ITNv7 Ch 6

MAC sublayer

controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium.

cyclic redundancy check (CRC) value

A transmitting node creates a logical summary of the contents of the frame

At each hop along the path, a router performs the following Layer 2 functions:

Accepts a frame from a medium De-encapsulates the frame Re-encapsulates the packet into a new frame Forwards the new frame appropriate to the medium of that segment of the physical network

Bus

All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Legacy Ethernet networks were often bus topologies using coax cables because it was inexpensive and easy to set up.

Examples of contention-based access methods include the following:

Carrier sense multiple access with collision detection (CSMA/CD) used on legacy bus-topology Ethernet LANs Carrier sense multiple access with collision avoidance (CSMA/CA) used on Wireless LANs The image shows three PCs connected to an Ethernet hub. Two of the PCs are sending frames simultaneously.

Full-duplex communication

Both devices can simultaneously transmit and receive on the shared media. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but they can operate in half-duplex if connecting to a device such as an Ethernet hub. Click play in the figure to see the animation showing full-duplex communication. full duplex communication between a server and a hub

Half-duplex communication

Both devices can transmit and receive on the media but cannot do so simultaneously. WLANs and legacy bus topologies with Ethernet hubs use the half-duplex mode. Half-duplex allows only one device to send or receive at a time on the shared medium. Click play in the figure to see the animation showing half-duplex communication. half duplex communication between a server and a hub

CMSA/CA uses a method similar to CSMA/CD to detect if the media is clear. CMSA/CA uses additional techniques. In wireless environments it may not be possible for a device to detect a collision.

CMSA/CA does not detect collisions but attempts to avoid them by waiting before transmitting. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.

Data

Contains the frame payload (i.e., packet header, segment header, and the data).

Some multiaccess networks require rules to govern how devices share the physical media. There are two basic access control methods for shared media:

Contention-based access Controlled access

The data link layer does the following:

Enables upper layers to access the media. The upper layer protocol is completely unaware of the type of media that is used to forward the data.

Ring

End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated, unlike in the bus topology. Legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks used ring topologies.

Data link layer protocols include:

Ethernet 802.11 Wireless Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay

The data link protocol is responsible for NIC-to-NIC communications within the same network. Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts:

Header Data Trailer

Control

Identifies special flow control services such as quality of service (QoS). QoS gives forwarding priority to certain types of messages. For example, voice over IP (VoIP) frames normally receive priority because they are sensitive to delay.

Type

Identifies the Layer 3 protocol in the data field.

Physical topology

Identifies the physical connections and how end devices and intermediary devices (i.e, routers, switches, and wireless access points) are interconnected. The topology may also include specific device location such as room number and location on the equipment rack. Physical topologies are usually point-to-point or star.

Media Access Control (MAC)

Implements this sublayer (IEEE 802.3, 802.11, or 802.15) in hardware. It is responsible for data encapsulation and media access control. It provides data link layer addressing and it is integrated with various physical layer technologies.

The data link layer provides the addressing used in transporting a frame across a shared local media. Device addresses at this layer are referred to as physical addresses. Data link layer addressing is contained within the frame header and specifies the frame destination node on the local network.

It is typically at the beginning of the frame, so the NIC can quickly determine if it matches its own Layer 2 address before accepting the rest of the frame. The frame header may also contain the source address of the frame.

Controlled access

In a controlled-based multiaccess network, each node has its own time to use the medium. These deterministic types of legacy networks are inefficient because a device must wait its turn to access the medium.

Contention-based access

In contention-based multiaccess networks, all nodes are operating in half-duplex, competing for the use of the medium. However, only one device can send at a time. Therefore, there is a process if more than one device transmits at the same time.

Error Detection

Included after the data to form the trailer.

Error detection

Includes a trailer used to detect transmission errors.

Addressing

Indicates the source and destination nodes on the media.

Engineering organizations that define open standards and protocols that apply to the network access layer (i.e., the OSI physical and data link layers)

Institute of Electrical and Electronics Engineers (IEEE) International Telecommunication Union (ITU) International Organization for Standardization (ISO) American National Standards Institute (ANSI)

Examples of multiaccess networks that use co

Legacy Token Ring Legacy ARCNET

Addressing

Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.

WANs traditionally used other types of protocols for various types of point-to-point, hub-spoke, and full-mesh topologies.

Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay Asynchronous Transfer Mode (ATM) X.25

Logical topology

Refers to the way a network transfers frames from one node to the next. This topology identifies virtual connections using device interfaces and Layer 3 IP addressing schemes.

If two devices transmit at the same time, a collision will occur. For legacy Ethernet LANs, both devices will detect the collision on the network. This is the collision detection (CD) portion of CSMA/CD.

The NIC compares data transmitted with data received, or by recognizing that the signal amplitude is higher than normal on the media. The data sent by both devices will be corrupted and will need to be resent.

high bandwidth technology is usually not cost-effective for WANs that cover large geographic areas (cities or multiple cities, for example).

The cost of the long-distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity.

Frame delimiting

The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.

The Layer 2 protocol that is used for a particular network topology is determined by the technology used to implement that topology.

The technology used is determined by the size of the network, in terms of the number of hosts and the geographic scope, and the services to be provided over the network.

Mesh Topology

This topology provides high availability but requires that every end system is interconnected to every other system. Therefore, the administrative and physical costs can be significant. Each link is essentially a point-to-point link to the other node.

Wireless

These Layer 2 protocols are now being replaced in the WAN by Ethernet.

Logical Link Control (LLC)

This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame.

Hub and Spoke Model

This is a WAN version of the star topology in which a central site interconnects branch sites through the use of point-to-point links. Branch sites cannot exchange data with other branch sites without going through the central site.

Point to point (PPP)

This is the simplest and most common WAN topology. It consists of a permanent link between two endpoints.

Frame start and stop indicator flags

Used to identify the beginning and end limits of the frame.

Examples of contention-based access networks include the following:

Wireless LAN (uses CSMA/CA) Legacy bus-topology Ethernet LAN (uses CSMA/CD) Legacy Ethernet LAN using a hub (uses CSMA/CD)

computer networks

a node is a device that can receive, create, store, or forward data along a communications path. A node can be either an end device such as a laptop or mobile phone, or an intermediary device such as an Ethernet switch.

topology of a network

is the arrangement, or the relationship, of the network devices and the interconnections between them.

data link layer

prepares network data for the physical network. The data link layer is responsible for network interface card (NIC) to network interface card communications.

LLC sublayer

takes the network protocol data, which is typically an IPv4 or IPv6 packet, and adds Layer 2 control information to help deliver the packet to the destination node.