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This is particularly inefficient for small data transactions, as the process has to be repeated every time. It is possible that all devices on a network could have valid and correct IP addresses but that a specific host fails to respond to a message sent to it by a client program residing on another host. A typical scenario could be a supervisory computer sending a command to a PLC. Before assuming that the PLC is defective, one has to ascertain that the supervisory computer is, in fact, using the correct IP address when trying to communicate with the PLC. This can only be ascertained by using a protocol analyzer and capturing the communication or attempt at communication between the computer and the PLC. The packets exchanged between the computer and PLC can be identified by means of the MAC addresses in the appropriate Ethernet headers.

It then sets the SYN flag to ‘1’ and transmits the header to the appropriate IP address and port number. Assuming that the chosen sequence number was 132, this action would then be abbreviated as SYN 132. TCP acknowledges data received on aper segmentbasis, although several consecutive segments may be acknowledged at the same time. Since TCP chooses the segment size in such a way that it fits into the data field of the Data Link layer protocol, each segment is associated with a different frame. Before any data transmission takes place, both sender and receiver (e.g. client and server) have to agree on the Initial Sequence Numbers to be used.

3 Types Of Vpn

The DNS client resolver software can implement a caching function by storing the results from the name resolution operation. In this way the resolver can resolve a future query by looking up the cache rather than actually contacting the name server. Cache entries are given a Time To Live so that they are purged from the cache after a given amount of time. A network connected to the Internet needs access to at least one Primary Name Server and one Secondary Name Server, both capable of performing naming operations for the registered domain names on the Internet. In the case of the Internet, the number of domain names is so large that the namespace is distributed across multiple Primary and Secondary servers in different countries. For example, all theco.zadomain names are hosted across several name servers located in South Africa.

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3 2 Handheld Cable Testers

Letter, numeral, punctuation, control code or any other symbol contained in a message. A data path shared by many devices, with one or more conductors for transmitting control signals, data or power. An intermediate temporary storage device used to compensate for a difference in data rate and data flow between two devices . Baseband operation is the direct transmission of data over a transmission medium without the prior modulation of a high frequency carrier signal. This cable, sometimes called the drop cable, was used to attach terminals to the transceiver unit in 10Base5 Ethernet systems. A false lower frequency component that appears in analog data reconstructed from sampled data that was acquired by sampling the original signal at too low a rate.

Nodes like this are typically performing tasks such as video-conferencing or database access, which require a large bandwidth. The solution to the problem is to give these nodes separate switch connections or to group them together on a faster 100Base-T or 1000Base-T segment. A stub cable is an abandoned telephone cable leading from a punch-down block to some other point. This does not create a problem for telephone systems, but if the same Cat3 telephone cabling is used to support 10BaseT, then the stub cables may cause signal reflections that result in bit errors. Again, a high quality cable tester only will be able to detect this problem.

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A layer may have more than one entity – for example a protocol entity and a management entity. Entities in adjacent layers interact through the common upper and lower boundaries by passing physical information through Service Access Points . A SAP could be compared to a predefined ‘postbox’ where one layer would collect data from the previous layer. The relationship between layers, entities, functions and SAPs is shown in Figure 2.6. Packet switched services generally support two types of services viz.

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2 1 Multicast Ip

Some of the protocols used for VPNs are based on PPP, which was traditionally used for RAS access with authentication by the Password Authentication Protocol . Adding VPN hardware in an existing LANIn the arrangement shown in Figure 12.8A, the inbound VPN traffic is also made to go through the firewall before accessing the tunnel server. This may require the firewall to be upgraded as VPN traffic increases. In figure 12.8B, the VPN traffic is handled directly by the tunnel server whereas non-VPN traffic is sent through the firewall.

SMTP is used by e-mail clients such as Outlook Express or Eudora to send messages and files to an e-mail server on a remote network, usually that of an ISP. SNMPv1 and SNMPv2 are, however, not secure, with the result that hackers can easily exploit them to gain unauthorized access to SNMP-enabled devices. These weaknesses have been addressed in SNMPv3, which supports authentication, privacy and access control. WINS is not a general TCP/IP Application layer protocol, but rather a Microsoft Windows-specific utility with the primary role of NetBIOS name registration and resolution on TCP/IP. However, while DNS resolves TCP/IP host names to static IP addresses, WINS resolves NetBIOS names to dynamic addresses assigned by DHCP. On a small network one name server may be sufficient and the name server software may run on a machine already used for other server purposes .

4 Medium Access Control

A typical speed at which a WAN interconnects varies between 9600 bps for a leased line to 40 Gbps for SDH/SONET. On a LAN the software controlling the transfer of messages among the devices on the network must deal with the problem of sharing the common resources of the network without conflict or corruption of data. Since many users can access the network at the dead or alive 2 rtp same time, some rules must be established by which devices can access the network, when, and under what conditions. These rules are covered under the general heading of Media Access Control. In order to synchronize their transmissions, the transmitting and receiving devices need to agree on the length of the code elements to be used, known as the bit time.

The second part of the address enables the postal system to route the letter towards its destination post office from anywhere in the world. This is a connection-oriented protocol that offers vastly improved protection and error control. This protocol, the TCP component of TCP/IP, is the heart of the TCP/IP suite of applications. It provides a very reliable method of transferring data in byte format, between applications.

A typical IP header is 20 octets, allowing some space for headers of higher-level protocols. TheType of Service field is intended to provide an indication of the parameters of the quality of service desired. These parameters are used to guide the selection of the actual service parameters when transmitting a datagram through a particular network. Instead, whenever an IPv4 network address is assigned to an organization, it is done in the form of a 32-bit network address and a corresponding 32-bit mask, usually expressed as a prefix. The MAC or hardware address discussed earlier is unique for each node, and has been allocated to that particular node e.g.

• A software image of a field device describing the objects supplied by it e.g. measured data, events, status etc, which can be accessed by another network.
• In order to achieve its intended goals, TCP makes use of ports and sockets, connection oriented communication, sliding windows, and sequence numbers/acknowledgments.
• This number uniquely identifies each datagram sent by a host and is normally incremented by one for each datagram sent.
• This task is performed on the basis of the IP addresses embedded in the header of each IP datagram.
• During an RTSP session, an RTSP client may open and close many reliable transport connections to the server to issue RTSP requests.
• Applications such as TELNET and RLOGIN need minimum delay since they transfer small amounts of data.
• Instead, it uses UDP, with time-out and retransmission mechanisms to ensure data delivery.
• The irony of the matter is that all practical Gigabit Ethernet systems run in full duplex mode and this restriction is therefore irrelevant.
• This is analogous to the efficiencies of the mail system as discussed in the following paragraph.

Positive acknowledgment philosophyThe sliding window form of positive acknowledgment is used by TCP, as it is very time consuming waiting for individual acknowledgments to be returned for each packet transmitted. Hence the idea is that a number of packets are transmitted before the source may receive an acknowledgment to the first message . As long as acknowledgments are received, the window slides along and the next packet is transmitted. Obviously there is a need to get some sort of acknowledgment back to ensure that there is a guaranteed delivery.

10 Terminal Servers

Some 10BaseT hubs feature 50 pin connectors to conserve space on the hub. Alternatively, some building wire systems use 50 pin connectors on the wiring panels but the hub equipment has RJ-45 connectors. This consists of a 50 pin connector connected to a length of 25-pair cable, which is then broken out as a set of 12 small cables, each with an RJ-45 connector.

Again, the signals propagate down the cable, and some short time later they ‘collide’. Almost immediately, node 2’s transceiver recognizes that the signals on the cable are corrupted, and the logic incorporated in the NIC asserts a collision detect signal. This causes node 2 to send a jam signal of 32 bits, and then stop transmitting. The standard allows any data to be sent as long as, by design, it is not the value of the CRC field of the frame.

3 Routers

Unlike bridges and layer 2 switches, routers operate at layer 3 of the OSI model, viz. They therefore ignore address information contained within the Data Link layer header and, instead, delve deeper into each frame and extract the address information contained in the Network layer. Virtual LANs using switchesThe IEEE802.1p committee has defined a standard for packet-based LANs that supports layer 2 traffic prioritization in a switched LAN environment. IEEE802.1p is part of a larger initiative (IEEE802.1p/Q) that adds more information to the Ethernet header (as shown in Fig 10.11) to allow networks to support VLANs and traffic prioritization. Provided that a LAN is constructed around switches that support VLANs, individual hosts on the physical LAN can be grouped into smaller Virtual LANs , totally invisible to their fellow hosts. This modified frame is known as an IEEE 802.1Q tagged frame and is used for communication between the switches.

Routing calls through a Gatekeeper provides better performance in the network, as the Gatekeeper can make routing decisions based on a variety of factors, for example, load balancing among Gateways. The services offered by a Gatekeeper are defined by RAS and include address translation, admissions control, bandwidth control, and zone management. A Gatekeeper is a logical component of H.323 but can be implemented as part of a Gateway or MCU. The need for a Quality of Service on the Internet arose because of the increasing number of time-sensitive applications involving voice and video.

File transfer requires a reliable transport mechanism, and therefore TCP connections are used. The FTP process running making the file transfer request is called the FTP client, while the FTP process receiving the request is called the FTP server. As in the case of TCP, this header is used for computational purposes only, and discarded after the computation.

Author: Adam Thompson