Coaxial and Optical Fiber Transmission Quiz

Transmission Medium & LAN Technologies - Lecture 3

• The lecture focuses on LAN wiring, hardware components, medium sharing, and LAN protocols.
• The objectives are to discuss LAN wiring, identify major hardware components, how the medium is shared among devices, and LAN protocol operations.
• The lecture covers guided transmission media, specifically twisted pair and coaxial cable. Also, it discusses optical fiber.
• Lastly, it introduces LAN technologies like Ethernet. Specific standards, including Ethernet, token bus, token ring, and DQDB, are included.
• The lecture also describes random access protocols like CSMA/CD.

Twisted Pair

• Twisted pair consists of two insulated copper wires twisted in a spiral pattern.
• The twisted pair acts as a single communication link, reducing crosstalk.
• Twisted pair is usually installed in buildings during construction due to ease of work.
• Analog signals need amplifiers every 5 km to 6 km. Digital signals need repeaters every 2–3 km.
• Twisted pair has limited distance and bandwidth (1 MHz for analog point-to-point signaling).
• Higher data rates are possible for short distances (up to 10 Gbps).
• Susceptible to interference and noise (EMI).

Twisted Pair - Transmission Characteristics

• Analog signals require amplifiers every 5–6 km.
• Digital signals require repeaters every 2–3 km.
• Limited distance, Bandwidth (1 MHz for analog point-to-point signaling).
• High data rates possible for very short distances (up to 10 Gbps).
• Susceptible to interference and noise.

Advantages and Disadvantages of Twisted Pair

• Advantages: Common guided medium for analog and digital signals, commonly used in telephone networks, less expensive, and easier to work with.
• Disadvantages: Sensitive to EMI and eavesdropping (especially unshielded), unsuitable for very high-speed data transmission, and some standards are unstable.

Unshielded Twisted Pair (UTP) & Categories

• UTP cable is rated by its data-carrying capacity.
• Category 1: Analog and digital voice (telephone), low-speed data.
• Category 2: Integrated Services Digital Network (ISDN) and medium-speed data.
• Category 3: High-speed data, LAN traffic (up to 10 Mbps).
• Category 4: LAN traffic (up to 16 Mbps).
• Category 5: LAN technologies like 100 Mbps Ethernet.
• Category 6: LAN technologies like 10 Gbps Ethernet.

Coaxial Cable

• Structure: Central copper core, insulating material, braided outer conductor, protective plastic covering.
• Superior frequency characteristics than twisted pair (performance limited by attenuation/noise).
• Analog signals require amplifiers every few kilometers. Better performance with increased frequency.
• Digital signals require repeaters every kilometer. Better data rates supported via higher-frequency transmission.
• High bandwidth possible.
• More resistant to EMI than twisted pair.
• Heavier, bulkier, potentially more expensive than twisted pair.

Coaxial Cable - Advantages and Disadvantages

• Advantages: Mature technology/standards leading to compatibility, better EMI resistance than twisted pair, higher bandwidths than twisted pair, sturdy/resists rough treatment.
• Disadvantages: Susceptible to EMI and eavesdropping like twisted pair, sometimes heavy, bulky, or expensive.

Optical Fiber

• Structure: Glass or plastic core surrounded by cladding, specially designed jacket. Small size and weight.
• Light transmission: Uses total internal reflection to transmit light. Light at less than critical angle is absorbed in jacket.
• Benefits: High capacity (hundreds of Gbps over tens of km), lower attenuation, electromagnetic isolation, greater repeater spacing (tens of kilometers), and smaller size and weight..
• Transmission characteristics: Uses total internal reflection to transmit light, effective wave guide (1014–1015 Hz). Supports infrared and visible light spectrums. Various light sources possible.
• Light sources (e.g., Light Emitting Diodes (LEDs) and Injection Laser Diodes (ILDs))
• Transmission modes: Step-index multimode, Graded-index multimode, and Single mode.

Optical Fiber - Advantages and Disadvantages

• Advantages: Immune to interference/eavesdropping, higher bandwidths possible, smaller/lighter than other options.
• Disadvantages: Network interfaces and cables relatively expensive, connections require precision manufacturing and careful handling, relatively complex to configure/install.

LAN Technologies

• Ethernet (IEEE 802.3): Well-known/widely used technology, employs bus topology, invented at Xerox in the early 1970s. DEC, Intel, and Xerox created a production standard.
• Ethernet standards: Original Ethernet (10 Mbps), Fast Ethernet (100 Mbps), Gigabit Ethernet (1 Gbps). Uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD).
• Sharing on Ethernet: Sending computer has exclusive use of cable during transmission. Other computers wait.
• Ethernet frame format: Preamble, destination address, source address, type, data, and CRC. Fields have specific sizes in bits. The Ethernet frame begins with a 64-bit preamble, containing alternating 1s and 0s which allows receiving hardware to synchronize with the incoming signal. 16 bit field of header specifies the type. 48-bit static addressing scheme used. Addresses are 6 bytes. Adapters receive frame w/ matching address. Otherwise, it discards frame. Checks whether it matches broadcast address, forwards data for net-layer protocol. Checks the Frame type. CRC checked at receiver.

Protocol definition

• Protocols define the format, order, of messages sent and received among network entities, and actions on msg transmission, receipt. Protocols define standards for computer network communications.

Access Networks

• Frequency Division Multiplexing (FDM): Different channels transmitted in different frequency bands for Cable-based access.

Random Access Protocols

• When a node has a packet to send, it transmits at full channel data rate (R) without coordination.
• Two or more transmitting nodes lead to a "collision."
• Protocols defining the how to detect collisions and recover via delayed retransmissions:
• CSMA
• CSMA/CD
• CSMA/CA

CSMA (Carrier Sense Multiple Access)

• Simple CSMA: Listen before transmitting, transmit if idle, defer transmission if busy.
• Human analogy: Don't interrupt others.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

• Collisions detected within a short time.
• Colliding transmissions aborted, reduces channel wastage.
• Collision detection easy in wired, difficult in wireless.
• Human analogy: The polite conversationalist.

CSMA: Collisions

• Collisions may still happen despite carrier sensing due to propagation delays.
• Two nodes might not hear each other starting transmission.
• Collision wastes the entire transmission time.
• Distance and propagation delay determine collision probability.

Carrier Sense On Multi-Access (CSMA) Algorithm

• Ethernet lacks a centralized controller. Check whether the cable is currently in use.
• If the cable is idle, transmit a frame.
• If the cable is busy, wait for the sender to finish.
• All computers attached to an Ethernet participate in a distributed coordination scheme.

Collision Detection (CD)

• Collision is interference/mixture of transmission signals.
• Monitoring a cable during transmission is collision detection (CD).
• Collision prevents a correct reception of either frame in a garbled transmission.
• CSMA/CD reduces time lost due to collisions. Sending stations immediately stop transmission when a collision is detected.

Binary Exponential Backoff with CSMA/CD

• To avoid collisions, delay after collision, retransmit after random delay.
• Max delay (d): computer selects random delay less than d.
• To avoid a collision sequence, stations double the delay range after each collision.
• Random delay between 0 and d (1st collision), 0 and 2d (2nd collision), and so on. Larger range for a few collisions makes the process more random. The range from which to select a random value gets larger after a few collisions.

Binary Exponential Backoff

• Stations resend when they collide.
• Mean random delay doubles up to 10 attempts.
• Remains same for 6 further attempts.
• After 16 failed attempts, station gives up and reports the error. A 1-persistent algorithm with binary exponential backoff which is efficient over a wide range of loads.

Why does CSMA/CD use a random delay?

• Randomization prevents multiple stations from transmitting simultaneously, leading to potential collisions.
• The protocol defines a maximum delay and a random delay less than it for each station after a collision. In most cases, when two stations choose a random value, the station with the smallest delay will proceed to send and the network will return to normal operation.

Token Bus (IEEE 802.4)

• A token ring operates as a single shared medium.
• To send, a device must wait for permission (a token).
• Once permission is granted, it controls the ring.
• As it transmits a frame, the bits pass around the ring.
• A frame is copied and forwarded to other stations, until it reaches the intended destination.
• The originating station receives a copy to confirm reception.

Token Passing in Token Ring (802.5)

• Token rings use a token to regulate transmission, and the stations wait for turn (token) to communicate.
• The token passes around the ring, and a station will wait for the token to become available before sending.
• A three-byte token is sent whenever the network is inactive.
• The station receiving data will make a copy, and checks for errors before sending a confirmation back to the sender.

Priority and Reservation

• After token release, next station with data to send takes control of the ring.
• Busy token can be reserved by waiting stations regardless of their position on the ring.
• Each station has a priority code.
• Higher priority stations can remove lower priority reservations, and equal priority stations use a first-come, first-serve approach.

References

• Comer, Douglas E. Computer Networks and Internets. Prentice Hall, 3rd ed.
• Comer, Douglas E. Computer Networks and Internets. Prentice Hall, 4th ed.
• Stallings, William. Data and Computer Communications. 8th ed.

Test your knowledge on coaxial and optical fiber cables, including their advantages, disadvantages, and performance factors. This quiz covers aspects of digital signals, Ethernet standards, and the role of Light Emitting Diodes in fiber applications. Enhance your understanding of how these technologies function and their significance in modern communication.