern networks)

Most common form of data network

Advantages

• Flexibility (easy to add to, move and change)

• Network failure is minimised if a segment breaks

Disadvantages

• May use more cable than bus or ring

Example implementations

• Ethernet (traditional bus)

  • 10BaseT to wiring closet

  • 10Base5 / 10BaseT multi-port repeaters

Only used to connect the most important sites to each other

Advantages

• Troubleshooting and failure isolation is easy

• Fault tolerance maximised by rerouting traffic around failed links

Disadvantages

• Difficult to install and reconfigure

• Expensive (redundant connections and wasted bandwidth)

Operates at the physical layer of the OSI model

Intelligent repeaters

• Regenerates digital signal, so immune to attenuation over distance

• Normal repeater will amplify noise, so limits their number

Amplifiers

• Used by broadband networks to extend signal transmission

Typically used on bus networks

• Place the repeater so the two segments are roughly the same length

Problems

• Signal quality (noise is also amplified)

• Time delays (may cause time-out errors over distance)

• Network traffic (they don't filter traffic, so do nothing to reduce traffic)

• Node limitations (Nodes added through a repeater add to overall node total on the subnet)

Backbone

• a high-speed network link connecting only segments

• Only data for another segment traverses the backbone

Message Switching

• 'Store and Forward'

• Messages use the same path, but connection isn't maintained

• Devices must have sufficient memory and power to accept and store information

• Used by:

  • Email

Packet Switching

• Info is broken down into packets which contain data, destination address, and sequence information.

• Packets can travel over any route and don't have to follow each other

• Destination node reassembles info from packets

• Not good for audio/video, etc.

• Economical, i.e. Doesn't hog bandwidth

• Used by:

  • Ethernet, FDDI, the Internet

Ethernet

• Originally developed by Xerox, Intel, and DEC in the late 70's

• Fixed frame length – padded

• Higher protocol info (e.g. TCP/IP) In place of length field

IEEE 802.3

• Contains a 2-byte length field containing length of the frame

Other differences

• Pinouts

• Signal Quality Error (SQE) signal, aka. the heartbeat

Most common implementation of connection access

Carrier Sensing: listens for someone talking

Multiple Access: all have concurrent access to media

Collision Detection: system repeats message if two are sent simultaneously

Causes of transmission failure

• Bad cabling or improper cable length

• Improper termination

• Collisions

Collisions

• Node transmits when it determines no other node is transmitting

• System listens while transmitting and if detects other transmissions, it quits for random time then retransmits when carrier is clear

• Most network OS will track retransmissions – giving good idea of collision rates

10Base5

• Uses Thicknet coaxial cable

• Without repeater can carry signal 500m (1,640ft)

10BaseT

• Uses UTP cable

• Can carry signal 100m between workstation and hub

Capable of only 230kbps throughput

Not easily supported by non-Macs

Newer Macs don't even have LocalTalk port, but can use LocalTalk via an Ethernet adapter

Uses CSMA/CA (Collision Avoidance)

• When node wants to transmit it sends out an alert, so if there is a collision, lengthy retransmissions are avoided

Uses TP wiring, same as telephone

Maximum segment length 1,000ft (305m)

Up to 32 nodes can be networked before data errors begin

Uses a star, or more commonly a bus physical topology

To connect:

• Connect from serial port to transceiver (known as a teleconnector)

• Connect teleconnector to wall jack with TP cable, which leads to the networks horizontal wiring

• Teleconnector contains a resistor to avoid signal bounce

Protocols

• As standard, will be peer-to-peer

• As standard, will use AppleTalk protocol

• Can use Apple version of TCP/IP (MacTCP)

• Can use other TCP/IP, as long as the packets are wrapped as AppleTalk

• Can use AppleShare and a server to be client/server setup

Token Ring is a popular token passing scheme

Features of token passing

• Only one active token on the ring at any one time

• Tokens travel at thousands of mph (fibre optic)

• Token (data frame) passes from system to system

• System can attach data when token is free (empty – Token Bit is 0)

• Each system receives and regenerates the token (each node acts as a repeater)

Features

• Logical ring usually wired as physical star

• Transfer rate of 4-16 Mbps

• UTP, STP or fibre optic cable

• Deterministic (possible to predict passage of token, so popular for time critical apps)

• When token returns to source with ack from destination, source relinquishes token allowing others to put data on network (changes to Frame Status (FS) byte)

• Monitors

  • Active monitor is first to be powered on and inserts on the ring

  • responsible for

    • verifying token is on the ring and regenerating it if it is not

    • removing continuously circulating frames

  • All other stations are standby monitors

  • If the active monitor fails/is removed, the standby monitor with the highest address becomes the active monitor

  • Every >7 seconds the active monitor signals other nodes to identify itself

Number of workstations on MSAU

• STP – 260

• UTP – 72

Each ring can have up to 33 MSAUs

Efficiency

• Better than Ethernet for small/medium network with heavy traffic

• Ethernet better for large network with light traffic (because of direct routing)

If a node has data to send, it appends it to the token, making it a frame

A frame

• is a unit of data transmission

• includes

  • delimiters

  • control characters

  • information

  • checking characters

Defines a 'failure domain'

• the reporting station

• it's Nearest Upstream Active Neighbour (NAUN)

• everything in between

When identified the NAUN removes itself from the ring and does a self test, then reattaches to the ring if possible

If not, the beaconing station removes itself from the ring, does a self-test, then reattaches to the ring if possible

If that is unsuccessful, manual intervention is required

Speed of 100 Mbps

• Faster than Token Ring / Ethernet at the time, though they did catch up so FDDI isn't as coveted now

• First network transport system to reach 100 Mbps threshold

• Therefore, to be found in backbones from late 80s / early 90s

Often used to connect LANs in separate buildings

FDDI links can span up to 62 miles

Uses physical ring topology and is similar to token ring but

• uses two complete rings

• if primary ring fails, secondary ring will do data transmission

Advantages

• Fibre optics make it reliable and secure

• Works well with 100BaseTX technology

Disadvantages

• Cost (up to ten times more per switch port than Fast Ethernet)

Developed by Bell Labs in '83, but took ten years of standards

Runs over copper/fibre, e.g. SONET and T-carriers

Used by large data carriers, e.g. WANs, ISPs, phone companies

Uses virtual circuits

• example of circuit switching

• switches determine the optimal path between source and dest, THEN the network transmits data

• cf. Ethernet uses packet switching

Circuit switching allows ATM to offer QoS guarantees

• 4 levels, from 'best effort' to 'guaranteed real-time'

• e.g. A company might use ATM with high QoS for voice calls across a state

Can support multiple higher-level protocols (TCP/IP, AppleTalk, IPX/SPX, etc.)

Uses LAN Emulation (LANE) to integrate with Ethernet and Token Ring networks

• encapsulates incoming Ethernet/Token Ring frames and converts them to cells for the ATM network

Threatened by the faster Gigabit Ethernet

May not have a power source, but most do

Takes signal in on one port and passes it out every other port (all nodes see the signal, like on a physical bus topology)

May have lights for power, network traffic, link state per port and collision

May provide a BNC connector for connecting to a:

• 10Base2 backbone

• AUI port for connecting to a transceiver

Resets cable length

• e.g. for 10BaseT networks, 100m cable can be attached between the node and the active hub

Some hubs can resynchronise data from NICs that are not in standard timing, helping prevent data loss and improving performance

Some hubs will alert you to the above condition in a NIC

Only sends packets out the port of the destination

Good method of avoiding collisions

• Each computer is free to transmit when it wants

• The switch will buffer packets and when computers transmit simultaneously

Speed auto-sense

• Allows connection of 10Mbps and 100Mbps NICs

• Will buffer and retransmit at required speed

May support

• TCP/IP Simple Network Management Protocol (SNMP)

• logging in to

  • enable/disable/reset ports

  • monitor traffic in real-time

• built in routing or bridging

Multistation Access Units (MSAU) support up to 8 nodes

Maximum 12 MSAUs per ring

Local Ring Hub allows 4 node connections on 1 MSAU port cable

Maximum 64 to 72 nodes per ring recommended for optimal performance

Maximum distances

2-twisted pair cable is used between MSAU and device (one from MSAU to device, one from device to MSAU)

Unused ports are bypassed to maintain the internal ring

Provides a backup (redundant) ring to take over in case of a cable breakage

Used to be called MAUs but got confused with Media Access Unit (transceiver). Be wary of usage.

Patch cables are connected between the patch panel and a hub or MSAU, activating the wall jack

Be sure to include in calculation of cable length:

• Patch cable from computer to wall jack

• Cable run from wall jack to patch panel

• Patch cable from patch panel to hub/MSAU

Has AUI port on one side and one of the following on the other

• 10Base2, 10Base5, RJ-45, fibre optic

Example:

• a transceiver might connect a 10Base5 backbone to a twisted pair hub

• Media is different (coax to TP), but access method is still Ethernet

When used in token ring, transceivers are known as 'media filters'