A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as a...More
RJ45 Patch Cords
Black Copper Cat5e and Cat6 are twisted pair cables that use copper wires, typically 4 twisted pairs in each cable. The specification for Cat6 features more stringent specifications for crosstalk and system noise, and provides performance of up to 250 MHz. Cat5e, in contrast, performs up to 100 MHz. This was often achieved using a spline (a longitudinal separator) in the wiring, which isolates each of the four pairs of twisted wire. However, this made Cat6 cables more rigid; newer cables use other methods to reduce noise and are more flexible. Regardless of whether a spline is used, a cable that meets Cat6 specifications provides significantly lower interference or near end crosstalk (NEXT) in the transmission. It also improves equal level far end crosstalk (ELFEXT), return loss and insertion loss compared with Cat5e. The result is less noise, fewer errors and higher data rates in the transmission of the signal.
Cable testing provides a level of assurance that the installed cabling links provide the desired transmission capability to support the data communication desired by the users.
Buy high quality Sunkit SK-868E 6P8P Hand Network Cable Crimping Pliers from China Crimping Tool with free shipping to worldwide and competitive price. ... Description; Specifications; Downloads. Sunkit SK-868E 6P8P Hand Network ...
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, or related standards. The AP usually connects to a router as a standalone device, but it can also be an integral component of the router itself.
Wi-Fi, also spelled Wifi or WiFi, is a technology that allows an electronic device to exchange data or connect to the internet wirelessly using 2.4 GHz UHF and 5 GHz SHF radio waves. The name is a trademark name, and was stated to be a play on the audiophile term Hi-Fi. The Wi-Fi Alliance defines Wi-Fi as any "wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards". However, since most modern WLANs are based on these standards, the term "Wi-Fi" is used in general English as a synonym for "WLAN". Only Wi-Fi products that complete Wi-Fi Alliance interoperability certification testing successfully may use the "Wi-Fi CERTIFIED" trademark. Many devices can use Wi-Fi, e.g., personal computers, video-game consoles, smartphones, some digital cameras, tablet computers and digital audio players. These can connect to a network resource such as the Internet via a wireless network access point. Such an access point (or hotspot) has a range of about 20 meters (66 feet) indoors and a greater range outdoors. Hotspot coverage can comprise an area as small as a single room with walls that block radio waves, or as large as many square kilometres achieved by using multiple overlapping access points. Depiction of a device sending information wirelessly to another device, both connected to the local network, in order to print a document. Wi-Fi can be less secure than wired connections (such as Ethernet) because an intruder does not need a physical connection. Web pages that use SSL are secure but unencrypted internet access can easily be detected by intruders. Because of this, Wi-Fi has adopted various encryption technologies. The early encryption WEP, proved easy to break. Higher quality protocols (WPA, WPA2) were added later. An optional feature added in 2007, called Wi-Fi Protected Setup (WPS), had a serious flaw that allowed an attacker to recover the router's password. The Wi-Fi Alliance has since updated its test plan and certification program to ensure all newly certified devices resist attacks.
D-Link's products are geared towards the networking and communications market. Its business products include switches, surveillance network cameras, firewalls, iSCSI SANs and business wireless, while consumer products cover consumer wireless devices, broadband devices, and the Digital Home devices (which include media players, storage, and surveillance camera/NVR). It was the first wired and wireless networking company to launch the green technology D-Link Green. D-Link first applied the power saving technology to its unmanaged switches and later to its wireless routers. The technology works by setting any ports that are not in use to standby mode to reduce power to idle ports, in addition to fanless and variable speed fans. "Green" switches can also detect cable length and adjust power output accordingly. D-Link recently released the Boxee Box, which is a collaboration with the popular media streaming software known as Boxee. D-Link provides its customers with the actual hardware while the software is based on the Boxee software. The Boxee Box was released on November 10, 2010.
The Connectix 24 Way Keystone Patch Panels offers a 1u high density solution Features: Suitable for Connectix Express assemblies Integrated cable management tray Earth connection Robust all steel construction Keystone Jack Features: Connectix Tool-less Keystone Modules ensure a quick and easy to terminate installation. The snap-fit to terminate design means that a punch down tool is not required. The module offers a high performance transmission that meets or exceeds Cat5e standards. They have been designed to be installed into most UK and European keystone applications, including standard or angled faceplate products, patch panels or surface boxes. The loading cap is colour standards to ensure a quick and easy installation. Features: Quick and easy to install Snap-fit to terminate Suitable for all standard keystone applications Integrated cable tie position Category 5e hardware performance Black colour coded wiring terminals
A network switch (sometimes known as a switching hub) is a computer networking device that is used to connect devices together on a computer network by performing a form of packet switching. A switch is considered more advanced than a hub because a switch will only send a message to the device that needs or requests it, rather than broadcasting the same message out of each of its ports. A switch is a multi-port network bridge that processes and forwards data at the data link layer (layer 2) of the OSI model. Some switches have additional features, including the ability to route packets. These switches are commonly known as layer-3 or multilayer switches. Switches exist for various types of networks including Fibre Channel, Asynchronous Transfer Mode, InfiniBand, Ethernet and others. The first Ethernet switch was introduced by Kalpana in 1990. Overview A switch is a device used on a computer network to physically connect devices together. Multiple cables can be connected to a switch to enable networked devices to communicate with each other. Switches manage the flow of data across a network by only transmitting a received message to the device for which the message was intended. Each networked device connected to a switch can be identified using a MAC address, allowing the switch to regulate the flow of traffic. This maximises security and efficiency of the network. Because of these features, a switch is often considered more "intelligent" than a network hub. Hubs neither provide security, or identification of connected devices. This means that messages have to be transmitted out of every port of the hub, greatly degrading the efficiency of the network. Network design An Ethernet switch operates at the data link layer of the OSI model to create a separate collision domain for each switch port. With four computers (e.g., A, B, C and D) on four switch ports, any pair (e.g. A and B) can transfer data back and forth while the other pair (e.g. C and D) also do so simultaneously, and the two conversations will not interfere with one another. In full duplex mode, these pairs can also overlap (e.g. A transmits to B, simultaneously B to C, and so on). In the case of using a repeater hub, they would all share the bandwidth and run in half duplex, resulting in collisions which would require retransmissions. Applications The network switch plays an integral part in most modern Ethernet local area networks (LANs). Mid-to-large sized LANs contain a number of linked managed switches. Small office/home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as a residential gateway to access small office/home broadband services such as DSL or cable Internet. In most of these cases, the end-user device contains a router and components that interface to the particular physical broadband technology. User devices may also include a telephone interface for VoIP. Microsegmentation Segmentation is the use of a bridge or a switch (or a router) to split a larger collision domain into smaller ones in order to reduce collision probability and improve overall throughput. In the extreme, i. e. microsegmentation, each device is located on a dedicated switch port. In contrast to an Ethernet hub, there is a separate collision domain on each of the switch ports. This allows computers to have dedicated bandwidth on point-to-point connections to the network and also to run in full-duplex without collisions. Full-duplex mode has only one transmitter and one receiver per 'collision domain', making collisions impossible. Role of switches in a network Switches may operate at one or more layers of the OSI model, including the data link and network layers. A device that operates simultaneously at more than one of these layers is known as a multilayer switch. In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the layers mentioned. While layer-2 functionality is adequate for bandwidth-shifting within one technology, interconnecting technologies such as Ethernet and token ring is easier at layer 3. Devices that interconnect at layer 3 are traditionally called routers, so layer-3 switches can also be regarded as (relatively primitive) routers. Where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall, network intrusion detection, and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules. In other cases, the switch is used to create a mirror image of data that can go to an external device. Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems and packet sniffers. Layer-specific functionality Main article: Multilayer switch A modular network switch with three network modules (a total of 24 Ethernet and 14 Fast Ethernet ports) and one power supply. While switches may learn about topologies at many layers, and forward at one or more layers, they do tend to have common features. Other than for high-performance applications, modern commercial switches use primarily Ethernet interfaces. At any layer, a modern switch may implement power over Ethernet (PoE), which avoids the need for attached devices, such as a VoIP phone or wireless access point, to have a separate power supply. Since switches can have redundant power circuits connected to uninterruptible power supplies, the connected device can continue operating even when regular office power fails. Layer 1 (Hubs versus higher-layer switches) A network hub, or repeater, is a simple network device. Repeater hubs do not manage any of the traffic that comes through them. Any packet entering a port is flooded out or "repeated" on every other port, except for the port of entry. Since every packet is repeated on every other port, packet collisions affect the entire network, limiting its capacity. A switch creates the – originally mandatory – Layer 1 end-to-end connection only virtually. Its bridge function selects which packets are forwarded to which port(s) on the basis of information taken from layer 2 (or higher), removing the requirement that every node be presented with all data. The connection lines are not "switched" literally, it only appears like this on the packet level. "Bridging hub", "switching hub", or "multiport bridge" would be more appropriate terms. There are specialized applications where a hub can be useful, such as copying traffic to multiple network sensors. High end switches have a feature which does the same thing called port mirroring. By the early 2000s, there was little price difference between a hub and a low-end switch. Layer 2 A network bridge, operating at the data link layer, may interconnect a small number of devices in a home or the office. This is a trivial case of bridging, in which the bridge learns the MAC address of each connected device. Single bridges also can provide extremely high performance in specialized applications such as storage area networks. Classic bridges may also interconnect using a spanning tree protocol that disables links so that the resulting local area network is a tree without loops. In contrast to routers, spanning tree bridges must have topologies with only one active path between two points. The older IEEE 802.1D spanning tree protocol could be quite slow, with forwarding stopping for 30 seconds while the spanning tree reconverged. A Rapid Spanning Tree Protocol was introduced as IEEE 802.1w. The newest standard Shortest path bridging (IEEE 802.1aq) is the next logical progression and incorporates all the older Spanning Tree Protocols (IEEE 802.1D STP, IEEE 802.1w RSTP, IEEE 802.1s MSTP) that blocked traffic on all but one alternative path. IEEE 802.1aq (Shortest Path Bridging SPB) allows all paths to be active with multiple equal cost paths, provides much larger layer 2 topologies (up to 16 million compared to the 4096 VLANs limit), faster convergence, and improves the use of the mesh topologies through increase bandwidth and redundancy between all devices by allowing traffic to load share across all paths of a mesh network. While layer 2 switch remains more of a marketing term than a technical term, the products that were introduced as "switches" tended to use microsegmentation and Full duplex to prevent collisions among devices connected to Ethernet. By using an internal forwarding plane much faster than any interface, they give the impression of simultaneous paths among multiple devices. 'Non-blocking' devices use a forwarding plane or equivalent method fast enough to allow full duplex traffic for each port simultaneously. Once a bridge learns the addresses of its connected nodes, it forwards data link layer frames using a layer 2 forwarding method. There are four forwarding methods a bridge can use, of which the second through fourth method were performance-increasing methods when used on "switch" products with the same input and output port bandwidths: Store and forward: The switch buffers and verifies each frame before forwarding it. Cut through: The switch reads only up to the frame's hardware address before starting to forward it. Cut-through switches have to fall back to store and forward if the outgoing port is busy at the time the packet arrives. There is no error checking with this method. Fragment free: A method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64 bytes of the frame, where addressing information is stored. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames that are in error because of a collision will not be forwarded. This way the frame will always reach its intended destination. Error checking of the actual data in the packet is left for the end device. Adaptive switching: A method of automatically selecting between the other three modes. While there are specialized applications, such as storage area networks, where the input and output interfaces are the same bandwidth, this is not always the case in general LAN applications. In LANs, a switch used for end user access typically concentrates lower bandwidth and uplinks into a higher bandwidth. Layer 3 Within the confines of the Ethernet physical layer, a layer-3 switch can perform some or all of the functions normally performed by a router. The most common layer-3 capability is awareness of IP multicast through IGMP snooping. With this awareness, a layer-3 switch can increase efficiency by delivering the traffic of a multicast group only to ports where the attached device has signaled that it wants to listen to that group. Layer 4 While the exact meaning of the term layer-4 switch is vendor-dependent, it almost always starts with a capability for network address translation, but then adds some type of load distribution based on TCP sessions. The device may include a stateful firewall, a VPN concentrator, or be an IPSec security gateway. Layer 7 Layer-7 switches may distribute loads based on Uniform Resource Locator URL or by some installation-specific technique to recognize application-level transactions. A layer-7 switch may include a web cache and participate in a content delivery network. Rack-mounted 24-port 3Com switch Types of switches Form factor Desktop, not mounted in an enclosure, typically intended to be used in a home or office environment outside of a wiring closet. Rack-mounted, a switch that mounts in an equipment rack. Chassis, with swappable module cards. DIN rail–mounted, normally seen in industrial environments. Configuration options Unmanaged switches – these switches have no configuration interface or options. They are plug and play. They are typically the least expensive switches, and therefore often used in a small office/home office environment. Unmanaged switches can be desktop or rack mounted. Managed switches – these switches have one or more methods to modify the operation of the switch. Common management methods include: a command-line interface (CLI) accessed via serial console, telnet or Secure Shell, an embedded Simple Network Management Protocol (SNMP) agent allowing management from a remote console or management station, or a web interface for management from a web browser. Examples of configuration changes that one can do from a managed switch include: enabling features such as Spanning Tree Protocol or port mirroring, setting port bandwidth, creating or modifying Virtual LANs (VLANs), etc. Two sub-classes of managed switches are marketed today: Smart (or intelligent) switches – these are managed switches with a limited set of management features. Likewise "web-managed" switches are switches which fall into a market niche between unmanaged and managed. For a price much lower than a fully managed switch they provide a web interface (and usually no CLI access) and allow configuration of basic settings, such as VLANs, port-bandwidth and duplex. Enterprise Managed (or fully managed) switches – these have a full set of management features, including CLI, SNMP agent, and web interface. They may have additional features to manipulate configurations, such as the ability to display, modify, backup and restore configurations. Compared with smart switches, enterprise switches have more features that can be customized or optimized, and are generally more expensive than smart switches. Enterprise switches are typically found in networks with larger number of switches and connections, where centralized management is a significant savings in administrative time and effort. A stackable switch is a version of enterprise-managed switch. Typical switch management features Linksys 48-port switch HP Procurve rack-mounted switches mounted in a standard Telco Rack 19-inch rack with network cables Turn particular port range on or off Link bandwidth and duplex settings Priority settings for ports IP Management by IP Clustering MAC filtering and other types of "port security" features which prevent MAC flooding Use of Spanning Tree Protocol SNMP monitoring of device and link health Port mirroring (also known as: port monitoring, spanning port, SPAN port, roving analysis port or link mode port) Link aggregation (also known as bonding, trunking or teaming) allows the use of multiple ports for the same connection achieving higher data transfer rates VLAN settings. Creating VLANs can serve security and performance goals by reducing the size of the broadcast domain 802.1X network access control IGMP snooping Traffic monitoring on a switched network Unless port mirroring or other methods such as RMON, SMON or sFlow are implemented in a switch, it is difficult to monitor traffic that is bridged using a switch because only the sending and receiving ports can see the traffic. These monitoring features are rarely present on consumer-grade switches. Two popular methods that are specifically designed to allow a network analyst to monitor traffic are: Port mirroring — the switch sends a copy of network packets to a monitoring network connection. SMON — "Switch Monitoring" is described by RFC 2613 and is a protocol for controlling facilities such as port mirroring. Another method to monitor may be to connect a layer-1 hub between the monitored device and its switch port. This will induce minor delay, but will provide multiple interfaces that can be used to monitor the individual switch port.
What is CAT 7 cable? A Category 7 cable, more commonly known as a CAT 7 or Cat-7 cable, is used for the cabling infrastructure of Gigabit Ethernet. A CAT 7 cable offers performance of up to 600MHz. Put simply, a CAT 7 cable is what we recommend you use when wiring your smart home What’s the difference between CAT 5, CAT 6 & CAT 7 cable? You might be wondering whether it’s worth your while to consider CAT 7 cable. CAT 7 cable differs from preceding Ethernet cable standards including CAT 5 and CAT 6 in several ways, however, one of the greatest advantages of CAT 7 cable is the shielding of its twisted pairs, which significantly improves noise resistance. CAT 7 cable, whilst being the more expensive option, is also considered the most durable, and has a longer lifespan than CAT 5 and CAT 6, improving its overall return on investment, and is the best choice for wiring with the future in mind. Does anyone use CAT 7? Yep! In fact, it’s one of our most popular products. If you’re a Twitter user, just type in the hashtag #LiveInstall and you’ll see the bright green cable popping up in lots of smart home installations. Isn’t CAT 7 much more expensive? CAT 7 is going to cost you more, there’s no getting around that at the moment. However, the average price for 305m / 1000ft of CAT 6 cable in Karachi, Pakistan (taking the average price of the first four Google Shopping results for CAT 6 305m), so in terms of price, there’s around Rs. 10,000 difference per 305m length compared to the CAT 7. Our Branch office in Karachi provide Cat7 Cable to Azad Kashmir, Bagh, Bhimber, khuiratta, Kotli, Mangla, Mirpur, Muzaffarabad, Plandri, Rawalakot, Punch, Balochistan, Amir Chah, Bazdar, Bela, Bellpat, Bagh, Burj, Chagai, Chah Sandan, Chakku, Chaman, Chhatr, Dalbandin, Dera Bugti In state of Balochistan, we are planning to expand our network with these cities: Dhana Sar, Diwana, Duki, Dushi, Duzab, Gajar, Gandava, Garhi Khairo, Garruck, Ghazluna, Girdan, Gulistan, Gwadar, Gwash, Hab Chauki, Hameedabad, Harnai, Hinglaj, Hoshab, Ispikan, Jhal, Jhal Jhao, Jhatpat, Jiwani, Kalandi, Kalat, Kamararod, Kanak, Kandi, Kanpur, Kapip, Kappar We can deliver cat7 cable with cat6 cable in Karodi, Katuri, Kharan, Khuzdar, Kikki, Kohan, Kohlu, Korak, Lahri, Lasbela, Liari, Loralai, Mach, Mand, Manguchar, Mashki Chah, Maslti, Mastung, Mekhtar, Merui, Mianez, Murgha Kibzai, Musa Khel Bazar, Nagha Kalat, Nal, Naseerabad, Nauroz Kalat, Nur Gamma, Nushki, Nuttal, Ormara, Palantuk, Panjgur, Pasni, Piharak, Pishin, Qamruddin Karez, Qila Abdullah, Qila Ladgasht Black Copper Network Cable Cat5e & Cat7 is famous in Qila Safed, Qila Saifullah, Quetta, Rakhni, Robat Thana, Rodkhan, Saindak, Sanjawi, Saruna, Shabaz Kalat, Shahpur, Sharam Jogizai, Shingar, Shorap, Sibi, Sonmiani, Spezand, Spintangi, Sui, Suntsar, Surab, Thalo, Tump, Turbat, Umarao, pirMahal, Uthal, Vitakri, Wadh, Washap, Wasjuk, Yakmach, Zhob, Federally Administered Northern Areas/FANA We are already sent 3M Cat6 cable to these places - Astor, Baramula, Hunza, Gilgit, Nagar, Skardu, Shangrila, Shandur, Federally Administered Tribal Areas/FATA, Bajaur, Hangu, Malakand, Miram Shah, Mohmand, Khyber, Kurram, North Waziristan, South Waziristan, Wana, NWFP, Abbottabad, Ayubia, Adezai, Banda Daud Shah, Bannu, Batagram, Birote, Buner, Chakdara, Charsadda, Chitral, Dargai, Darya Khan, Dera Ismail Khan we are planning to open a branch office of Lan Cable in Drasan, Drosh, Hangu, Haripur, Kalam, Karak, Khanaspur, Kohat, Kohistan, Lakki Marwat, Latamber, Lower Dir, Madyan, Malakand, Mansehra, Mardan, Mastuj, Mongora, Nowshera, Paharpur, Peshawar, Saidu Sharif, Shangla, Sakesar, Swabi, Swat, Tangi, Tank, Thall, Tordher, Upper Dir, Punjab, Ahmedpur East, Ahmed Nager Chatha, Ali Pur, Arifwala, Attock, Basti Malook, Bhagalchur Ethernet Wire Good selling in these area's Bhalwal, Bahawalnagar, Bahawalpur, Bhaipheru, Bhakkar, Burewala, Chailianwala, Chakwal, Chichawatni, Chiniot, Chowk Azam, Chowk Sarwar Shaheed, Daska, Darya Khan, Dera Ghazi Khan, Derawar Fort, Dhaular, Dina City, Dinga, Dipalpur, Faisalabad, Fateh Jang, Gadar, Ghakhar Mandi Rj45 Lan Cable Cat7 Demands very high in Gujranwala, Gujrat, Gujar Khan, Hafizabad, Haroonabad, Hasilpur, Haveli Lakha, Jampur, Jhang, Jhelum, Kalabagh, Karor Lal Esan, Kasur, Kamalia, Kamokey, Khanewal, Khanpur, Kharian, Khushab, Kot Addu, Jahania, Jalla Araain, Jauharabad, Laar, Lahore, Lalamusa, Layyah, Lodhran, Mamoori, Mandi Bahauddin, Makhdoom Aali, Mandi Warburton, Mailsi, Mian Channu, Minawala, Mianwali, Multan, Murree, Muridke, Muzaffargarh, Narowal, Okara, Renala Khurd, Rajan Pur, Pak Pattan, Panjgur Networking Cable is low cost in Pattoki, Pirmahal, Qila Didar Singh, Rabwah, Raiwind, Rajan Pur, Rahim Yar Khan, Rawalpindi, Rohri, Sadiqabad, Safdar Abad – (Dhaban Singh), Sahiwal, Sangla Hill, Samberial, Sarai Alamgir, Sargodha, Shakargarh, Shafqat Shaheed Chowk, Sheikhupura, Sialkot, Sohawa, Sooianwala, Sundar (city), Talagang, Tarbela, Takhtbai, Taxila, Toba Tek Singh, Vehari, Wah Cantonment, Wazirabad Our Head Office is cover Networking Wire in Sindh, Ali Bandar, Baden, Chachro, Dadu, Digri, Diplo, Dokri, Gadra, Ghanian, Ghauspur, Ghotki, Hala, Hyderabad, Islamkot, Jacobabad, Jamesabad, Jamshoro, Janghar, Jati (Mughalbhin), Jhudo, Jungshahi, Kandiaro, Karachi, Kashmor, Keti Bandar, Khairpur, Khora, Klupro, Khokhropur, Korangi, Kotri, Kot Sarae, Larkana, Lund, Mathi, Matiari, Mehar, Mirpur Batoro, Mirpur Khas, Mirpur Sakro, Mithi, Mithani Technocity Branch office send cat7 Cable in Moro, Nagar Parkar, Naushara, Naudero, Noushero Feroz, Nawabshah, Nazimabad, Naokot, Pendoo, Pokran, Qambar, Qazi Ahmad, Ranipur, Ratodero, Rohri, Saidu Sharif, Sakrand, Sanghar, Shadadkhot, Shahbandar, Shahdadpur, Shahpur Chakar, Shikarpur, Sujawal, Sukkur, Tando Adam, Tando Allahyar, Tando Bago, Tar Ahamd Rind, Thatta, Tujal, Umarkot, Veirwaro, Warah The average house will usually require at least 750 metres / 2500ft of cable, which is three boxes of cable. That works out at a £60 / $ 74 difference between the average CAT 6 price in Pakistan and the CAT 7 Cable Price in Pakistan… not as bad as perhaps as it first appears, especially when you consider the costs involved in the grand scheme of building a house. Cat7 Cable Famous Brand in Pakistan is 3M, Systimax, Black Copper, Commscope & AMP
Are you an IT professional, network administrator or someone who deals with a lot of wires? Do you want to quickly find a specifc cable from a handful of messy undocumented / untagged patch cables? Here's the solution.
Key Specifications/Special Features: Network cable tester for RJ45 Can test correspondingly double-twisted cables 1, 2, 3, 4, 5, 6, 7, 8 and G, meanwhile can judge wrong connection, short circuit and open circuit