Networks
Week COIT13147 2 Underlying Technologies
Transmission Media
Transmission Media can be divided into two broad categories: guided and unguided OR Wireless and Wired OR Wireless and Nothing OR Wireless and Cabled
Guided Media
Those that provide a conduit from one device to another The signal is directed and contained by the physical limits of the medium. Twisted Pair Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) Coaxial Cable Optical Fiber
Twisted Pair
Unshielded Twisted Pair (UTP)
UTP
EIA standards Cat 1 Basic twisted pair cabling used in telephone systems. Cat 2 Bandwidth up to 4 MHz. Cat 3 at least three twists per foot, Bandwidth up to 16 MHz. Cat 4 at least three twists per foot, Bandwidth up to 20 MHz. Cat 5 Bandwidth up to 100 MHz.
Shielded Twisted Pair (STP)
Coaxial Cable
Optical Fiber
Unguided Media
Radio bands (8 bands) less then 300 KHz used for long-range navigation 300 KHz 3 GHz radio communication 3 GHz 30 GHz microwave point-to-point communication for terrestrial and satellite microwaves 30 GHz 300 GHz used for space communication 300 GHz 100 THz infrared frequencies
Local Area Networks (LANs)
Ethernet Fast Ethernet Token Ring FDDI
Ethernet
The access mechanism - Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Before a station transmits data, it listens to the medium. If no other station is transmitting, the station can send its data. If a collision occurs, all stations ignore the data received. The sending stations wait for a period of time (generated randomly) before resending data.
Ethernet Specifications
Addressing Each station has its own NIC (network interface card). Ethernet NICs come with a six byte physical address. Data Rate Ethernet LANs can support data rates between 1 and 10 Mbps.
Ethernet Frames
Frame format
Ethernet Implementation
All Ethernet LANs are configured as LOGICAL buses, although physically they can be bus or star topologies. Each frame is transmitted to every station on the link but read only by the station to which it is addressed. 10Base5 Thick Ethernet 10 Base2 Thin Ethernet 10Base-T Twisted Pair Ethernet Fast Ethernet
Collision Domains
The collision domain of Ethernet is 2,500 meters. This collision domain is calculated to achieve the data rate of 10 Mbps using the CSMA/CD access method. For CSMA/CD to work a station should be able to sense the collision before the whole frame is sent on the transmission media. The minimum size of an Ethernet frame is 72 bytes or 576 bits (header/trailer (26) + minimum data (46)).
Collision Domains
To send 576 bits at a data rate of 10 Mbps takes 57.6 microseconds. Before the last bit is sent the first bit must have reached the end of the DOMAIN, and if there is a collision it must be sensed by the sender. So a collision must be detected during 57.6 microseconds. This time is adequate to allow a signal to make a round-trip of 5,000 meters.
Collision Domain Questions
Why do we use 5,000 meters? If we increase the data rate, what happens to the collision domain?
Collision Domains
We decrease the round-trip time without changing the minimum size of the frame. So it takes less time for the same 576 bits to travel round-trip on the network. With a speed of 100 Mbps, the round trip time reduces to 5.76 microseconds. What does this mean for the collision domain which is calculated on how far a message can go without sending the last bit before you can detect a collision?
Collision Domains
The collision domain must be decreased 10 times from 2,500 m to 250 meters. If the only change is an increase in the data rate then the collision domain is decreased by the same factor. e.g. If the data rate is increased by a factor of 10; the collision domain is by decreased a factor of 10.
Token Ring
Logical ring topology. Requiring stations to take turns sending data. Each station can transmit only one frame during each turn.
Access Method
Token is passed from station to station in sequence until it encounters a station with data to send. Captures the token and sends its data frame. Each intermediate station examines the destination address. The intended recipient copies the message, checks for errors, and changes four bits in the last byte of the frame to indicate that the address has been recognized and the frame copied.
Access Method
The full packet continues around the ring until it returns to the station that sent it. The sender receives the frame, examines the address-recognized bits. If they are set it knows the frame was received. The sender discards the used data frame and releases the token back to the ring. Addressing 6 byte address of NIC card Data Rate - 4 or 16 Mbps.
Frame Formats
Data Frame
Token Frame Abort Frame
Implementation
STP sections linking each station to its immediate neighbors. One disabled or disconnected node could stop the flow of traffic around the entire network. Each station is connected to an automatic switch. The switch bypasses an inactive station. When a station comes on, a signal sent by the NIC moves the switch and brings the station into the ring.
Implementation
Fiber Distributed Data Interface (FDDI)
Data rates of 100 Mbps high speed alternative. When FDDI was designed speeds of 100 Mbps required fiber-optic cable now you can use copper cable. Also uses Token Passing.
Access Methods
In FDDI access is limited by time. Each station keeps a timer that shows when the token should leave the station. If earlier it can keep the token and send data until the scheduled leaving time. If later than this time, it should let the token pass to the next station and wait for its next turn. Addressing 2 to 6 byte address Data Rate - 100 Mbps
Frame Format
Data Frame No access Control
Token frame the same as for Token Ring
Implementation
Switching
A switched network consists of a series of interlinked nodes, called switches. Switches create temporary connections between two or more devices linked to the switch but not to e...