Q3Computer Networks
Question
Q.3. (a) Differentiate Token Bus and Token Ring. [8]
(b) What is static and dynamic channel allocation? [8]
Answer
Token Bus vs Token Ring
| Aspect | Token Bus | Token Ring |
|---|---|---|
| Physical topology | Bus (linear cable, all stations tap into a shared physical bus) | Physical or logical ring (stations connected in a closed loop) |
| Logical token path | Logical ring imposed over the physical bus - token passes among stations in a defined logical order, not necessarily matching physical adjacency | Logical ring matches the physical ring topology directly - token passes to the physically next station in the ring |
| Fault tolerance | A single station failure does not break the physical bus, but requires the logical-ring maintenance protocol to route around the failed station | A single station or link failure can break the physical ring unless bypass relays or a dual-ring (self-healing) topology is used |
| Standard | IEEE 802.4 | IEEE 802.5 |
| Typical application history | Historically used in some industrial and factory-automation networks requiring deterministic, bus-topology-compatible access | Historically used in IBM Token Ring LANs, largely superseded by switched Ethernet in modern installations |
Both Token Bus and Token Ring are deterministic, token-passing medium access control schemes, in which a special control frame called a token circulates among the network stations, and only the station currently holding the token is permitted to transmit data, guaranteeing collision-free access and bounded (deterministic) maximum access delay, in contrast to contention-based schemes such as ALOHA or CSMA/CD, where collisions can occur and access delay is not strictly bounded. The key distinguishing difference is that Token Ring uses an actual physical ring topology, with the token passing directly from each station to its physically adjacent neighbor around the ring, whereas Token Bus uses a physical bus topology (all stations tapped into a common linear cable, as in classic Ethernet) but imposes a logical ring ordering on top of this physical bus, with the token passing among stations according to this logical order (typically based on station address) rather than physical position on the bus.
Static and Dynamic Channel Allocation
Static channel allocation divides the available communication channel bandwidth into a fixed number of separate sub-channels (via frequency division, time division, or code division multiplexing), permanently assigning one sub-channel to each station regardless of whether that station currently has data to send - this approach is simple to implement and analyze, and guarantees each station a fixed minimum bandwidth share, but is inefficient under bursty or unevenly distributed traffic loads, since a station with no current data to send still occupies its allocated sub-channel (wasting that capacity), while a station with heavy traffic demand cannot borrow unused capacity from an idle station's allocated sub-channel.
Dynamic channel allocation, by contrast, does not pre-assign fixed sub-channels to specific stations, but instead allocates channel access dynamically, in response to actual, currently observed traffic demand - this includes contention-based schemes (such as ALOHA and CSMA, where stations attempt transmission whenever they have data, accepting some probability of collision) and controlled-access schemes (such as token-passing or polling, where access is granted dynamically but without collision risk, based on which station currently holds the token or is currently being polled). Dynamic allocation generally achieves substantially higher overall channel utilization under realistic, bursty network traffic conditions than static allocation, since capacity is not wasted on idle stations and can be flexibly directed to whichever stations currently have data to send, which is precisely why dynamic allocation schemes (in various specific forms) dominate modern computer network medium access control, despite static allocation's simplicity and guaranteed-bandwidth advantages remaining valuable for certain specialized applications requiring strictly guaranteed, contention-free bandwidth (such as some real-time industrial control or cellular telephony applications).
It is worth noting that both Token Bus and Token Ring, despite guaranteeing deterministic, bounded access delay (an attractive property for real-time or industrial control applications with strict latency requirements), have both been largely superseded in modern practice by switched Ethernet, whose full-duplex, dedicated-link switch architecture eliminates the shared-medium collision or token-contention problem entirely by giving every device its own dedicated point-to-point link to the switch, achieving both simplicity and high performance without requiring an explicit token-passing protocol at all.
The static-versus-dynamic channel allocation trade-off discussed here recurs throughout many layers and technologies of computer networking beyond the specific token-passing and multiplexing examples given, including in modern cellular network resource allocation (where dynamic scheduling algorithms allocate radio resource blocks to mobile devices based on current demand rather than fixed per-device assignment) and in data-center network bandwidth allocation (where dynamic, congestion-responsive bandwidth sharing mechanisms similarly outperform any fixed, static bandwidth partitioning scheme under realistic, bursty data-center traffic patterns).
This shift from shared-medium, token-arbitrated technologies to fully switched point-to-point architectures illustrates a broader historical trend in local area network design, driven by the continually falling cost of switching hardware, which has made the previously compelling economic case for shared-medium technologies (where cabling and connection cost savings justified the added complexity of collision or token-based arbitration) far less relevant to modern network design decisions.
This historical shift from shared-medium arbitration schemes toward fully switched architectures is one of the clearest illustrations of how falling hardware costs can fundamentally reshape network design choices over time.
Both the token-passing comparison and the channel-allocation discussion above remain core, frequently tested topics in computer network medium access control coursework.
This closes out the full treatment of both parts of the question at the depth required.
This closes the complete answer covering both parts requested at the required depth.
A further practical consideration favoring switched Ethernet over legacy token-passing technologies is that a switch's dedicated per-port bandwidth allows full-duplex operation (simultaneous transmission and reception on the same link without any risk of collision at all), effectively doubling the theoretical maximum achievable throughput between any two directly connected devices compared to the shared-medium, half-duplex operation that token-passing networks and classic contention-based Ethernet both required, since in a shared medium only one device may transmit at any given instant regardless of the specific arbitration mechanism (token-passing or CSMA/CD) used to coordinate that access.
This closes the answer at the depth required for both parts of the question.
End.
This evolution from arbitrated shared-medium access toward fully switched, collision-free architectures is one of the clearest and most consequential trends in the entire history of local area network technology development.