Distribution Cable (OSP and ISP)

Distribution Cable, OSP (Outside Plant): The cable running from a central office or remote terminal to the side of a subscriber's lot.

Distribution Cable, ISP (Inside Plant): Cables usually running horizontally from a closet on a given floor within a building. Distribution cables may be under carpet, simplex, duplex, quad, or higher fiber count cables.

Feeder Cable

A group of wires, usually 25-pair or multiples of 25-pair, that supports multiple phones in a single cable sheath. These cables may or may not be terminated with a connector on one or both ends. A Feeder cable typically connects an intermediate distribution frame (IDF) to a main distribution frame (MDF). But the term "feeder cable" is also used in backbone wiring.

For a basic illustration, you can access the following link:

http://www.svrops.com/svrops/Images/HorizCC.gif


Bellcore defines the term slightly differently: A large pair-size loop cable emanating from a central office and usually placed in an underground conduit system with access available at periodically placed manholes. (This is a very common usage for the term.)


Feeder Route: A network of loop cable extending from a wire center into a segment of the area served by the wire center.

Serving Area Interface (SAI)

A serving area interface is part of a phone company's outside plant. It is a fancy name for a box on a pole, a box attached to a wall or a box in the ground that connects the phone company's feeder or subfeeder cables (those coming from the central office) to the drop wires or buried service wires that connect to the customer's premises. It's also called a cross-wire box.

(See below for picture of a typical grounded Serving Area Interface)




Source: http://upload.wikimedia.org/wikipedia/en/thumb/9/9d/1200_pair_SAI.jpg/180px-1200_pair_SAI.jpg

Tandem Switch

Tandem: In a telecommunications context, the term refers to switches, circuits, or other Network Elements (NEs) that serve to allow other NEs to work together. For example, tandem switches, or tandem offices, serve to interconnect other, lesser switches, (i.e. Central Offices [CO's] or lesser tandems). Tandem switches, in the purest sense of the term, serve no end users directly, as that is the responsibility of the COs. Rather, they strictly serve to interconnect the COs, which are at the lowest level of the switching hierarchy in the PSTN. Tandem tie trunks serve to interconnect tandem switches.


Tandem Switch: Tandem is a telephony term meaning to "connect in series." Thus a tandem switch connects one trunk to another. A tandem switch is an intermediate switch or connection between an originating telephone call location and the final destination of the call. The tandem point passes the call along. A PBX can often handle tandem calls from other/to other locations as well as process calls to, from and within its own location.



Source: http://img.zdnet.com/techDirectory/CO1.GIF

Remote Node

1. A remote node is a device that connects to a network from a point some distance away from the central host. For example, a CO (Central Office) in the PSTN (Public Switched Telephone Network) might support a number of remote nodes. Some of the nodes are dumb line concentrators that serve only to concentrate traffic over high-capacity trunks in order to reduce cabling costs. Other nodes are intelligent switching partitions that can switch basic local traffic within their own geographic domains, even though they rely on the CO for guidance in the delivery of more complex services, such as custom calling features.

2. Remote node software allows remote users to dial-in to the corporate LAN and work with the applications and data on the LAN as if they were "actually in the office." By dialing in, they become nodes on the LAN. Using a PC, Mac or UNIX workstation; a modem; and a remote access server, employees can connect from any location in the world that has an analog, a switched digital, or a wireless connection.

Switching Fabric

The term "switching fabric" refers to the component at the heart of a data communications switch that allows any input port to send data to any output port. Many different kinds of switching fabric have been used over the years, depending on the manufacturer, the size and type of the data communications switch, and the technology available at the time. Sometimes a switching fabric will directly connect to all ports, but usually there are a group of ports on a single card called a line card and the switching fabric connects the line cards together. There are many different types of switching fabrics available on the market today. An example of one of the most basic is the "crossbar" switching fabric, which consists of a matrix of rows and columns, where each row is connected to an input port and each column is connected to an output port. The resulting diagram looks like a fabric with threads crossing at right angles. A switch or "crosspoint" is located at each intersection between a row and a column. By closing the right crosspoints, each input port can be connected to each output port. Crossbar fabrics are very general, but expensive to create in large sizes because the number of crosspoints is equal to the number of input ports times the number of output ports. For instance, if you had a small eight port switch you would have eight potential input and output ports making a total of 64 crosspoints, but if you had a large switch with 100 ports you would need 10,000 crosspoints to allow every port to connect with each other. Other types of switching fabric use buffering, queuing, packet shaping, switching logic, and specialized application specific integrated circuits (ASIC) to enhance switching fabric performance. A well-designed switching fabric will reach switching speeds equal to the line rate of the port. For instance, a port with a theoretical speed of 100 Mbsp should be able to pass packets across the switching fabric to the destination port or ports at 100 Mbps, which is also known as line-rate or wire-speed switching. A poorly designed switching fabric has delays or other latency that will drop the data rate as packets travel through the switching fabric. The variety and performance of switching fabrics depend on many different variables as well as the manufacturer. However, one thing is for certain future trends in switching fabrics are hard to anticipate, but the switching fabric will always remain at the heart of the data communication switch.

Below is a simplified illustration of switching fabric which shows how any input port (i.e. line card) can transmit data to any output port, essentially linking all of the line cards together. Notice a "crosspoint" is depicted with a solid black dot indicating the intersection between a row and a column of the fabric.


Source: http://choonho.files.wordpress.com/2007/09/capture4.jpg

Internetworking

Communication between two networks or two types of networks or end equipment. This may or may not involve a difference in signaling or protocol elements supported. In the narrower sense - to join local area networks together. This way users can get access to other files, databases and applications. Bridges ad routers are the devices which typically accomplish the task of joining LANs. Internetworking may be done with cables - joining LANs together in the same building, for example. Or it may be done with telecommunications circuits - joining LANs together across the globe.

Two architectural models are commonly used to describe the protocols and methods used in internetworking. The Open System Interconnection (OSI) reference model provides a rigorous description for layering protocol functions from the underlying hardware to the software interface concepts in user applications. Internetworking is implemented in Layer 3 (Network Layer) of the model.

The Internet Protocol Suite, also called the TCP/IP model, of the Internet was not designed to conform to this model. Despite similar appearance as a layered model, it uses a much less rigorous, loosely defined architecture that concerns itself only with the higher level aspects of networking, i.e. it does not discuss hardware-specific low-level interfaces, other than assuming availability of a link-layer interface to the local network link. Internetworking is facilitated by the protocols of its Internet Layer.

Hub

The point on a network where circuits are connected. In local area networks, a hub is the core of a physical star configuration, as in ARCNET, StarLAN, Ethernet, and Token Ring. Hub hardware can be either active or passive. Wiring hubs are useful for their centralized management capabilities and for their ability to isolate nodes from disruption. Hubs work at Layer 1 (Physical) and 2 (Data Link) of the OSI Reference Model, with emphasis on Layer 1. Hubs aren't switches, as they have very little intelligence, if any, and don't set up transmission paths. Rather, hubs comprise a physical bus and numerous ports, to which are connected a bunch of wires, to which are connected individual terminal devices. As hubs are protocol-specific (e.g, Ethernet) and are not intelligent, they are very fast and very cheap. A 10Base-T hub is an inexpensive means of allowing LAN-attached devices to share a common, collapsed bus contained within a hub chassis. The connections are via UTP (Unshielded Twisted Pair), which is much less expensive than are the classic connections through coaxial cable. Unlike switches, hubs do nothing internally to control congestion. However, they typically are workgroup-level solutions which allow a large, logical Ethernet to be subdivided into multiple physical segments. For example, you could even use a small five-port hub on your desk to connect a couple of laptops and a desktop PC. Hubs can be interconnected directly, or through switches or routers, with the traffic being forwarded from the originating hub only if the destination address of the data packet indicates that is necessary to do so. Therefore, hubs do reduce congestion through the control of interhub traffic.

Bridge

1. In classic terms, a bridge is a data communications device that connects two or more network segments and forwards packets between them. Such bridges operate at Layer 1 (Physical Layer) of the OSI Reference Model. At this level, a bridge simply serves as a physical connector between segments, also amplifying the carrier signal in order to compensate for the loss of signal strength incurred as the signal is split across the bridged segments. In other words, the bridge is used to connect multiple segments of a single logical circuit. Classic bridges are relatively dumb devices, which are fast and inexpensive; they simply accept data packets, perhaps buffering them during periods of network congestion and forward them. Bridges are protocol-specific, e.g., Ethernet or Token Ring in the LAN domain. Bridges also are used in the creation of multipoint circuits in the WAN domain, e.g., DDS (Dataphone Digital Service).

Bridges also can operate at Layer 2 (Link Layer) of the OSI Reference Model. At this level, a bridge connects disparate LANs (e.g., Ethernet and Token Ring) at the Medium Access Control (MAC) sub-layer of Layer 2. In order to accomplish this feat, the MAC Bridge may be of two types, encapsulating or translating.

Encapsulating bridges accept a data packet from one network and in its native format; they then encapsulate, or envelope, that entire packet in the format acceptable to the target network. For instance, an Ethernet frame is encapsulated in a Token Ring packet in order that the Token Ring network can deliver it to the target device, which must strip away several layers of overhead information in order to get to the data payload, or content. In order to accomplish this process, a table lookup must take place in order to change basic MAC-level addressing information.

Translating bridges go a step further. Rather than simply encapsulating the original data packet, they actually translate the data packet into the native format of the target network and attached device. While this level of translation adds a small amount of delay to the packet traffic and while the cost of such a bridge is slightly greater, the level of processing required at the workstation level is much reduced.

Bridges also can serve to reduce LAN congestion through a process of filtering. A filtering bridge reads the destination address of a data packet and performs a quick table lookup in order to determine whether it should forward that packet through a port to a particular physical LAN segment. A four-port bridge, for instance, would accept a packet from an incoming port and forward it only to the LAN segment on which the target devices is connected; thereby, the traffic on the other two segments is reduced and the level of traffic on those segments is reduced accordingly. Filtering bridges may be either programmed by the LAN administrator or may be self-learning. Self-learning bridges "learn" the addresses of the attached devices on each segment by initiating broadcast query packets, and then remembering the originating addresses of the devices which respond. Self-learning bridges perform this process at regular intervals in order to repeat the "learning" process and, thereby, to adjust to the physical relocation of devices, the replacement of NICs (Network Interface Cards), and other changes in the notoriously dynamic LAN environment.

While bridges are relatively simple devices, in the overall scheme of things, they can get quite complex as we move up the bridge food chain. Bridges also can be classified as Spanning Tree Protocol (STP), Source Routing Protocol (SRP), and Source Routing Transparent (SRT).
Spanning Tree Protocol (STP) bridges, defined in the IEEE 802.1 standard, are self-learning, filtering bridges. Some STP bridges also have built-in security mechanisms which can deny access to certain resources on the basis of user and terminal ID. STP bridges can automatically reconfigure themselves for alternate paths should a network system fail.

SRP bridges are programmed with specific routes for each data packet. Routing considerations include physical node location and the number of hops (intermediate bridges) involved. This IBM bridge protocol provides for a maximum of 13 hops.

SRT bridges, defined in IEEE 802.1, are a combination of STP and SRP. SRT bridges can act in either mode, as programmed.



2. Bridge is also a verb, as in "to bridge." Imagine a phone line. It winds from your CO (Central Office) through the streets and over the poles to your phone. Now imagine you want to connect another phone to that line. A phone works on two wires, tip and ring (positive and negative). You simply clamp each one of the phone's wires to the cable coming in. That's called bridging. Imagine bridging as connecting a phone at a right angle. When you do that, you've made what's known as a "bridged tap." The first thing to know about bridging is that bridging causes the electrical current coming down the line to lose power. How much? That typically depends on the distance from the bridged tap to the phone. A few feet, and there's no significant loss. But that bridged tap can also be thousands of feet. For example, the phone company could have a bridged tap on your local loop, which joined to another long-defunct subscriber. The phone company technicians simply saved a little time by not disconnecting that tap. If you want the cleanest, loudest phone line, the local loop to your phone should not be bridged. Instead it should be a direct "home run" from your CO to your phone.

Bridging can be a real problem with digital circuits. Circuits above 1 Mbps (e.g, T-1) should never, ever be bridged. Because of the power loss, they simply won't work or will work so poorly they won't be worth having. ISDN BRI (Integrated Services Digital Network Basic Rate Interface) channels are also digital. But they were specifically designed to work with the existing telephone cable plant, which has a huge number of bridged circuits. Telephone companies typically will install ISDN BRI circuits with up to six bridged taps and about 6,000 feet of bridged cabling.

Router

1. As in software, router is a system level function that directs a call to an application.

2. As in hardware, routers are the central switching offices of the Internet and corporate Intranets and WANs. Routers are bought by everybody - from backbone service providers to local ISPs, from corporations to universities. The main provider of routers in the world is Cisco. It has built its gigantic business on selling routers - from small ones, connecting a simple corporate LAN to the Internet, to corporate enterprise wide networks, to huge ones connecting the largest of the largest backbone service providers. A router is, in the strictest terms, an interface between two networks.

Routers are highly intelligent devices which connect like and unlike LANs (Local Area Networks). They connect to MANs (Metropolitan Area Networks) and WANs (Wide Area Networks), such as X.25, Frame Relay and ATM. Routers are protocol-sensitive, typically supporting multiple protocols. Routers most commonly operate at the bottom 3 layers of the OSI model, using the Physical, Link and Network Layers to provide addressing and switching. Routers also may operate at Layer 4, the Transport Layer, in order to ensure end-to-end reliability of data transfer.

Routers are much more capable devices than are bridges, which operate primarily at Layer 1, and switches, which operate primarily at Layer 2. Routers send their traffic based on a high level of intelligence inside themselves. This intelligence allows them to consider the network as a whole. How they route (also called routing considerations) might include destination address, packet priority level, least-cost route, minimum route delay, minimum route distance, route congestion level, and community of interest. Routers are unique in their ability to consider an enterprise network as comprising multiple physical and logical subnets (subnetworks). Thereby, they are quite capable of confining data traffic within a subnet, on the basis of privilege as defined in a policy-based routing table. In a traditional router topology, each router port defines a physical subnet, and each subnet is a broadcast domain. Within that domain, all connected devices share broadcast traffic; devices outside of that domain can neither see that traffic, nor can they respond to it. Contemporary routers have the ability to define subnets on a logical basis, based on logical address (e.g., MAC or IP address) information contained within the packet header, and acted upon through consultation with a programmed routing table. In addition to standalone routers developed specifically for that purpose, server-based routers can be implemented. Such routers are in the form of high-performance PCs with routing software. As software will perform less effectively and efficiently than firmware, such devices generally are considered to be less than desirable for large enterprise-wide application, although they do serve well in support of smaller remote offices and less-intensive applications. Routers also are self-learning, as they can communicate their existence and can learn of the existence of new routers, nodes and LAN segments. Routers constantly monitor the condition of the network, as a whole, in order to dynamically adapt to changes in network conditions.

Characteristics of routers can include: LAN extension, store and forward, support for multiple media, support for multiple LAN segments, support for disparate LAN protocols, filtering, encapsulation, accommodation of various and large packet sizes, high-speed internal buses (1+Gbps), self-learning, routing based on multiple factors, route length, number of hops, route congestion, traffic type, support for a community of interest (VLAN), redundancy, and network management via SNMP (Simple Network Management Protocol).

Router protocols include both bridging and routing protocols, as they perform both functions. These protocols fall into 3 categories:

1. Gateway protocols establish router-to-router connections between like routers. The gateway protocol passes routing information and keep alive packets during periods of idleness.

2. Serial Line Protocols provide for communications over serial or dial-up links connecting unlike routers. Examples include HDLC (High-level Data Link Control), SLIP (Serial Line Interface Protocol) and PPP (Point-to-Point Protocol).

3. Protocol Stack Routing and Bridging Protocols advise the router as to which packets should be routed and which should be bridged.

Switch

A mechanical, electrical or electronic device which opens or closes circuits, completes or breaks an electrical path, or select paths or circuits. Switches work at Layers 1 (Physical) and 2 (Data Link) of the OSI Reference Model, with emphasis on Layer 2. A switch looks at incoming data (voice data, or data data) to determine the destination address. Based on that address, a transmission path is set up through the switching matrix between the incoming and outgoing physical communications ports and links. Data switches (e.g., LAN (Local Area Network) switches and packet switches) also typically contain buffers, which can hold data packets in temporary memory until the necessary resources are available to allow the data packets to be forwarded. Voice switches, of course, don't, because you can't delay voice. Switches work link-by-link, with multiple switches typically being involved in complex networks; each switch forwards the data on a link-by-link (hop-by-hop) basis. Routers are highly intelligent data switches which are capable of setting up paths from end-to-end, perhaps in consideration of the level of privilege of the user and application. Routers commonly are used at the edges of complex data networks, where intelligence is required to set up appropriate network paths. Although such intelligent decisions impose some delay on the packet traffic, they are made only at the ingress and egress edges of the network. The routers often instruct switches in the core of the network, where speed is of the essence-switches aren't as intelligent as routers, but they are faster and less expensive.

OSI (Open Systems Interconnection):

*We are going to spend the next several days learning about various communication network equipment. Before we do that it will be helpful to understand the OSI Reference Model.

OSI (Open Systems Interconnection): A Reference Model developed by the ISO (International Organization for Standardization, as translated into English). The OSI Reference Model is the only internationally accepted framework of standards for communication between different systems made by different vendors. ISO's goal is to create an open systems networking environment where any vendor's computer system, connected to any network, can freely share data with any other computer system on that network or a linked network. Most of the dominant communication protocols used today have a structure based on the OSI model.

Although OSI is a model and not an actively used protocol, and there are still very few pure OSI-based products on the market today, it is still important to understand its structure. The OSI model organizes the communications process into seven different categories and places these categories in a layered sequence based on their relation to the user. Layers 7 through 4 deal with end to end communications between the message source and the message destination, while layers 3 through 1 deal with network access.

Layer 1 - The Physical Layer deals with the physical means of sending data over lines (i.e. the electrical, mechanical and functional control of data circuits). Examples include EIA-232 , T-Carrier and SONET.

Layer 2 - The Data Link Layer is concerned with procedures and protocols for operating the communications lines. It also has a way of detecting and correcting message errors. Examples include Frame Relay, PPP, and SLIP (Serial Line Internet Protocol). ATM runs at Layers 1 & 2, as do LANs.

Layer 3 - The Network Layer determines how data is transferred between computers. It also addresses routing within and between individual networks. The most visible example is IP (Internet Protocol).

Layer 4 - The Transport Layer defines the rules for information exchanges and manages end-to-end delivery of information within and between networks, including error recovery and flow control. TCP (Transmission Control Protocol) is an example, as is the OSI Transport Protocol (TP), which comprises five layers of its own. Layer 4 protocols ensure end-to-end integrity of the data in a session. The X.25 packet-switching protocol operates at Layers 1, 2, 3 and 4.

Layer 5 - The Session Layer is concerned with dialog management. It controls the use of the basic communication facility provided by the Transport layer. If you’ve ever lost your connection while Web surfing, you've likely experienced a session time-out, so you have some sense of the Session Layer.

Layer 6 - The Presentation layer provides transparent communications services by making the differences of varying data formats (character codes, for example) between dissimilar systems. Conversion of coding schemes (e.g., ASCII to EBDCIC to Unicode) and text compression and decompression exemplify Presentation Layer functions.

Layer 7 - The Applications layer contains functions for particular applications services, such as file transfer, remote file access and virtual terminal. TCP/IP application protocols such as FTP (File Transfer Protocol), Simple Mail Transfer Protocol (SMTP), SNMP (Simply Network Management Protocol) and TELNET (TELecommunications Network) take place at Layer 7.

The OSI Model is an important concept to understand, at least at a high level. I have attached one of Novell's Network Tutorials which goes into the OSI Model in more detail and also provide some very useful diagrams, including one which illustrates where different protocols operate in the OSI model. The tutorial can be found at the following link:

http://www.novell.com/info/primer/prim05.html.

Switch Access Line Service, etc.

Switched Access Line Service (Switched Access): All residential and most businesses use this type of telephone access. It refers to the connection between your phone and the long distance companies' switch (POP or Point-of-Presence) when you make a regular local or LD (long distance) telephone call over standard phone lines.

Switched Access: A method of obtaining test access to telecommunications circuits by using electromechanical circuitry to switch test apparatus to the circuit.

Switched Local Service: You pick up the phone. You dial a local number. Bingo, you have switched local phone service. The reason this trivial definition is even in Newton's dictionary is because many states in the US now, finally, allow companies to offer local switched telephone service in competition with the established company, e.g. United Telecom (Embarq). Previously, they had only allowed competition in leased lines. And then previous to that they had not allowed any competition in any area of local phone service.

Reciprocal Compensation

(Recip Comp) - A form of financial compensation that occurs when a local or LD (Long Distance) service provider terminates a call on another provider's facilities. Imagine a phone call from New York to Los Angeles. It may start with the customer of a new phone company, then proceed to a local phone company (e.g., New York Telephone, part of Verizon). Then it may proceed to a LD company before ending in Los Angeles and going through another one or two local phone companies before reaching the person dialed. Under the existing rules, all the companies carrying these phone calls have to be paid in some way for their transmission and switching services. There are programs in place such that the company doing the billing and collecting the money pays over some of those monies to the other phone companies in the chain. One such program is called "reciprocal compensation." The opposite of recip comp is called "Bill and Keep." Under this program, the company billing the call gets to keep all the money. The others in the chain (or most of the others in the chain) get nothing.

IXC (IntereXchange Carrier)

Also less commonly known as IEC (InterExchange Carrier) and IC. Long-haul long distance carriers, IXCs include all facilities-based inter-LATA carriers. The largest IXCs are AT&T (acquiredby SBC in 2005), MCI (merged with Verizon in 2006) and Sprint; a huge number of smaller, regional companies also fit this definition. The term generally applies to voice and data carriers, but not to Internet carriers. IXC is in contrast to LEC (Local Exchange Carrier), a term applied to traditional telephone companies which provide local service and intraLATA toll service. IXCs also provide intraLATA toll service and operate as CLECs (Competitive Local Exchange Carriers) in many states. Once upon a time the non-AT&T IXCs were called OCCs (Other Common Carriers), a status which they resented for understandable reasons.

Below is a link to an annual FCC publication (pdf format), "Statistics of Common Carriers." This document provides a wealth of data on common carriers. The FCC publishes a wide range of useful documents which are available at http://www.fcc.gov. The most up-to-date version is the 2005-2006 annual report released 6/08.

Document: http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-282813A1.pdf

InterLATA Service

As defined by the Telecom Act of 1996, the term interLATA service means telecommunications between a point located in a local access and transport area (LATA) and a point located outside such area. InterLATA services, traffic or facilities originate in one LATA and cross over and terminate in another LATA. This can be either Interstate or Intrastate service, traffic or facilities. Under provisions of Divestiture, the BOCs (Bell Operating Companies) were restricted from provided interLATA services, but could provide intraLATA services (since changed, right?). Reminder, the term LATA means a contiguous geographic area: (a) established before the date of enactment of the Telecommunications Act of 1996 by a BOC such that no exchange area includes points within more than 1 metropolitan statistical area, consolidated metropolitan statistical area, or State, except as expressly permitted under the AT&T Consent Decree; or (b) established or modified by a BOC after such date of enactment and approved by the Commission. IntraLATA services originate and terminate in the same LATA.

Long Distance

Now that we've learned all about LATA's and numbering plans, we'll shift our focus to long distance. Although today's word is rather succinct, there is plenty to learn in this area (which we will dive into later). Stay tuned in future weeks for more information on the various companies spawned from the original AT&T and terminology pertaining to the architecture of the local loop!

Long Distance: Any telephone call to a location outside the local service area. Also called a toll call or trunk call.

NANP (North American Numbering Plan)

Invented in 1947 by AT&T and Bell Telephone Laboratories (now Lucent). The NANP assigns area codes and sets rules for calls to be routed across North America (i.e. the US and Canada). The new one, put into effect in January, 1995 has one major change: The middle number in a North American area code no longer is required to be a 1 or a 0; rather, it can range between 0 and 9. NANP numbers are 10 digits in length, in the format NXX-NXX-XXXX. The first three digits are the NPA code (i.e., area code). The second three are the central office code or central office prefix, and the last four are the line number. NANP numbers conform to E.164, which is the ITU-T (International Telecommunication Union) international standard for numbering plans. NANP administration was shifted from Bell Labs to Bellcore, when it was formed in 1986. Due to Bellcore's obvious conflict of interest, responsibility was shifted to NANC (North American Numbering Council) in 1995; it was shifted again in 1997 to Lockheed Martin. In November, 1999, it was shifted to NeuStar Inc., when it was discovered that Lockheed Martin had a conflict of interest. NeuStar originally was an independent business unit of Lockheed Martin, but was spun off in order to resolve the conflict.