Unit 2: High-speed LANs and routing
Computer Networks notes · PTU syllabus (PGCA1910)
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Unit summary
Faster LANs and smarter routing keep networks fast as traffic grows. This unit covers FDDI, Fast Ethernet, HIPPI, Fibre Channel, the IEEE 802.x standards, static and dynamic routing and routing algorithms, and the causes of congestion with congestion control strategies.
After this unit you can
- Describe high-speed LAN technologies
- Summarise the IEEE 802 standards
- Compare static and dynamic routing and apply routing algorithms
- Explain causes of congestion and its control
PTU syllabus topics
- FDDI
- Fast Ethernet
- HIPPI
- Fiber Channel
- IEEE 802.x standards
- static vs dynamic routing and routing algorithms
- causes of congestion and congestion control strategies
Routes set
Manually by the admin
Automatically by protocols
Adapts to failure
No
Yes
Overhead
None
Uses bandwidth and CPU
Examples
Small networks
RIP, OSPF, BGP
Topic 1
FDDI
- Fibre Distributed Data Interface: a 100 Mbps token-ring LAN over optical fibre, up to about 200 km and 1,000 stations, often used as a campus backbone in the 1990s.
- Dual counter-rotating rings: a primary and a secondary ring; if a link fails, the rings wrap to form one ring, giving fault tolerance. Stations are dual-attached (DAS) or single-attached (SAS). Uses timed token rotation and supports synchronous and asynchronous traffic. Replaced by Fast and Gigabit Ethernet.
Topic 2
Fast Ethernet
100BASE-TX
Two pairs of Cat 5 UTP
100 m
100BASE-T4
Four pairs of Cat 3 UTP
100 m
100BASE-FX
Two multimode fibres
2 km (full-duplex)
- IEEE 802.3u (1995): 100 Mbps with the same frame format and CSMA/CD as classic Ethernet, so existing software works; uses hubs or switches in a star; auto-negotiation selects 10 or 100 Mbps and half or full duplex.
Topic 3
HIPPI and Fibre Channel
Full form
High-Performance Parallel Interface
—
Speed
800 Mbps (later 1.6 Gbps; HIPPI-6400 at 6.4 Gbps)
1 Gbps originally; now 32, 64 and 128 Gbps
Medium
Parallel copper cables up to 25 m; fibre extenders
Optical fibre or copper
Use
Supercomputers and high-speed peripherals in the 1990s
Storage area networks (SANs) connecting servers to disk arrays
Topology
Point-to-point through crossbar switches
Point-to-point, arbitrated loop, switched fabric
Topic 4
IEEE 802.x standards
- 802.1
- Bridging, VLANs (802.1Q), network management
- 802.2
- Logical link control (LLC)
- 802.3
- Ethernet (CSMA/CD) and its faster versions
- 802.4
- Token bus
- 802.5
- Token ring
- 802.11
- Wireless LAN (Wi-Fi 6 is 802.11ax, Wi-Fi 7 is 802.11be)
- 802.15
- Wireless PAN — Bluetooth (802.15.1), Zigbee (802.15.4)
- 802.16
- Broadband wireless (WiMAX)
- The 802 standards cover the physical and data link layers; the data link layer is split into LLC (common to all) and MAC (specific to each medium).
Topic 5
Static and dynamic routing
Routes
Entered manually by the administrator
Learnt and updated automatically by routing protocols
Adapts to failures
No
Yes
Overhead
None on links and CPU
Protocol messages and computation
Suits
Small, stable networks; default routes
Large, changing networks
Examples
ip route commands
RIP, OSPF, BGP
Topic 6
Routing algorithms
- Routing algorithm: decides the output line for each incoming packet. Static (non-adaptive) routes are fixed in advance; dynamic (adaptive) routes change with traffic and topology (distance vector — RIP; link state — OSPF).
- Optimality principle: if router J is on the optimal path from I to K, then the optimal path from J to K lies on the same route. Hence optimal routes from all sources to a destination form a sink tree.
- 1Mark the source with distance 0, others infinity
- 2Pick the unvisited node with the smallest distance and make it permanent
- 3Update distances of its neighbours via this node
- 4Repeat until all nodes are permanent
- 5Read paths by following predecessors
Example
Links A–B 2, A–C 5, B–C 1, B–D 4, C–D 1. From A: B = 2; C = min(5, 2 + 1) = 3; D = min(2 + 4, 3 + 1) = 4. Shortest path A → B → C → D with cost 4.
- Other algorithms: flooding (send on every line; robust but wasteful), distance vector (Bellman–Ford; count-to-infinity problem), link state, hierarchical routing.
Knowledge
Distances to destinations, learnt from neighbours
Full topology map flooded to all routers
Algorithm
Bellman–Ford
Dijkstra
Convergence
Slow; count-to-infinity problem
Fast
Examples
RIP
OSPF, IS-IS
Topic 7
Causes of congestion
- Too much traffic
- Input exceeds capacity of links or routers
- Slow processors
- Routers cannot handle queues quickly
- Insufficient buffers
- Packets dropped, then retransmitted, adding load
- Bursty traffic
- Many sources send at once
- Low-bandwidth links
- Fast networks feed into slow links
- Effect: as load nears capacity, delay rises sharply and throughput falls (congestion collapse) as retransmissions add to the load.
Topic 8
Congestion control strategies
- Congestion: too many packets in the subnet, so performance degrades — queues overflow and packets are lost.
Idea
Good design so congestion does not occur
Monitor, detect and act after congestion starts
Policies
Retransmission, out-of-order caching, acknowledgement, discard and admission policies
Choke packets, backpressure, explicit congestion notification
Examples
Traffic shaping with leaky bucket and token bucket
Load shedding, random early detection (RED)
- Leaky bucket: outputs packets at a constant rate whatever the input burst. Token bucket: tokens accumulate at a fixed rate; bursts are allowed up to the bucket size.
- TCP congestion control: slow start, congestion avoidance (additive increase, multiplicative decrease), fast retransmit and fast recovery.
Key terms
- FDDI
- 100 Mbps dual fibre token ring
- Fast Ethernet
- 100 Mbps Ethernet (IEEE 802.3u)
- Fibre Channel
- High-speed technology for storage area networks
- Dynamic routing
- Routes learnt automatically by protocols
- Congestion
- Excess traffic degrading network performance
Quick revision
- FDDI dual rings; Fast Ethernet TX, T4, FX; HIPPI; Fibre Channel SANs.
- 802.1, .2, .3, .4, .5, .11, .15, .16; LLC and MAC.
- Static vs dynamic routing; shortest path, flooding, distance vector, link state.
- Causes of congestion; open- and closed-loop control.
- Leaky bucket, token bucket, choke packets, TCP slow start.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Why does FDDI use two rings?
- Q2.Name the three media types of Fast Ethernet.
- Q3.What is Fibre Channel used for?
- Q4.Which IEEE standard defines Wi-Fi?
- Q5.Distinguish static and dynamic routing.
- Q6.State two causes of congestion.
Long-answer questions
- Q1.Explain FDDI and Fast Ethernet.
- Q2.Explain HIPPI, Fibre Channel and the IEEE 802 standards.
- Q3.Explain routing algorithms with an example.
- Q4.Explain the causes of congestion and congestion control strategies.
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