The physical, datalink and network layers are the----------------------support layers

Data Link Layer

The data link layer handles such tasks as gathering up sets of bits for transmission as packets and making sure the packets get from one end to the other. In addition, recognizing that physical layer transmission sometimes introduces errors, the data link layer handles error detection (and sometimes correction).

When the OSI model was first introduced, data communication was point to point. With time, data communication has become much more multipoint to multipoint, so the data link layer was divided to recognize multipoint needs. With the split of the data link layer, the traditional functions of the layer went to the upper portion, called the logical link layer. A new sublayer, media access control (MAC), was defined. The MAC layer routes packets from a sender to a receiver along a common path. It makes sure the message arrives at the intended recipient. The MAC layer adds a physical address, defining the intended recipient machine, and controls shared access to a resource. For example, control of TDMA operation (see Section 4.3.1) is a MAC layer function.

Data Link Layer

The data link layer takes the data bits and “frames,” and creates packets of the data to guarantee reliable transmission. This layer adds source and destination addresses to the data stream as well as information to detect and control transmission errors. The data link layer has two sublayers. One is the logical link control (LLC) sublayer, which essentially maintains the communications link between two devices on the network. The other is the media access control (MAC) sublayer which manages the transmission of data between two devices. The network card on a PC has a MAC address, essentially a unique address for every device on a local area network.

The details of the data link layer can be specified differently and are reflected in various network types (Ethernet, token ring, etc.). Each network type has its own method of addressing, error detection, control of network flow, and so forth.

(2) Data-link layer

The data-link layer is responsible for transferring messages (or frame) from a given node to all other nodes in the CAN network. This layer handles bit stuffing and checksums for error handling, and after sending a message, waits for acknowledgment from the receivers. It is subdivided into two further layers:

the logical link control layer (LLC), which accepts messages by a filtering process; overload notification and recovery management tasks will be taken care of by this layer;

the medium access control layer (MAC), which carries out data encapsulation, frame coding and arbitration, media access management, error detection and signaling and acknowledgment tasks.

(iii) Communication services

The data-link layer provides the application layer with send and request data with reply, and send data with no acknowledge communication services. For the former, the master issues a command or sends data to the slave and receives a reply within a defined time span. This reply either consists of an acknowledgement (brief acknowledgement) or is the requested data. In the send data with no acknowledge, the data are sent to a whole group of slaves. This permits event-controlled synchronization, where all slaves set their outputs simultaneously (synchronous mode) or update their input data simultaneously (freeze mode). A master-controlled bus assignment for slave replies is not possible in this case so that SDN telegrams remain unacknowledged. The access of the application to these basic forms of communication, as well as the various data-link services based on them is granted via so-called service access points which are used by the higher layers (user interfaces in some of Profibus networks) to perform all communication tasks of the respective application program.

21.5.3 IEEE 802.11 Data Link Layer

The data link layer within 802.11 consists of two sublayers: logical link control (LLC) and media access control (MAC). 802.11 uses the same 802.2 LLC and 48-bit addressing as the other 802 LAN, allowing for simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLAN. The sublayer above MAC is the LLC, where the framing takes place. The LLC inserts certain fields in the frame such as the source address and destination address at the head end of the frame and error handling bits at the end of the frame.

The 802.11 MAC is similar in concept to 802.3, in that it is designed to support multiple users on a shared medium by having the sender sense the medium before accessing it. For the 802.3 Ethernet LAN, the carrier sense multiple access with collision detection (CSMA/CD) protocol regulates how Ethernet stations establish access to the network and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN. In an 802.11 WLAN, collision detection is not possible due to the near/far problem (see Chapter 11). To detect a collision, a station must be able to transmit and listen at the same time, but in radio systems the transmission drowns out the ability of a station to hear a collision.

4.2.1 Data Link Layer

Some data link layer protocols offer capabilities to save energy. This is especially important for mobile devices (like laptops or sensor nodes) that are equipped with and rely on (rechargeable) battery packs. One example for such a data link protocol that could easily be enhanced for energy-aware mechanisms are the IEEE 802.11 standards. 802.11 is a protocol widely used in the field of wireless data transmission. A lot of energy has to be used for the transmission of data, but also for receiving and even for just sensing the wireless channel. In infrastructure mode, a wireless node can notify the access point of a 802.11 network that it will go to sleep mode. This indication can be done by setting a special flag inside the 802.11 header and thereby telling the access point that the node will not listen for any further frames until the next beacon frame is sent. A beacon frame is usually sent every 100 ms, and the wakeup time of a node is approximately 250 μs. When the access point receives a frame that indicates sleep mode of a specific node, it will buffer all packages that it should send to the node and transmit it as soon as the node becomes available again. Of course, if there are no frames that were buffered in the mean time, the node could go to sleep again and save some energy [8].

13.23.6.4 Data Link Layer

The data link layer considers data transfer between two nodes where they can share the same link. The MAC protocol mainly consists of fairness, bandwidth utilization, flow control, frame synchronization, and error control, which are the keywords for effective data communication. Correction and error detection are done in the data link layer. The fixed data block sizes are agreed between sender and receiver before sending data. The 8-bit cyclic redundancy check (CRC) (72) is used for error detection. There are a few recovery techniques in WSNs, such as simple packet combining, hybrid automatic repeat request (ARQ) (72), and forward error detection. Timeout and positive–negative acknowledgment used through an ARQ as feedback for the sender. The design of the MAC protocol consists of energy, topology, and network topology to minimize energy by extending the network lifetime. It also prevents packet collisions, overhearing, and excessive retransmission.

Berkeley MAC protocol (73): The Berkeley MAC (B-MAC) is a protocol that controls low-power processing, collision avoidance, and high channel utilization. Clear channel assessment, packet backoff, link layer acknowledgment, low-power listening function by a B-MAC protocol. It supports the link layer acknowledgement. An acknowledgment packet is sent after receiving a packet by the receiver. An adaptive preamble sampling scheme, such as low-power listening, is used to reduce power consumption. It performs both in the awake and sleep period cycling.

Collaborative protocol (74): The collaborative protocol consists of an event MAC (E-MAC) and a network MAC (N-MAC). It can prevent redundant transmission as well as correlation of data at the MAC layer. For transmitting data, a single representative sensor node works when the other sensor nodes are backed off for a time. Furthermore, the E-MAC protocol filters out correlated packets and N-MAC protocol routes those packets to the sink. Energy savings, latency, and packet drop rate features of this protocol.

Power-consumed distributed MAC protocol (75): This protocol combines carrier sense multiple access with collision avoidance (CSMA)/CA) and multichannel spread spectrum techniques. In this network, a unique channel and code are assigned across each node's two-hop neighbors. This protocol is used to avoid collisions and minimize energy waste. It also introduces a low-power wakeup radio and normal data radio operation to save energy. Thus, the energy consumption of channel monitoring is significantly reduced.

Traffic-adaptive medium-access protocol (76): The traffic-adaptive medium-access protocol (TRAMA) is used to increase channel utilization and energy efficiency. The nodes start in random access mode and transmit the data at random slots. TRAMA consists of three parts: NP, schedule exchange protocol, and adaptive election algorithm. To gather neighbor updates, small signaling packets are sent out by the NP. Those packets are used to maintain connectivity between the neighbors. At the time of scheduled access, the schedule information is broadcasted by the scheduled access protocol. A schedule is generated by the node after calculating the schedule interval. The last adaptation election algorithm can determine the state of node. To save energy, the nodes are switched to sleep mode.

Z-MAC protocol (77): The Z-MAC is a hybrid MAC protocol that consists of with high-channel utilization and low latency under high contention. It can combine strength and enhance the contention resolution of the TDMA and CSMA. It can reduce the collision between two hop neighbors, thus limiting cost. This protocol can change time synchronization failures in the network. For slight assignment and channel reassignment, an efficient and scalable channel scheduling algorithm of this protocol is used. A node can transmit the packet data when the channel is clear. The main objective is to reuse a slot when the data is not transmitted. To improve timing failures, channel conditions, and slot assignment failures, the CSMA, TDMA, and Z-MAC can be mixed.

Power reservation-based protocol (78): This protocol is used for the issue of energy conservation and adaptation to traffic. The slot reservation, schedule establishment, and data transmission function by the TDMA-like frame structure. The probability of successful data transmission depends on adopting the TDMA with fixed frame size. Moreover, the frame size will decrease while the number of failures is small. By increasing throughput, the nodes can transmit at a high data rate.

20.5.2 Data Link Layer

The data link layer is responsible for multiplexing data streams, data frame detection, medium access, and error control. It ensures reliable point-to-point and point-to-multipoint connections in a communication network. The MAC protocol in a wireless multihop self-organizing sensor network should achieve two goals. The first is the creation of the network infrastructure. Since thousands of sensor nodes are densely scattered in a sensor field, MAC must establish communication links for data transfer. This forms the basic infrastructure needed for wireless communication hop to hop and gives the sensor network self-organizing ability. The second objective is to fairly and efficiently share communication resources between sensor nodes. Since the environment is noisy and sensor nodes can be mobile, the MAC protocol must be power-aware and be able to minimize collision with neighbors' broadcasts. Table 20.1 provides a qualitative overview of MAC protocols for sensor networks.

Table 20.1. MAC protocols for sensor networks.

MAC protocolChannel access modeSensor network specificsPower conservationSelf-organizing Media Access Control for Sensor networks (SMACS) and Eaves-drop-and-Register (EAR) Algorithm [20]Fixed allocation of duplex time slots at fixed frequencyExploitation of large available bandwidth compared to sensor data rateRandom wake-up during setup and turning radio off while idleHybrid TDMA/FDMA [18]Centralized frequency and time divisionOptimum number of channels calculated for minimum system energyHardware-based approach for system energy minimizationCSMA-based [21]Contention-based random accessApplication phase shift and pretransmit delayConstant listening time for energy efficiency

Regardless of which type of MAC scheme is used for sensor networks, it certainly must have built-in power-saving mechanisms and strategies for proper management of node mobility or failure. The most obvious means of power conservation is to turn the transceiver off when it is not required. Although this power-saving method seemingly provides significant energy gains, an important point that must not be overlooked is that sensor nodes communicate using short data packets. The shorter the packets, the more the dominance of startup energy. Operation in a power-saving mode is energy efficient only if the time spent in that mode is greater than, a certain threshold. There can be a number of such useful modes of operation for the wireless sensor node, depending on the number of states of the microprocessor, memory, A/D converter, and transceiver. Each of these modes should be characterized by its power consumption and latency overhead, which is the transition power to and from that mode. The main features of sensor MAC are periodic listen and sleep, collision and overhearing avoidance, and message passing. The duration of the sleep and awake cycles are application-dependent and they are set the same for all nodes.

Since a sensor node has limited power resources, forward error correction (FEC) is more feasible than automatic repeat request (ARQ), which in a multihop sensor network environment is limited by additional retransmission energy cost and overhead.

56.5.2 Bridges

As more and more devices become attached to the LAN, the utilisation increases. As the amount of traffic on the network approaches the prescribed operating limits of the LAN, network efficiency begins to decrease.

For example, on a non-contention LAN, such as an IEEE 802.5 Token Passing Ring, the station must wait a finite time before the token makes its way around the ring to it, thereby enabling it to transmit. This minimum time to transmission increases with the number of attached devices. Similarly, on a contention LAN, such as an IEEE 802.3 CSMA/CD, as the number of devices contending for control of the medium increases, the number of collisions on the network also rises, thereby reducing its overall performance.

Analysis of the nature of traffic on the LAN, in most cases, led to the conclusion that effectively segmenting the network into two or more separate LANs could increase overall network performance.

This would certainly be the case when the network could be segmented into heavy users, (e.g. CAD/CAM users, computer rooms), and light users, (e.g. word processing). (See Figure 56.17.)

A further advantage of such segmentation is that failures on a particular segment of the LAN can be contained within that segment.

The solution to the congestion problem led to the development of the bridge, or to be more precise, the local bridge.

Since a local bridge could be used to create two networks out of a single network, it followed that it could also be employed to create one logical extended network out of two, (or more), locally dispersed LANs.

56.5.2.1 Local bridges

Bridges operate at the Data Link Layer, (Layer 2), of the OSI Model, or more specifically at the Media Access Control, (MAC), sublayer. (See Figure 56.18.)

Figure 56.18. OSI model of a bridge

Upon receipt of a data packet, bridges examine the source and destination address of the data packet. If the destination device is on a network other than that of the source device, then the bridge will ‘FORWARD’ the packet onto the extended network. If the destination device address is on the same network segment as the source device, then the bridge will not forward the packet; instead it will block its path onto the extended network effectively keeping it local. In this way the bridge acts as a ’ FILTER’ of data packets.

By means of ‘Filtering and Forwarding’ bridges can create one single logically unified network out of several locally discrete LANs, while at the same time limiting the flow of unnecessary traffic between them.

Organisations that had local area networks within their geographically dispersed operating sites soon began to express a desire to interconnect all their remote LANs together, to effectively build a single organisation-wide area network, irrespective of geography.

Manufacturers responded to this requirement with two types of solutions, remote bridges and routers.

Manufacturers offered as a simple, ‘transparent’ solution, bridges that incorporated wide area interfaces to provide access to different types of media, such as leased or switched analog telephone lines, 48, 56, 64kbit/s lines, or E-1 or T-1 lines.

Which layers are support layers?

The physical layer, data link layer and the network layer are the network support layers. The layers manage a physical transfer of data from one device to another. Session layer, presentation layer, and application layer are the user support layers.

What is physical and datalink layer?

The data link layer is an interface between the network and physical layer. It is further subdivided into two modules: Medium Access Control (MAC) and Logical Link Control (LLC). The MAC module plays a critical role in conserving network life by efficiently allocating medium access to the contending nodes.

Which layer links the network support layer?

The Transport layer links the network support layers and user support layers.

What are responsibilities of physical data link and network layer?

The network layer uses network addresses (typically Internet Protocol addresses) to route packets to a destination node. The data link layer establishes and terminates a connection between two physically-connected nodes on a network. It breaks up packets into frames and sends them from source to destination.