Cisco Chapter 4 Network Access

Frame

A packet prepared for transport across the local media by encapsulating it with a header and a trailer. Fields: Frame start and stop indicator flags - Used to identify the beginning and end limits of the frame. Addressing - Indicates the source and destination nodes on the media. Type - Identifies the Layer 3 protocol in the data field. Control - Identifies special flow control services such as quality of service (QoS). QoS is used to give forwarding priority to certain types of messages. Data link frames carrying voice over IP (VoIP) packets normally receive priority because they are sensitive to delay. Data - Contains the frame payload (i.e., packet header, segment header, and the data). Error Detection - These frame fields are used for error detection and are included after the data to form the trailer.

Wireless

Carries electromagnetic signals that represent the binary digits of data communications using radio or microwave frequencies. The signals are patterns of microwave transmissions. Types: WiFi, Bluetooth, and WiMax

Media Access Control Methods(Multi-Access Networks)

Contention-based access - All nodes operating in half-duplex compete for the use of the medium, but only one device can send at a time. However, there is a process if more than one device transmits at the same time. Ethernet LANs using hubs and WLANs are examples of this type of access control. Figure 1 shows contention-based access. Controlled access - Each node has its own time to use the medium. These deterministic types of networks are inefficient because a device must wait its turn to access the medium. Legacy Token Ring LANs are an example of this type of access control. Figure 2 shows controlled access.

3 basic forms of network media

Copper cable Fiber-optic cable Wireless

Duplex communication

Direction of data transmission between two devices. Types:Half-duplex communication - Both devices can transmit and receive on the media but cannot do so simultaneously. The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-duplex allows only one device to send or receive at a time on the shared medium and is used with contention-based access methods. Figure 1 shows half-duplex communication. Full-duplex communication - Both devices can transmit and receive on the media at the same time. 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 can operate in half-duplex if connecting to a device such as an Ethernet hub. Figure 2 shows full-duplex communication.

Common Physical WAN Topologies

Point-to-Point - This is the simplest topology that consists of a permanent link between two endpoints. For this reason, this is a very popular WAN topology. Hub and Spoke - A WAN version of the star topology in which a central site interconnects branch sites using point-to-point links. Mesh - This topology provides high availability, but requires that every end system be 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.

Data Link

Prepares network data for the physical network. Sub Layers: Logical Link Control (LLC) - This upper sublayer communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media. Media Access Control (MAC) - This lower sublayer defines the media access processes performed by the hardware. It provides data link layer addressing and access to various network technologies.

Physical LAN Topologies

Star - End devices are connected to a central intermediate device. Early star topologies interconnected end devices using Ethernet hubs. However, star topologies now use Ethernet switches. The star topology is easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot. Extended Star - In an extended star topology, additional Ethernet switches interconnect other star topologies. An extended star is an example of a hybrid topology. 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. Bus topologies using coax cables were used in legacy Ethernet networks because it was inexpensive and easy to set up. Ring - End systems are connected to their respective neighbor forming a ring. Unlike the bus topology, the ring does not need to be terminated. Ring topologies were used in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.

Topology

The arrangement or relationship of the network devices and the interconnections between them. Types:Physical - Refers to the physical connections and identifies how end devices and infrastructure devices such as routers, switches, and wireless access points are interconnected. Usually point-to-point or star. Logical - Refers to the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network. These logical signal paths are defined by data link layer protocols. Is relatively simple while shared media offers different access control methods.

Bandwidth

The capacity of a medium to carry data. Units: kilobits/second

Throughput

The measure of the transfer of bits across the media over a given period of time. Major factors that influence include: The amount of traffic The type of traffic The latency created by the number of network devices encountered between source and destination

Media Access Control(MAC)

The technique used for getting the frame on and off the media. Control Methods: Topology - How the connection between the nodes appears to the data link layer. Media sharing - How the nodes share the media. The media sharing can be point-to-point, such as in WAN connections, or shared such as in LAN networks.

Fiber-optic cable

Transmits data over longer distances and at higher bandwidths than any other networking media. The signals are patterns of light. Types:Single-mode fiber (SMF): Consists of a very small core and uses expensive laser technology to send a single ray of light, as shown in Figure 1. Popular in long-distance situations spanning hundreds of kilometers, such as those required in long haul telephony and cable TV applications. Multimode fiber (MMF): Consists of a larger core and uses LED emitters to send light pulses. Specifically, light from an LED enters the multimode fiber at different angles, as shown in Figure 2. Popular in LANs because they can be powered by low-cost LEDs. It provides bandwidth up to 10 Gb/s over link lengths of up to 550 meters.

Copper Media

Types: Unshielded Twisted-Pair (UTP), Shielded Twisted-Pair (STP), and Coaxial

Unshielded twisted-pair (UTP)

Terminated with RJ-45 connectors, is used for interconnecting network hosts with intermediate networking devices, such as switches and routers. The signals are patterns of electrical pulses. Types:Ethernet Straight-through: The most common type of networking cable. It is commonly used to interconnect a host to a switch and a switch to a router. Ethernet Crossover: A cable used to interconnect similar devices. For example to connect a switch to a switch, a host to a host, or a router to a router. Rollover: A Cisco proprietary cable used to connect a workstation to a router or switch console port.