Medium Access Control Sublayer Notes

Concise version

Medium Access Control Sublayer

• This sublayer addresses the channel allocation problem and covers multiple access protocols, Ethernet, wireless LANs, broadband wireless, Bluetooth, RFID, and data link layer switching.

MAC Sublayer

The MAC sublayer decides who sends next on a multi-access link and is crucial for LANs.

Channel Allocation Problem

• Fixed Channel Allocation:

  • Divides bandwidth using FDM, TDM, CDMA, etc.

  • Static allocation, which is inefficient for bursty traffic.

• Dynamic Allocation:

  • Gives the channel to a user when needed.

  • Can be N times as efficient for N users.

  • Schemes vary based on assumptions:

    • Independent traffic: Allows analysis but is often not a good model.

    • Single channel: No external coordination.

    • Observable collisions: Needed for reliability.

    • Continuous or slotted time: Slotting may improve performance.

    • Carrier sense: Can improve performance if available.

Multiple Access Protocols

• ALOHA

• CSMA (Carrier Sense Multiple Access)

• Collision-free protocols

• Limited-contention protocols

• Wireless LAN protocols

ALOHA

• Pure ALOHA:

  • Users transmit frames whenever they have data.

  • Users retry after a random time after collisions.

  • Efficient and low-delay under low load.

• Collisions occur when other users transmit during a vulnerable period twice the frame time.

• Slotted ALOHA:

  • Synchronizing senders to slots can reduce collisions.

  • Twice as efficient as pure ALOHA.

  • Efficiency up to $1/e$ (37%) for random traffic models.

  • Low load wastes slots, high loads causes collisions.

CSMA (Carrier Sense Multiple Access)

• Improves on ALOHA by sensing the channel.

  • User doesn’t send if the channel is sensed as busy.

• Variations on what to do if the channel is busy:

  • 1-persistent: Sends as soon as the channel is idle (greedy).

  • Nonpersistent: Waits a random time, then tries again.

  • p-persistent: Sends with probability $p$ when idle.

• CSMA outperforms ALOHA; less persistence is better under high load.

CSMA with Collision Detection (CSMA/CD)

• Improvement: Detect and abort collisions.

• Reduced contention times improve performance.

• Collision time is much shorter than frame time.

Collision-Free Protocols

• Avoid collisions entirely.

• Senders must know when it is their turn to send.

• Bit-Map Protocol:

  • Sender sets a bit in contention slot if they have data.

  • Senders send in turn; everyone knows who has data.

• Token Ring:

  • Token sent around ring defines the sending order.

  • Station with token may send a frame before passing.

  • Can be used without a ring (e.g., token bus).

• Binary Countdown:

  • Stations send their address in contention slot ($\log N$ bits instead of $N$ bits).

  • Medium ORs bits; stations give up when they send a “0” but see a “1”.

  • Station that sees its full address is next to send.

Limited-Contention Protocols

• Divide stations into groups with only a small number likely to send.

• Avoids wastage due to idle periods and collisions.

• Adaptive Tree Walk:

  • Tree divides stations into groups (nodes) to poll.

  • Depth-first search under nodes with poll collisions.

  • Start search at lower levels if >1 station expected.

Wireless LAN Protocols

• Wireless has complications compared to wired:

  • Nodes may have different coverage regions.

    • Leads to hidden and exposed terminals.

  • Nodes can’t detect collisions while sending.

    • Makes collisions expensive and to be avoided.

• Hidden Terminals:

  • Senders that cannot sense each other but collide at the receiver.

  • Want to prevent; loss of efficiency.

• Exposed Terminals:

  • Senders who can sense each other but still transmit safely to different receivers.

  • Desirable concurrency; improves performance.

• MACA (Multiple Access with Collision Avoidance):

  • Protocol grants access for A to send to B:

    • A sends RTS (Request to Send) to B; B replies with CTS (Clear to Send).

    • A can send with exposed but no hidden terminals.

    • Stations hearing RTS defer for CTS; stations hearing CTS defer for data.

Ethernet

• Classic Ethernet

• Switched/Fast Ethernet

• Gigabit/10 Gigabit Ethernet

Classic Ethernet

• Physical Layer:

  • One shared coaxial cable to which all hosts are attached.

  • Up to 10 Mbps, with Manchester encoding.

  • Hosts ran the classic Ethernet protocol for access.

• MAC:

  • 1-persistent CSMA/CD.

  • Random delay (backoff) after collision is computed with BEB (Binary Exponential Backoff).

  • Frame format is still used with modern Ethernet (DIX, IEEE 802.3).

  • Collisions can occur and take as long as $2t$ to detect (where $t$ is the propagation time).

  • Leads to a minimum packet size for reliable detection.

• Performance:

  • Efficient for large frames, even with many senders.

  • Degrades for small frames (and long LANs).

  • Example: 10 Mbps Ethernet, 64-byte minimum frame.

Switched/Fast Ethernet

• Hubs wire all lines into a single CSMA/CD domain.

• Switches isolate each port to a separate domain.

  • Much greater throughput for multiple ports.

  • No need for CSMA/CD with full-duplex lines.

• Switches can be wired to computers, hubs, and switches.

• Fast Ethernet extended Ethernet from 10 to 100 Mbps.

  • Twisted pair (with Cat 5) dominated the market.

Gigabit / 10 Gigabit Ethernet

• Switched Gigabit Ethernet is now common.

  • With full-duplex lines between computers/switches.

• Gigabit Ethernet commonly runs over twisted pair.

• 10 Gigabit Ethernet is being deployed where needed.

• 40/100 Gigabit Ethernet is under development.

Wireless LANs (802.11)

• 802.11 architecture/protocol stack

• 802.11 physical layer

• 802.11 MAC

• 802.11 frames

802.11 Architecture/Protocol Stack

• Wireless clients associate to a wired AP (Access Point) in infrastructure mode.

• Ad-hoc mode exists with no AP, but it is rare.

• MAC is used across different physical layers.

• Different IEEE standards and their properties:

  • WiFi 7 (802.11be): 40000 Mbit/s, TBA, 2.4/5/6 GHz

  • WiFi 6E (802.11ax): 600 to 9608 Mbit/s, 2020, 6 GHz

  • WiFi 6 (802.11ax): 2019, 2.4/5 GHz

  • WiFi 5 (802.11ac): 433 to 6933 Mbit/s, 2014, 5 GHz

  • WiFi 4 (802.11n): 72 to 600 Mbit/s, 2018, 2.4/5 GHz

802.11 Physical Layer

• NICs are compatible with multiple physical layers (e.g., 802.11 a/b/g)

• Examples:

  • 802.11b: Spread spectrum, 2.4 GHz, 11 Mbps

  • 802.11g: OFDM, 2.4 GHz, 54 Mbps

  • 802.11a: OFDM, 5 GHz, 54 Mbps

  • 802.11n: OFDM with MIMO, 2.4/5 GHz, 600 Mbps

802.11 MAC

• CSMA/CA inserts backoff slots to avoid collisions.

• MAC uses ACKs/retransmissions for wireless errors.

• Virtual channel sensing with the NAV and optional RTS/CTS avoids hidden terminals.

• Different backoff slot times add quality of service.

  • Short intervals give preferred access (e.g., control, VoIP).

  • MAC has other mechanisms too (e.g., power save).

802.11 Frames

• Frames vary depending on their type (Frame control).

• Data frames have 3 addresses to pass via APs.

Broadband Wireless (802.16)

• 802.16 architecture / protocol stack

• 802.16 physical layer

• 802.16 MAC

• 802.16 frames

802.16 Architecture/Protocol Stack

• Wireless clients connect to a wired base station (like 3G).

• MAC is connection-oriented; IP is connectionless.

  • Convergence sublayer maps between the two.

802.16 Physical Layer

• Based on OFDM.

• Base station gives mobiles bursts (subcarrier/time frame slots) for uplink and downlink.

802.16 MAC

• Connection-oriented with base station in control.

• Clients request the bandwidth they need.

• Different kinds of service can be requested:

  • Constant bit rate (e.g., uncompressed voice).

  • Real-time variable bit rate (e.g., video, Web).

  • Non-real-time variable bit rate (e.g., file download).

  • Best-effort for everything else.

802.16 Frames

• Frames vary depending on their type.

• Connection ID instead of source/destination addresses.

Bluetooth

• Bluetooth Architecture

• Bluetooth Applications / Protocol Stack

• Bluetooth Radio / Link Layers

• Bluetooth Frames

Bluetooth Architecture

• Piconet master is connected to slave wireless devices.

• Slaves may be asleep (parked) to save power.

• Two piconets can be bridged into a scatternet.

Bluetooth Applications / Protocol Stack

• Profiles give the set of protocols for a given application.

• 25 profiles, including headset, intercom, streaming audio, remote control, personal area network, etc.

Bluetooth Radio / Link Layers

• Radio layer:

  • Uses adaptive frequency hopping in the 2.4 GHz band.

• Link layer:

  • TDM with timeslots for master and slaves.

  • Synchronous CO for periodic slots in each direction.

  • Asynchronous CL for packet-switched data.

  • Links undergo pairing (user confirms passkey/PIN) to authorize them before use.

Bluetooth Frames

• Time is slotted.

• Enhanced data rates send faster but for the same time.

• Addresses are only 3 bits for 8 devices.

Data Link Layer Switching

• Uses of Bridges

• Learning Bridges

• Spanning Tree

• Repeaters, hubs, bridges, .., routers, gateways

• Virtual LANs

Uses of Bridges

• Common setup is a building with centralized wiring.

• Bridges (switches) are placed in or near wiring closets.

Learning Bridges

• A bridge operates as a switched LAN (not a hub).

• Computers, bridges, and hubs connect to its ports.

• Backward learning algorithm picks the output port:

  • Associates source address on frame with input port.

  • Frame with destination address sent to learned port.

  • Unlearned destinations are sent to all other ports.

• Needs no configuration.

• Forgets unused addresses to allow changes.

• Bandwidth-efficient for two-way traffic.

• Bridges extend the Link layer:

  • Use but don’t remove Ethernet header/addresses.

  • Do not inspect Network header.

Spanning Tree

• Bridge topologies with loops and only backward learning will cause frames to circulate forever.

• Need spanning tree support to solve the problem.

• Subset of forwarding ports for data is used to avoid loops.

• Selected with the spanning tree distributed algorithm.

Repeaters, Hubs, Bridges, Switches, Routers, & Gateways

• Devices are named according to the layer they process.

• A bridge or LAN switch operates in the Link layer.

Detailed Version

Medium Access Control Sublayer

• The Medium Access Control (MAC) sublayer is a crucial component of the data link layer in network communication. It specifically addresses the channel allocation problem, which involves efficiently sharing a communication channel among multiple devices or nodes. This sublayer encompasses a variety of multiple access protocols designed to manage access to the shared medium. Key areas covered by the MAC sublayer include:

  • Multiple Access Protocols: Such as ALOHA, CSMA (Carrier Sense Multiple Access), collision-free protocols, limited-contention protocols, and wireless LAN protocols.

  • Ethernet: Both classic and modern implementations, including switched, fast, and Gigabit Ethernet.

  • Wireless LANs (802.11): Protocols and standards for wireless local area networks.

  • Broadband Wireless (802.16): Technologies and protocols for broadband wireless access.

  • Bluetooth: Short-range wireless communication technology.

  • RFID (Radio-Frequency Identification): Technology for identifying and tracking tags attached to objects.

  • Data Link Layer Switching: Techniques for forwarding data frames between network segments.

MAC Sublayer

The MAC sublayer plays a pivotal role in determining which device or node gets to transmit data next on a multi-access link. This decision-making process is particularly critical for Local Area Networks (LANs), where multiple devices share the same communication medium. The primary function of the MAC sublayer is to ensure that data transmission is coordinated and efficient, minimizing the potential for collisions and maximizing the utilization of the available bandwidth.

Channel Allocation Problem

The channel allocation problem arises from the need to share a limited communication channel among multiple users or devices. Efficiently allocating this channel is essential for maximizing network performance and minimizing delays. Two main approaches to channel allocation exist:

Fixed Channel Allocation:

• This approach divides the available bandwidth into fixed slots or channels, using techniques like Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), or Code Division Multiple Access (CDMA).

• Fixed channel allocation is a static allocation method, which means that the bandwidth is pre-assigned to users or devices, regardless of their actual needs. This can be inefficient, especially when dealing with bursty traffic, where users may have periods of high activity followed by periods of inactivity. During inactive periods, the assigned bandwidth remains unused, leading to wasted resources.

Dynamic Allocation:

• Dynamic allocation, on the other hand, assigns the channel to a user only when they need to transmit data. This approach can be significantly more efficient than fixed allocation, especially in scenarios with bursty traffic.

• Dynamic allocation schemes can be $N$ times as efficient for $N$ users compared to fixed allocation methods, as they allow bandwidth to be shared more flexibly and efficiently.

• The effectiveness of dynamic allocation schemes depends on several assumptions:

• Independent Traffic: Assumes that the traffic generated by each user is independent of the others. This assumption simplifies analysis but might not accurately reflect real-world scenarios where traffic patterns can be correlated.

• Single Channel: Assumes that there is only one communication channel available. This simplifies the coordination process but may limit performance in scenarios with high traffic demand.

• Observable Collisions: Requires the ability to detect collisions when multiple users attempt to transmit data simultaneously. Collision detection is essential for ensuring reliability, as it allows the system to take corrective actions, such as retransmitting the data.

• Continuous or Slotted Time: Considers whether time is continuous or divided into discrete slots. Slotting time can improve performance by synchronizing transmissions and reducing the likelihood of collisions.

• Carrier Sense: Determines whether devices can sense the channel before transmitting. Carrier sense capability can significantly improve performance by allowing devices to avoid transmitting when the channel is busy, thus reducing the probability of collisions.

Multiple Access Protocols

Multiple access protocols are sets of rules and procedures that govern how multiple devices share a communication channel. These protocols are essential for coordinating access to the channel and minimizing interference. Common multiple access protocols include:

• ALOHA

• CSMA (Carrier Sense Multiple Access)

• Collision-free protocols

• Limited-contention protocols

• Wireless LAN protocols

ALOHA

ALOHA is a simple multiple access protocol that allows users to transmit data whenever they have data to send. There are two main versions of ALOHA:

• Pure ALOHA:

  • In pure ALOHA, users transmit frames whenever they have data to send, without any coordination or synchronization.

  • If a collision occurs (i.e., two or more users transmit simultaneously), the users involved retry their transmissions after a random amount of time. This randomization helps to avoid repeated collisions.

  • Pure ALOHA is efficient and offers low delay under low load conditions, as users can transmit data immediately without waiting for permission or synchronization.

• Collisions in pure ALOHA occur when other users transmit during a vulnerable period that is twice the frame time. This is because a collision can happen if another user starts transmitting up to one frame time before or after the start of the current transmission.

• Slotted ALOHA:

  • Slotted ALOHA is a variation of pure ALOHA that synchronizes senders to discrete time slots, which can reduce the likelihood of collisions.

  • In slotted ALOHA, time is divided into slots, and users are only allowed to begin transmitting at the beginning of a slot. This synchronization reduces the vulnerable period to one frame time, as collisions can only occur if two or more users start transmitting in the same slot.

  • Slotted ALOHA is twice as efficient as pure ALOHA because it reduces the vulnerable period by half.

  • The maximum efficiency of slotted ALOHA is approximately $1/e$ (37%) for random traffic models. This means that, under ideal conditions, slotted ALOHA can utilize up to 37% of the available bandwidth.

  • However, slotted ALOHA has its limitations. Under low load conditions, it can waste slots because users may not always have data to send at the beginning of each slot. Conversely, under high load conditions, it can still experience collisions, leading to reduced performance.

CSMA (Carrier Sense Multiple Access)

CSMA (Carrier Sense Multiple Access) is a multiple access protocol that improves upon ALOHA by requiring users to sense the channel before transmitting data. This helps to reduce collisions by allowing users to avoid transmitting when the channel is already in use.

• In CSMA, a user listens to the channel to determine whether it is currently busy. If the channel is sensed as idle, the user may proceed with transmission. However, if the channel is sensed as busy, the user must defer transmission according to a specific rule.

• There are several variations of CSMA, each with its own rule for what to do when the channel is busy:

  • 1-persistent CSMA: In this variation, the user continuously senses the channel and transmits as soon as the channel becomes idle. This approach is considered "greedy" because the user is always ready to transmit as soon as possible.

  • Nonpersistent CSMA: In nonpersistent CSMA, the user waits a random amount of time after sensing a busy channel before trying again. This randomization helps to reduce the likelihood of repeated collisions.

  • p-persistent CSMA: This variation is used in slotted systems. When the channel is idle, the user transmits with a probability $p$. If the user does not transmit (with probability $1-p$), it defers to the next slot.

• CSMA generally outperforms ALOHA because it reduces the number of collisions. Less persistence (i.e., waiting longer before retransmitting) tends to be better under high load conditions, as it helps to avoid congestion and reduce the likelihood of collisions.

CSMA with Collision Detection (CSMA/CD)

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is an enhancement to CSMA that allows users to detect collisions while they are transmitting data. This enables them to abort the transmission early, reducing the amount of time wasted on transmitting colliding frames.

• In CSMA/CD, a user listens to the channel while transmitting. If a collision is detected (i.e., the user senses that another user is also transmitting), the user immediately stops transmitting and sends a jamming signal to ensure that all other users are aware of the collision.

• The time it takes to detect a collision is typically much shorter than the frame time, so aborting collisions early can significantly improve performance.

• Reduced contention times result in improved performance compared to standard CSMA.

Collision-Free Protocols

Collision-free protocols are designed to completely avoid collisions by ensuring that senders know when it is their turn to transmit data. These protocols require some form of coordination or synchronization among the senders.

• Bit-Map Protocol:

  • In the bit-map protocol, each sender is assigned a bit in a contention slot. If a sender has data to transmit, it sets its bit in the contention slot.

  • After the contention slot, senders transmit in turn, based on the order of their assigned bits. Because everyone knows which senders have data to transmit, collisions are avoided.

• Token Ring:

  • The token ring protocol uses a special token that is passed around a ring. A station that wants to transmit must first capture the token.

  • The station with the token may send a frame before passing the token to the next station in the ring.

  • The token ring protocol can be used without a physical ring topology (e.g., token bus).

• Binary Countdown:

  • In the binary countdown protocol, stations send their address in a contention slot. The length of the contention slot is $\log N$ bits, where $N$ is the number of stations.

  • The medium performs a bitwise OR operation on the bits sent by the stations. If a station sends a "0" but sees a "1" on the medium, it gives up. The station that sees its full address on the medium is the next to transmit.

Limited-Contention Protocols

Limited-contention protocols divide stations into groups, with only a small number of stations likely to contend for the channel at any given time. This approach combines the advantages of both contention-based and collision-free protocols.

• By limiting the number of stations that contend for the channel, these protocols avoid the wastage that can occur due to idle periods and collisions.

• Adaptive Tree Walk:

  • The adaptive tree walk protocol uses a tree structure to divide stations into groups or nodes. The protocol polls the nodes of the tree to determine which stations have data to transmit.

  • The protocol performs a depth-first search under nodes where poll collisions occur. This allows the protocol to efficiently identify the stations that are contending for the channel.

  • The protocol can start the search at lower levels of the tree if it expects that more than one station will have data to transmit. This adaptive approach helps to optimize performance under different load conditions.

Wireless LAN Protocols

Wireless LANs (WLANs) present unique challenges compared to wired LANs, including:

• Nodes may have different coverage regions, leading to hidden and exposed terminal problems.

• Nodes cannot detect collisions while sending, making collisions more expensive and something to be avoided.

• Hidden Terminals:

  • Hidden terminals are senders that cannot sense each other but whose transmissions collide at the receiver. This can lead to loss of efficiency.

• Exposed Terminals:

  • Exposed terminals are senders who can sense each other but who can still transmit safely to different receivers. Allowing these concurrent transmissions improves performance.

• MACA (Multiple Access with Collision Avoidance):

  • The MACA protocol is designed to avoid collisions in wireless LANs. It works by having the sender (A) request permission to send to the receiver (B).

  • Sender A sends an RTS (Request to Send) frame to receiver B. Receiver B replies with a CTS (Clear to Send) frame.

  • If A receives the CTS frame, it can proceed with the transmission. This approach avoids hidden terminals because stations that hear the RTS frame defer for the CTS frame, and stations that hear the CTS frame defer for the data.

Ethernet

Ethernet is a widely used networking technology for LANs. There are several variations of Ethernet, including:

• Classic Ethernet

• Switched/Fast Ethernet

• Gigabit/10 Gigabit Ethernet

Classic Ethernet

• Physical Layer:

  • Classic Ethernet used a shared coaxial cable to which all hosts were attached.

  • It supported data rates of up to 10 Mbps and used Manchester encoding.

  • Hosts ran the classic Ethernet protocol for access to the shared cable.

• MAC:

  • Classic Ethernet used 1-persistent CSMA/CD for medium access.

  • After a collision, a random delay (backoff) is computed using the Binary Exponential Backoff (BEB) algorithm.

  • The frame format used in classic Ethernet is still used with modern Ethernet (DIX, IEEE 802.3).

  • Collisions can occur and take as long as $2t$ to detect, where $t$ is the propagation time. This leads to a minimum packet size for reliable collision detection.

• Performance:

  • Classic Ethernet is efficient for large frames, even with many senders.

  • However, its performance degrades for small frames (and long LANs).

  • For example, a 10 Mbps Ethernet with a 64-byte minimum frame size.

Switched/Fast Ethernet

• Hubs wire all lines into a single CSMA/CD domain. Switches, on the other hand, isolate each port to a separate domain, providing much greater throughput for multiple ports.

• With full-duplex lines, there is no need for CSMA/CD.

• Switches can be wired to computers, hubs, and other switches.

• Fast Ethernet extended Ethernet from 10 to 100 Mbps. Twisted pair (with Cat 5) dominated the market.

Gigabit / 10 Gigabit Ethernet

• Switched Gigabit Ethernet is now common, with full-duplex lines between computers and switches.

• Gigabit Ethernet commonly runs over twisted pair.

• 10 Gigabit Ethernet is being deployed where needed.

• 40/100 Gigabit Ethernet is under development.

Wireless LANs (802.11)

• 802.11 architecture/protocol stack

• 802.11 physical layer

• 802.11 MAC

• 802.11 frames

802.11 Architecture/Protocol Stack

• Wireless clients associate to a wired AP (Access Point) in infrastructure mode.

• Ad-hoc mode exists with no AP, but it is rare.

• MAC is used across different physical layers.

• Different IEEE standards and their properties:

  • WiFi 7 (802.11be): 40000 Mbit/s, TBA, 2.4/5/6 GHz

  • WiFi 6E (802.11ax): 600 to 9608 Mbit/s, 2020, 6 GHz

  • WiFi 6 (802.11ax): 2019, 2.4/5 GHz

  • WiFi 5 (802.11ac): 433 to 6933 Mbit/s, 2014, 5 GHz

  • WiFi 4 (802.11n): 72 to 600 Mbit/s, 2018, 2.4/5 GHz

802.11 Physical Layer

• NICs are compatible with multiple physical layers (e.g., 802.11 a/b/g)

• Examples:

  • 802.11b: Spread spectrum, 2.4 GHz, 11 Mbps

  • 802.11g: OFDM, 2.4 GHz, 54 Mbps

  • 802.11a: OFDM, 5 GHz, 54 Mbps

  • 802.11n: OFDM with MIMO, 2.4/5 GHz, 600 Mbps

802.11 MAC

• CSMA/CA inserts backoff slots to avoid collisions.

• MAC uses ACKs/retransmissions for wireless errors.

• Virtual channel sensing with the NAV and optional RTS/CTS avoids hidden terminals.

• Different backoff slot times add quality of service. Short intervals give preferred access (e.g., control, VoIP).

• MAC has other mechanisms too (e.g., power save).

802.11 Frames

• Frames vary depending on their type (Frame control).

• Data frames have 3 addresses to pass via APs.

Broadband Wireless (802.16)

• 802.16 architecture / protocol stack

• 802.16 physical layer

• 802.16 MAC

• 802.16 frames

802.16 Architecture/Protocol Stack

• Wireless clients connect to a wired base station (like 3G).

• MAC is connection-oriented; IP is connectionless. Convergence sublayer maps between the two.

802.16 Physical Layer

• Based on OFDM.

• Base station gives mobiles bursts (subcarrier/time frame slots) for uplink and downlink.

802.16 MAC

• Connection-oriented with base station in control.

• Clients request the bandwidth they need.

• Different kinds of service can be requested:

  • Constant bit rate (e.g., uncompressed voice)

  • Real-time variable bit rate (e.g., video, Web)

  • Non-real-time variable bit rate (e.g., file download)

  • Best-effort for everything else

802.16 Frames

• Frames vary depending on their type.

• Connection ID instead of source/destination addresses.

Bluetooth

• Bluetooth Architecture

• Bluetooth Applications / Protocol Stack

• Bluetooth Radio / Link Layers

• Bluetooth Frames

Bluetooth Architecture

• Piconet master is connected to slave wireless devices.

• Slaves may be asleep (parked) to save power.

• Two piconets can be bridged into a scatternet.

Bluetooth Applications / Protocol Stack

• Profiles give the set of protocols for a given application.

• 25 profiles, including headset, intercom, streaming audio, remote control, personal area network, etc.

Bluetooth Radio / Link Layers

• Radio layer:

  • Uses adaptive frequency hopping in the 2.4 GHz band.

• Link layer:

  • TDM with timeslots for master and slaves.

  • Synchronous CO for periodic slots in each direction.

  • Asynchronous CL for packet-switched data.

  • Links undergo pairing (user confirms passkey/PIN) to authorize them before use.

Bluetooth Frames

• Time is slotted.

• Enhanced data rates send faster but for the same time.

• Addresses are only 3 bits for 8 devices.

Data Link Layer Switching

• Uses of Bridges

• Learning Bridges

• Spanning Tree

• Repeaters, hubs, bridges, .., routers, gateways

• Virtual LANs

Uses of Bridges

• Common setup is a building with centralized wiring.

• Bridges (switches) are placed in or near wiring closets.

Learning Bridges

• A bridge operates as a switched LAN (not a hub).

• Computers, bridges, and hubs connect to its ports.

• Backward learning algorithm picks the output port:

  • Associates source address on frame with input port.

  • Frame with destination address sent to learned port.

  • Unlearned destinations are sent to all other ports.

• Needs no configuration.

• Forgets unused addresses to allow changes.

• Bandwidth-efficient for two-way traffic.

• Bridges extend the Link layer: Use but don’t remove Ethernet header/addresses. Do not inspect Network header.

Spanning Tree

• Bridge topologies with loops and only backward learning will cause frames to circulate forever.

• Need spanning tree support to solve the problem.

• Subset of forwarding ports for data is used to avoid loops.

• Selected with the spanning tree distributed