--- title: "Short note: Network topologies and IBM Ring LAN" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:short-note-topologies-ibm-ring" source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-topologies-ibm-ring site: BCT Engineering --- # Short note: Network topologies and IBM Ring LAN Telecommunications · Chapter 8: Data Communication - Asked in: 2064 Poush (old course), 10 marks - Video: [Token ring network and how it works](https://www.youtube.com/watch?v=p72R2uGglnU) — Sunny Classroom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-topologies-ibm-ring - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question Write a short note on network topologies and IBM Ring LAN in the data communication. ## Short answer Network topology is the geometric arrangement of nodes and links. IBM Token Ring (IEEE 802.5) is a star-wired logical ring using an MAU; stations get the right to send by capturing a circulating token, so there are no collisions. It runs at 4 or 16 Mbps with differential Manchester coding. ## Answer #### Network topologies The **topology** of a network is the geometric arrangement of the nodes and the links that join them. It decides cost, reliability and the access method. ```text Bus: H---H---H---H Star: H H =============== \ / H--[Hub]--H Ring: H---H / \ | | H H H---H Mesh: every node linked to every other (n(n-1)/2 links) ``` | Topology | Description | Advantages | Disadvantages | |---|---|---|---| | Bus | All nodes on one cable with terminators | Cheap, easy to add nodes | Cable fault stops all; collisions | | Star | Each node on its own link to a hub/switch | Easy fault isolation, easy to manage | Hub is single point of failure | | Ring | Each node joined to two neighbours in a loop | No collisions (token), fair | Break stops ring (unless dual/MAU) | | Mesh | Point-to-point links between all pairs | Very reliable, private links | $n(n-1)/2$ links, costly | | Tree | Hierarchy of stars | Scalable, easy to expand | Root failure affects branches | | Hybrid | Mix, e.g. star-wired ring | Combines merits | More complex | #### IBM Ring LAN **IBM Token Ring** (IEEE 802.5) is a LAN in which stations form a logical ring and gain the right to send by capturing a circulating **token**; there are no collisions. Rates are **4 or 16 Mbps**, coding is differential Manchester. Physically it is a **star-wired ring** – a hybrid topology: ```text +---------------+ A -----| |----- B | MAU (hub) | D -----| ring inside |----- C +---------------+ Logical ring: A -> B -> C -> D -> A Each lobe = 2 pairs (send + receive) ``` - Stations connect to an **MAU** by STP/UTP lobes; relays in the MAU bypass a dead station. - Up to 260 stations per ring (STP). #### Frame and token format ```text Token (3 bytes): +----+----+----+ | SD | AC | ED | +----+----+----+ Data frame: +--+--+--+--+--+------+---+--+--+ |SD|AC|FC|DA|SA| Data |FCS|ED|FS| |1 |1 |1 |6 |6 | 0-n | 4 |1 |1 | +--+--+--+--+--+------+---+--+--+ AC byte = P P P T M R R R ``` - **SD / ED:** start and end delimiters (use code violations J, K of differential Manchester). - **AC (access control):** PPP = priority, T = token bit (0 token, 1 frame), M = monitor bit, RRR = reservation. - **FC:** frame control (data or MAC control frame). - **DA, SA:** 6-byte addresses; **FCS:** 32-bit CRC. - **FS (frame status):** A (address recognised) and C (frame copied) bits set by the receiver. #### Working (token passing) 1. A free token circulates round the ring; each station repeats it bit by bit (1-bit delay). 2. A station with data waits for the token, changes the T bit to 1 (token becomes a start-of-frame), and sends its frame. It may hold the token for at most the token holding time (10 ms). 3. Each station repeats the frame. The destination copies it and sets the A and C bits in FS. 4. The frame returns to the sender, which **removes it** from the ring (source stripping), checks A and C for delivery, and releases a new free token. 5. At 16 Mbps, **early token release** lets the sender release the token right after its frame, so two frames may be on the ring at once. **Priority:** a station with a high-priority frame writes its priority in the RRR bits of a passing frame; the next token is issued at that priority, so only stations with equal or higher priority may use it. #### Ring maintenance – active monitor One station is elected **active monitor**; others are standby monitors. - Detects a **lost token** (no token within a timeout) and issues a new one. - Removes **orphan/circulating frames**: it sets the M bit on each frame; a frame seen again with M = 1 is purged. - Supplies the master clock and a **latency buffer** so the ring is long enough to hold a 24-bit token. - Sends Active Monitor Present frames; **beacon** frames locate cable breaks. **Advantages:** deterministic, fair access, good at high load, priorities. **Disadvantages:** costly and complex compared with Ethernet. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many point-to-point links does a full mesh of n nodes need? A. n B. n^2 C. n(n-1)/2 D. 2n Answer: C. Every node is linked to every other, giving n(n-1)/2 links, which makes mesh costly. (Key point: Mesh links count) 2. What is the main disadvantage of a star topology? A. Difficult fault isolation B. Collisions on a shared cable C. Needs n(n-1)/2 links D. The hub is a single point of failure Answer: D. All nodes depend on the hub or switch, though fault isolation is easy. (Key point: Star topology weakness) 3. Why is IBM Token Ring called a hybrid, star-wired ring? A. Physically a star around an MAU but logically a ring B. Physically a bus but logically a star C. Physically a ring but logically a mesh D. It uses both bus and tree Answer: A. Stations are cabled to a MAU in a star, while inside the MAU the lobes form the logical ring. (Key point: Star-wired ring) 4. What does a station do to turn a free token into a data frame? A. Sets the M bit B. Changes the T bit from 0 to 1 C. Writes priority into RRR D. Sets the A and C bits Answer: B. T = 0 marks a token; changing it to 1 makes it the start of a frame. (Key point: Token and frame bit) 5. How does a station with a high-priority frame get the next token? A. It writes its priority in the RRR bits of a passing frame so the next token is issued at that priority B. It removes the active monitor C. It holds the token for longer than 10 ms D. It uses early token release Answer: A. Reservation bits make the next token carry that priority, so only equal or higher priority stations may use it. (Key point: Priority mechanism) ## Related in Telecommunications - [Short note: IBM Ring LAN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-ibm-ring-lan) — Telecommunications, chapter 8 - [Short note: Cambridge Ring LAN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-cambridge-ring) — Telecommunications, chapter 8 - [Flow control in data communication network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-network) — Telecommunications, chapter 8 ## Related in other subjects - [Cast steel ring current from B-H data](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Acast-steel-ring-magnetization-data) — Electrical Machine, chapter 1 - [Cast steel ring with air gap, B-H data](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Acast-steel-ring-bh-table) — Electrical Machines, chapter 1 - [Horton constants from double ring infiltrometer data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahorton-constants-double-ring-data) — Engineering Hydrology, chapter 3