communications processors

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Communications Processors Front-end processor => A small computer dedicated to communication management. It is attached to the main or host computer in a network. It performs such special communications processing as error control, formatting, editing, controlling and routing, and speed and signal conversion. Concentrator => A programmable telecommunication computer that collects and temporarily stores messages from terminals for batch transmission to the host computer.

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Page 1: Communications Processors

Communications Processors• Front-end processor => A small computer dedicated

to communication management. It is attached to the main or host computer in a network. It performs such special communications processing as error control, formatting, editing, controlling and routing, and speed and signal conversion.

• Concentrator => A programmable telecommunication computer that collects and temporarily stores messages from terminals for batch transmission to the host computer.

Page 2: Communications Processors

Communications Processors• Controller => A specialized computer that supervises

communications traffic between the CPU and the peripheral devices such as terminals and printers. It routes output from the CPU to the appropriate peripheral device

• Multiplexer => A device that enables a single communications channel to carry data transmissions from multiple sources simultaneously. The multiplexer divides the communications channel so that it can be shared by multiple transmission devices. The multiplexer can divide a high-speed channel into multiple channels of slower speed .

Page 3: Communications Processors

Local Area Network

Printer

Computer1

Computer2

NetworkGateway

Computer3

OtherNetwork

NetworkOperating System

Server

Page 4: Communications Processors

Local Area Network• LAN is used to connect PCs or resources (e.g. printers) in one

building or several buildings in close proximity.

• LAN has higher transmission capacity than PBX (Private Branch Exchange).

• LAN can transmit video and graphics.

• LAN is more expensive to install than PBX and less flexible. It requires new wiring each time a LAN is moved.

• The server acts as a librarian. It stores programs and data files for network users. The server determines who will get access to what and in what sequence.

• Sever can be powerful PCs with large hard-disk capacity, workstations, minicomputers, or mainframes.

• The network gateway connects the LAN to public networks, such as the telephone network, or to other corporate networks.

Page 5: Communications Processors

Local Area Network

• A gateway is a communications processor that can connect dissimilar networks by translating from one set of protocols to another.

• LAN can use twisted wire, coaxial , or fiber-optic cable.

• LAN also can use wireless technology.• The network operating system (NOS) can reside

on every computer in the network, or it can reside on a single server.

• The NOS routes and manages communications on the network and sharing of network resources.

Page 6: Communications Processors

Wide Area Network

• It spans a broad geographical distance, ranging from several miles to the span of entire continent.

• WAN may consist of a combination of switched and dedicated lines, microwave, and satellite communications.

• Switched lines are telephone lines that a person can access from his or her terminal to transmit data to the designated destination.

• Dedicated lines are continuously available for transmission. This lines can be leased or purchased from a common carriers or private communications media vendors.

• Most WANS are switched.

Page 7: Communications Processors

Value-Added Networks (VANS)

• Value-added networks are private, multipath, data-only, third-party-managed network that is used by multiple organizations on a subscription basis.

• The VAN is set up by a firm that is in charge of managing the network.

• The subscribers pay only for the amount of data they transmit plus a subscription fee.

• Customers do not have to invest in network equipment and software or perform their own error checking, editing, routing, and protocol conversion.

• The network can use twisted-pair lines, satellite links, and other communication channels leased by the value-added carrier.

Page 8: Communications Processors

LANs• The most popular LANs are Ethernet LANS, Token

Ring LANS, wireless LANs, and ATM LANs.

Ethernet LANs:• Ethernet is the most widely used local area network

protocol. • The protocol was designed in 1973 by Xerox with a

data rate of 10 Mbps (Traditional Ethernet) and a bus topology.

• Today it has a data rate of 100 Mbps (Fast Ethernet) and 1000 Mbps (Gigabit Ethernet).

• Ethernet is formally defined by IEEE 802.3 standard.

Page 9: Communications Processors

Ethernet LANs• Access Method: CSMA/CD for traditional Ethernet.

Factors related to CSMA/CD: minimum frame length, data transmission rate, collision domain.

• Topology: Stations on a traditional Ethernet can be connected together using physical bus or star topology, but the logical topology is always a bus => only one station at a time can use the medium.

• Layers: The data link layer has two sublayers: the logical link control (LLC) sublayer and the media access control (MAC) sublayer. LLC=>flow and error control, MAC=>CSMA/CD

• Frame: IEEE 802.3 specifies one frame type containing seven fields; preamble, SFD, DA, SA, length/type of PDU, Data, and the CRC.

Page 10: Communications Processors

Ethernet LANs• Addressing: Each station such as a PC, workstation, or printer on

an Ethernet network has its own network interface card (NIC). NIC provides the station with a 6-byte physical address. Three types of addresses in Ethernet: unicast, multicast, and broadcast.

• Implementations: The IEEE standard defines several implementations for traditional Internet.10BASE5 (thick Ethernet, uses bus topology, uses thick coaxial cable as transmission medium)10BASE2 (Thin Ethernet, uses bus topology, uses thin coaxial cable as transmission medium) 10BASE-T (twisted-pair Ethernet, uses a physical star topology, uses twisted-pair cable as transmission medium)10BASE-FL (fiber link Ethernet, uses a star topology, uses fiber-optic cable as transmission medium).

Page 11: Communications Processors

Ethernet LANs• Fast Ethernet: Fast Ethernet uses the same principle as

traditional Ethernet (CSMA/CD). The transmission rate is 100 Mbps. For CSMA/CD to work, we have to increase the minimum frame length or decrease the collision domain. In Fast Ethernet, the collision domain has to be decreased to 250 meters.

• Gigabit Ethernet: The transmission rate is 1000 Mbps. To achieve this rate, the MAC layer has two options: keeping CSMA/CD or dropping it. To keep CSMA/CD, we need to decrease collision domain or increase the minimum frame length. Since a collision domain of 25 meters is unacceptable, the minimum length of the frame is increased in a very elegant way. In second option, dropping CSMA/CD, every station is connected by two separate paths to the central hub. This is called full-duplex Ethernet with no collision and no CSMA/CD.

Page 12: Communications Processors

Token Ring LANs• Token ring is a protocol defined in IEEE project 802.5.• Access method: It uses a token (3 bytes) passing access method.• Token frame• Addressing: Like Ethernet, most Token Ring implementations

use a 6-byte address. Only difference is Token Ring sends the most significant bit of each byte first.

• Implementations: The ring in a Token Ring consists of a series of shielded twisted pair sections linking each station to its immediate neighbors. Configuring the network as a ring introduces a potential problem. One disabled node could stop the flow of traffic around the entire network. To solve this problem, each station is connected to an automatic switch. The switch can bypass an inactive station

Page 13: Communications Processors

Wireless LANs

• Wireless communication is one if the fastest growing technologies. The demand for mobile devices has led to a need for wireless wide and local area networks.

• Spread Spectrum: Frequency Hopping and Direct Sequence.• ISM Frequency Band: In 1985, the Federal Communications

Commission (FCC) modified the radio spectrum regulations for unlicensed devices. It authorizes wireless LANs to operate in Industrial, Scientific, and Medical (ISM) bands. The use of these band does not require license from FCC if the equipment operates under 1 W power. 902 – 928 MHz => I – band2.4 – 2.48 GHZ => S-band5.725 – 5.85 GHz => M-band

• Access Method: CSMA/CA• Architecture: Define architecture for only IEEE 802.11.

Page 14: Communications Processors

Point-to-Point WANs• The second type of network that we encounter in the

Internet is the point-to-point wide area network.• A point-to-point WAN connects two remote devices

using a line available from a public network such as a telephone network.

• These public companies normally provide the service at the physical layer, the user is responsible for the protocol at the data link layer.

• At the physical layer, the point-to-point connection between two devices can be accomplished using one of the services available today such as traditional modem technology with regular telephone line, a DSL line, cable modem, a T-line, or SONET.

Page 15: Communications Processors

Point-to-Point WAN: V.90 (56K) modem

• Many end users of the Internet are connected from home or small businesses to an ISP through a traditional modem.

• One current development is the V.90 (56K) modem which uses the existing telephone line.

• The subscriber is connected to the switching station of the telephone company and uses the connection from the switching station to the ISP computer (server).

• The connection is asymmetric; the user can download data at 56 kbps but upload at only 33.6 kbps.

Page 16: Communications Processors

Point-to-Point WAN: Digital Subscriber Line (DSL)

• The digital subscriber line (DSL) is a newer technology that uses the existing telecommunication networks such as the local loop telephone line (a connection between subscriber resident and the telephone company) to accomplish high-speed delivery of data, voice, and multimedia.

• DSL is a family of technologies. Five of them are ADSL, RADSL, HDSL, VDSL, and SDSL.

Page 17: Communications Processors

Point-to-Point WAN: Cable Modem

• Another technology used for remote connection is the cable modem.

• This technology uses the cable TV services available in most areas.

• The technology uses a 500 MHz coaxial cable to deliver TV channels to residential areas.

• Because each TV channel needs only 6 MHz, more than 75 channels can be simultaneously broadcast through the cable. Some of these channels can be used to transmit data through the cable TV provider to the Ethernet.

Page 18: Communications Processors

Point-to-Point WAN: T Lines

• T lines are standard digital telephone carriers designed originally to multiplex voice channels.

• Today, however, T lines can be used to carry data from a residence or an organization to the Internet.

• They can also be used to provide a physical link between nodes in a switched wide area network.

• T lines are commercially available in two data rates: T-1 (1.544 Mbps) and T-3 (44.736 Mbps).

• A T-3 line is equivalent to 28 T-1 lines.

Page 19: Communications Processors

Point-to-Point WAN: SONET

• The high bandwidths of fiber-optic cable are suitable for today’s highest data rate technologies (such ads video conferencing) and for carrying large numbers of lower-rate technologies at the same line.

• ANSI created a set of standards called Synchronous Optical Network (SONET) to handle the use of fiber-optic cables.

Page 20: Communications Processors

Switched WANs• The backbone networks in the Internet are usually a switched

WAN.• A switched WAN is a wide area network that covers a large area

(a state or a country) and provides access at several points to the user.

• Inside the network, there is a mesh of point-to-point networks that connects switches. The switches, multiple port connectors, allow the connection of several inputs and outputs.

• Switched WAN technology differs from LAN technology in many ways.

• Instead of a bus or star topology, switches are used to create multiple paths.

• LAN technology is a connectionless technology, Switch WAN is a connection-oriented technology.

• Three common switched WANs are: X.25 (almost obsolete), Frame Relay (will be in use for a few more years) ATM (prevalent).

Page 21: Communications Processors

Switched WANs: X.25

• X.25, introduced in the 1970s, was the first switched WAN to become popular both in Europe and the United States.

• Although still used in Europe, it is disappearing from the United States.

• It was mostly used as a public network to connect individual computers or LANs.

• It provides an end-to-end service.

Page 22: Communications Processors

Switched WANs: X.25• Although X.25 was used as the WAN to carry IP packets

from one part of the world to another, there was always a conflict between IP and X.25. IP is network layer protocol. An IP packet is supposed to be carried by a frame at the second (data link) layer. X.25, which was designed before the Internet, is a three-layer protocol; it has its own network layer. IP packets had to be encapsulated in the X.25 network packet to be carried from one side of the network to another. This is analogous to a person who has a car but has to load it in a truck to go from one point to another.

• Another problem with X.25 is that it was designed at a time when transmission media were not very reliable. For this reason, X.25 performs flow and error control at both the data link layer and the network layer. This makes transmission very slow.

• For all of these reasons, X.25 will most likely soon disappear from the Internet.

Page 23: Communications Processors

Switched WANs: Frame Relay• Frame Relay, a switched technology that provides

low-level service, was designed to replace X.25. Frame Relay has some advantages:

• High data rate: Frame Relay was originally developed to provide 1.544 Mbps data rate. Now it can provide 44.736 Mbps.

• Bursty Data: Some services offered by wide area network providers assume that the user has a fixed-rate need. For example, T1 line was designed for constant data rate 1.544 Mbps. This type of services are not suitable for bursy data rates. Bursty data rate requires bandwidth on demand. Frame relay accepts bursty data.

Page 24: Communications Processors

Switched WANs: Frame Relay

• Less overhead due to improved transmission media: The quality of transmission media has improved since the last decade. There is no need to have a WAN that spends time and resources double-checking errors. Frame Relay does not provide error checking in the data link layer. Instead it is done at the network and transport layer.

Page 25: Communications Processors

Asynchronous Transfer Mode (ATM)

• ATM technology parcels information into uniform cells, each with 53 groups of eight bytes, eliminating the need for protocol conversion.

• It can seamlessly and dynamically switch voice, data, images, and video between users.

• It can pass data between computers from different vendors.

• It permits data to be transmitted at any speed the network handles. ATM can transmit up to 2.5 GBPS.

• It can connect LAN and WAN together more easily.

Page 26: Communications Processors

Internetworking, Connectivity & Open System

• Internetworking => The linking of separate networks, each of which retains its own identity, into an interconnected network.

• Connectivity => The ability of computers and computer-based devices to communicate with one another and share information in a meaningful way without human intervention.

• Open Systems => Open systems promote connectivity because they can operate on different hardware platforms. They are built on public non-proprietary operating systems, user interfaces, application standards, and networking protocols.