Ring
1. Definition
In ring topology, each node is connected to exactly two neighbouring nodes, forming a closed loop. Data travels around the ring, typically in one direction, being regenerated by each node it passes through. Because every node forwards the data, the ring is a point-to-point chain closed on itself rather than a shared bus.
2. Structure
┌────────────► Node A ────────────► Node B ──────────┐
│ ▲ │
│ │ ▼
Node F data flows in ONE direction Node C
▲ (unidirectional ring) │
│ │
└── Node E ◄──────────── Node D ◄───────────────────┘
A dual ring (as in FDDI) adds a second, counter-rotating ring so that a break can be wrapped around and service continues.
3. How It Works — Token Passing
- A small control frame called the token circulates whenever the ring is idle.
- A node that wants to transmit captures the token, sends its data frame around the ring, and releases the token when done (Token Ring) or after sending (early token release).
- Each intermediate node regenerates and forwards the frame; the destination copies it and sets an acknowledgement bit.
- The frame finally returns to the sender, which removes it from the ring and frees the token.
- Because only the token holder transmits, there are no collisions, and every station gets a turn — access is deterministic and the maximum wait is bounded.
Standards: IEEE 802.5 Token Ring at 4 and 16 Mbps (physical star cabling through a MAU/MSAU, logical ring) and FDDI at 100 Mbps over dual fibre rings.
4. Advantages
- No collisions — deterministic, fair access with a guaranteed maximum waiting time; ideal for time-critical and real-time traffic.
- Performance degrades gracefully — as load increases, throughput stays near capacity instead of collapsing as in bus/Ethernet with CSMA/CD.
- Every node regenerates the signal, so the ring can span longer distances than a passive bus.
- Equal opportunity for all nodes (each gets the token in turn) — no single station can monopolise the medium.
- Simple to determine the order of transmission, and dual-ring designs give high fault tolerance with automatic wrap-around.
- Easy to locate a fault in a dual ring — the wrap point identifies the failed link or node.
5. Disadvantages
- Single break stops the ring in a single-ring network (of the cable or of a node's transceiver); dual ring or a bypass switch is needed to survive.
- Adding or removing a node disturbs the ring — in a physical ring the loop must be opened (in Token Ring, the MAU hides this from users).
- Data passes through every intermediate node, so delay grows with the number of nodes, and every node is a potential point of failure or snooping point.
- Slower than switched Ethernet — 4/16/100 Mbps vs 1-10 Gbps, which is why ring LANs are now obsolete in offices.
- Costlier and more complex hardware (network interface, MAU, dual-ring fibre ports) than a simple star with a switch.
- Troubleshooting is harder than in a star (no central diagnostics device) unless the ring is physically cabled as a star.
6. Quick Comparison
| Basis | Ring | Star | Bus |
|---|---|---|---|
| Access method | Token passing (deterministic) | Switch forwarding (no contention) | CSMA/CD (contention) |
| Collisions | None | None on full-duplex switched links | Possible |
| Effect of one failure | Ring breaks (unless dual ring) | Only that node affected | Whole trunk may fail |
| Adding a node | Disturbs the ring | Easy — plug into the switch | Disturbs the backbone |
| Performance under heavy load | Degrades gracefully | Stable | Degrades sharply |
| Cable requirement | n links in a loop | n links to the centre | 1 trunk + drops |
| Typical speed | 4-16 Mbps (Token Ring), 100 Mbps (FDDI) | 100 Mbps-100 Gbps | 10 Mbps (legacy) |
| Current status | Legacy; concepts live on in SONET/SDH and MPLS rings | Dominant LAN topology | Legacy in LANs; used in field buses |
7. Where Ring Ideas Live Today
Although Token Ring and FDDI LANs are retired, the ring remains the backbone shape of choice for telecom networks: SONET/SDH and metro fibre rings with protection switching, resilient packet rings (RPR), and MPLS ring topologies used by ISPs and metro Ethernet carriers — precisely because a dual ring survives a single fibre cut.
8. Exam Points
- Explain ring topology with a diagram, advantages and disadvantages — 5-7 marks, almost certain.
- Explain token passing and why a ring network has no collisions — 5 marks, frequent.
- Compare ring, star and bus topologies — 5-7 marks, very frequent.
- What is a dual ring? How does it improve reliability? — 3-5 marks.
- Why did ring LANs (Token Ring, FDDI) become obsolete? — 3 marks.
- State the standards and speeds associated with Token Ring and FDDI — 2-3 marks.