Remote Access Networks
Step 2 Monitor and Sniff All Traffic and Try to Find Patterns and Characteristics of the Traffic
It can sometimes take days or months, and hundreds of pages of sniffer information, logs, and analysis. Some examples follow. When analyzing the service and processes of Windows 2000, you can establish that a service automatically runs as soon as you start the computer. It is called remote procedure call (RPC) and provides the endpoint mapper and other miscellaneous RPC services. Sniffing the traffic shows the role of RPC in keeping the ISDN line up. The...
Seven Layer OSI Model
In the 1980s, the European-dominated International Standards Organization (ISO) began to develop its OSI networking suite. OSI has two major components an abstract model of networking (the basic reference model, or seven-layer model), and a set of concrete protocols. This model eventually grew into the Internet, to which the core Internet protocols adhere. This section discusses the layers of the OSI model. The physical layer is concerned with transmitting raw bits over a communication channel....
Troubleshooting PPP Network Control Protocols
NCPs are a set of protocols that negotiate the upper-layer protocols. Every negotiation is a series of requests and acknowledgments Vil IPCP I CONFREQ and Vi1 IPCP I CONFACK REQsent . During the negotiation process, you can see IPCP, IPXCP (these are not included in Example 12-29), CDPCP, and CCP negotiating IP, IPX, CDP, and COMPRESSION, respectively. The negotiation depends on the configuration and the process starts by establishing the virtual connection, which is a bundle of channels called...
ISDN Planes ISDN Layer Architecture
As with every technology, the ISDN architecture creates a framework for more informed decision making, including appropriate investments in network technologies, products, and services. The CCITT defines the ISDN architecture to consist of four planes The C-plane primarily deals with UNIs and establishing and tearing down the calls, and, the U-plane deals with User-Network data carried by the B channel. When troubleshooting ISDN, it is important to remember how the ISDN protocol architecture...
Ping and Privileged Extended Ping Commands
These commands are available for Cisco, Microsoft, and UNIX platforms. The ping utility can determine basic connectivity. It is based on the specifications for RFC 1256 and RFC 791. Ping is assigned a low priority. It is not an indicator of performance, and it is not used for other more sophisticated tests. During a ping test, the host sends an ICMP echo packet and receives a reply ICMP message if basic connectivity exists. NOTE An ICMPv6 is available for the IPv6 protocol (see RFC 1885). The...
Overview of Modems
In general, modulator-demodulators (modems) are devices that modulate digital signals into audible sounds and demodulate audible sounds into digital signals. Included in this section are some basic facts about telephone companies, how telephone companies deal with switched services, and the limitations that this places on data connections. Besides the information provided in Chapter 1, Remote Access Overview, and Chapter 3, The Cloud, this section also includes information on ways that telcos...
ISDN Services
ISDN is considered to be the first advancement from analog dialup service. The ISDN standard is well developed and there are a variety of solutions that make ISDN the most commonly available service offering from telecommunication companies. The T sector of the ITU-T is responsible for issuing protocols for the E-Series, I-Series, and Q-Series, and for components of the X-series that are related to ISDN, broadband ISDN (B-ISDN), and Signaling System 7 (SS7). E-Series protocols cover the...
IEEE 80211bThe Next Step
All previously mentioned, coding techniques for 802.11 provide a speed of 1 to 2 Mbps, which is lower than the widespread IEEE 802.3 standard speed of 10 Mbps. The only technique (with regards to FCC rules) that is capable of providing a higher speed is DSSS, which was selected as a standard physical layer technique that supports 1 to 2 Mbps and two new speeds of 5.5 and 11 Mbps. The original 802.11 DSSS standard specifies the 11-bit chipping, or Barker sequence, to encode all data sent over...
NAT and PAT
NAT and PAT are design solutions that create private address space, in the sense that these addresses only exist inside the private network and not in the public address space. These techniques allow designers of future network implementations to achieve some important goals, including the following Provide a firewall technique Reduce time and cost by simplifying IP address management In its simplest configuration, NAT operates on a router that connects two networks together. One of these...
PPP DialIn Configuration
All PPP dial-in configurations must have a set of the same elements, whether only one or one hundred modems are attached. The first key element is authentication. This can be done locally with a fixed username and password configured on the router or with an authentication, authorization, and accounting (AAA) server. AAA servers can run TACACS+ or RADIUS services for authentication. To apply any aaa configuration, you must first issue the command aaa new-model. This instructs the router to user...
XDSL Coding Techniques
The emergence of digital subscriber line (DSL) technology increased the need for more modulation techniques. Unlike other technologies, DSL uses a set of protocols that are suitable for the different types of DSL technologies. Asymmetric DSL (ADSL), in particular, refers to the family of coding schemes where the emerging Internet technologies usually require higher bandwidth in the downstream direction than in the upstream direction (see the section, xDSL Services in Chapter 1, Remote Access...
Line Code Errors
On both a PRI and a T1, time slots exist for each of the 64-kbps channels. (See Chapter 2, Telecommunication Basics, for more information.) As the data is received, a time slot generator places the bits into these time slots. There can never be more than seven 0s in a row, even across time slots, and this is where line coding must be considered. If eight 0s are in a row, the time slot generator converts the eight 0s to a combination of 0s and 1s, depending on the line code. (See Figure 2-10 for...
Layer 1 BRI
ISDN standards require the availability of three types of services for end-to-end connections by the local exchange carrier (LEC) Circuit-switched services on B channels Circuit-switched or packet-switched services on D channels Packet-switched services on B channels 238 Chapter 9 ISDN Technology Background To use them, the ITU-T defined BRI and PRI access interfaces, and the operation speed and the number and type of channels. The BRI is also referred to as 2B+D, and typically operates over...
Digitalization of the Signal and Pulse Code Modulation
In general, the process of digitizing the signal creates an adequate digital model for a nondigital process. The challenge is trying to represent a real-life scenario as an algorithm. In some cases, the task appears to be much easier because of the existence of repeated samples. Human voice transfer requires a 4-kHz frequency band. Based on the Nyquist sampling theorem, digital sampling must take place at least twice the highest frequency to reconstruct the analog signal accurately. As a...
ISDN Channels
In ISDN, the local loop carries only digital signals that represent signaling information and user data. The local loop typically carries voice, data, or video. The term channel, unlike other techniques, means a unidirectional conduit that carries signaling and user information. Three basic channels are defined by the standards D channel (Delta channel, signaling channel) The D channel is a packet-switched channel that carries signaling and control information for B channels between the...
LCP Problems and the Magic 22 Seconds
One of the most complicated issues is when everything works correctly and the router makes calls out, but after the router disconnects, the call results in one of the following If the disconnect is immediately after call out (less than one second), there's either a wrong phone number or it is not provisioned for data. The error messages in Chapter 12 provide more explanations about the possible cause. If the call disconnects in two seconds, the other party is not configured to be called from...
Provisioning of Enterprise Remote Access Services
A key function of IT planning and design includes provisioning of remote access services for employees. Remote work is defined as work conducted away from corporate facilities. The provisioning activities described in this section support remote workers in a home office (telecommuting), as opposed to another type of virtual office (for example, a satellite or hotel office), instead of commuting to a corporate facility. Cisco's internal remote access offering has changed over the last couple of...
Scenario 1 Authentication Time Outs Part I
A user calls in to report that his modem won't authenticate successfully. It seems to be stuck at the verifying username and password stage. First, you use the command debug ppp negotiation to debug the problem. 220 Chapter 8 Dial Troubleshooting Scenarios After debugging the problem, the output in Example 8-1 is produced. Example 8-1 Output from debug ppp negotiation for Authentication Time Outs Part I MagicNumber 0XBC6B9F91 (0X0506BC6B9F91) MagicNumber 0XBC6B9F91 (0X0506BC6B9F91) MagicNumber...
Step 3 Use a Cisco Knowledge Base to Remedy any Identified Problems
It's time to implement the existing Cisco knowledge base. Based on the available information on Cisco.com for an ACL, you can create the following solution. For IOS routers, the ACL might include the following The 77x version would include the following pattern 00 6f pattern 00 89 pattern 00 8a pattern 00 7b set set set set set set set set set set set set tcp139 tcp445 ip filte ip filte ip filte ip filte ip filte offset 2 offset 2 r icmp out out udp111 out udp137 out udp138 out udp123 from...
The Traceroute and Privileged Extended Traceroute Commands
Available as traceroute in Cisco and UNIX based-platforms, and tracert in Microsoft-based platforms. The traceroute (Cisco UNIX) and tracert (Microsoft) commands differ slightly. Cisco UNIX uses UDP packets, and Microsoft uses ICMP packets, which are more likely to be filtered. The traceroute command tests the connectivity of the path between two devices (hops). It narrows down connectivity issues, helps to discover routing loops, and determines baseline network layer performance on a...
About the Author
Plamen Nedeltchev was born in February 1954 in Silistra, Bulgaria. He graduated from high school as valedictorian in 1972. In 1980, he received an M.S. (Summa Cum Laude) from Saint Petersburg State Electro-Technical University. In 1989, he received his Ph.D. from the Bulgarian Academy of Science, Sofia, Bulgaria. Plamen worked as the chief information officer of VMT (a division of the Ministry of Transport of Bulgaria) in his country of origin. He joined Sprint E-Solutions in 1999 as a senior...
Amplitude Frequency and Phase Modulations
In general, modulation and demodulation are always related to one of the three characteristics of the signal amplitude, frequency, and phase or a combination of all or some of these characteristics. Modulation, or shifting, of any of these characteristics creates a recognizable, adequate (or sometimes called constellation) state of the signal, interprets these states as 1 or 0, and digitalizes them. In amplitude-shift keying (ASK), the 1 and 0 are represented by two different amplitudes, as...
Troubleshooting PPP Authentication
The next phase of PPP protocol is the authentication phase. By default, PPP authentication is optional and should only be configured after the circuit is known good and IP connectivity has been established. This reduces the number of variables that have to be dealt with in case of a problem. Usually, the protocol number C023- Password Authentication Protocol (PAP) or C223-Challenge Handshake Authentication Protocol (CHAP) is used. If you enter the debug ppp authentication command, you can see...
Troubleshooting Remote Access Networks
201 W 103rd Street Indianapolis, IN 46290 USA Troubleshooting Remote Access Networks Copyright 2003 Cisco Systems, Inc. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the publisher, except for the inclusion of brief quotations in a review. Printed in the United States of America 1 2 3 4 5 6 7 8 9...
Tandem Office and Tool Office
Trunks are not directly run to another LEC's facilities, but to more centrally located LEs that are intermediate points equally convenient for all parties. These LEs are equipped with a secondary switch called a tandem that provides switching trunk-to-trunk, instead of local loop-to-local loop or local loop-to-trunk. It is provisioned for equal access from both parties and for providing services to them. Tandem is a telephony term that means to connect in series. Thus, a tandem switch connects...
Frame Relay and Upper Layer Protocols
Flow control and error recovery functions can be performed by the full LAPF protocol. Typically, the full LAPF is not used in PVC circuits. Therefore, these functions must be performed by the higher-layer protocols, which are encapsulated in the PDU as a component of the information field of the frame format. This approach is the most commonly implemented today. To ensure the interoperability of Frame Relay and higher-layer protocols, two major issues must be addressed multiprotocol support and...
Network Control Protocol Phase of PPP
When the PPP phase is up, authentication has completed successfully. Then, Network Control Protocol (NCP) negotiates Layer 3 protocols, including IP Control Protocol (IPCP). In the case of dial, IPCP negotiates IP addresses for the peer IP address, the Domain Name System (DNS) servers, and the Windows Internet Naming Service (WINS) servers. The following explanations are based on the output from the command debug ppp negotiation. Remember that the output can be long and you see these lines only...
Pseudo Ternary Line Coding and the ST Interface
ISDN and the first layer of Frame Relay use a coding scheme called pseudo-ternary signaling, which is used by the S T interface. This technique provides DC balance and non-voltage drifting procedures for signaling by using positive and negative 0s. In pseudo-signaling, 0 is represented as a line signal of approximately 750 millivolts that alternates between positive and negative polarity, and the 1 represents the absence of voltage. As you can see from Figure 2-6, synchronization is important...
Satellite Services
Direct Broadband Satellite (DBS) is a typical service for satellite communications. The system is capable of delivering downstream data at a rate of 23 Mbps by using one-way communication. All solutions are based on MPEG I Layer II audio encoding and use extensive compression methods. The upstream directions in early deployment for data were based on PSTN-wired modems, which was not a successful solution, especially combined with VPN technology. The most common speed offering is 400 kbps to 1...
Ppp authentication chap
Second, use the debug command to monitor the authentication process 3640-isdn debug ppp negotiation After the LCP is open, the expected output should look like the output shown in Example 13-17. Example 13-17 Output After the LCP Is Open Jul 30 11 12 21 PDT Se2 1 5 LCP I CONFACK ACKsent id 119 len 30 AuthProto CHAP (0X0305C22305) MagicNumber 0X484CD7F7 (0X0506484CD7F7) MRRU 1524 (0X110405F4) EndpointDisc 1 Local Jul 30 11 12 21 PDT Se2 1 5 LCP State is Open The LCP opens successfully, PPP...
Troubleshooting the Data Link Layer
As previously mentioned, the second layer runs at least two protocols Link Access Procedure on the D channel (LAPD) and PPP on the B channel. LAPD is also known as Q921 it is the focus of this section. LAPD is used between the Cisco router and the telco switch to provide a full-duplex error-free link. This protocol is not end-to-end It can be used only to troubleshoot the connection between the local router (CPE) and the local switch. The troubleshooting of both protocols (LAPD and PPP) must be...
Analog Dialup Services
The analog modem dialup service gained popularity because of constant increases in speed and support from modem manufacturers. The initial offering started with speeds from 300 to 2400 bps, then to 9600 bps, 14.4 kbps, and so on. The term modem is an abbreviation for modulator-demodulator. Because the modem operates in the 4-kHz bandwidth, it requires analog to digital and digital to analog conversion on both ends. Part of this technology uses extended code correction and compression, which...
Step One Verify that the Modem Is Ready to Accept Calls
The first event to occur is that the modem must pick up the call. If it is an external modem, increase the volume so that you can hear it. Place a call from a telephone to the modem to make sure it answers. If it does not, check the cabling between the modem and the router. The type of cable can differ between modems, so the easiest way to check is through the show line line-number command. The line you must check is the following If the modem is ready to take an incoming call, but there is no...
Frame Switching
When discussing Frame Relay technology options in Chapter 14, Frame switching and a set of protocols were introduced (refer to Figure 14-2 and Figure 14-3). Typically, a router is configured as data communications equipment (DCE) if it is connecting directly to another router. It can also be configured as DCE when connected to a 90i D4 channel unit, which in turn is connected to a telephone company (telco) channel bank. A variety of configuration solutions exist for frame switching, including...
Scenario 3 Frequent Retrains and Disconnects
To the client, frequent retrains or speedshifts are reported as a bad connection, where data flow is interrupted often. In most cases, a client does not hear the modem retrain because the speaker is turned off after the first train-up is successful. If you encounter a problem such as this, you can easily determine if the modem is retraining frequently. Just look at the output of the show modem operational-status command for the modem that this client last connected to or is currently connected...
Step 1Configuring the Dialer Interface
As previously mentioned, the dialer interface is a non-physical interface. It defines the call destination. As is usual for every interface configuration, you have to first configure the interface, its type, and its number. Figure 11-2 shows the network setup and configuration information. Figure 11-2 Configuring the Dialer Interface for Cisco IOS-Based ISDN Router on the Remote User's Side and on the Core Router's Side Host name - 804-isdn ISDN switch-type NI ISDN SPID1 40857647400101 5764740...
Frame Relay and Bridging
When using Frame Relay bridging, there must be a full mesh of Frame Relay virtual circuits between the bridges of a remote bridge group. If the Frame Relay network is not a full mesh, the bridge network must be divided into multiple remote bridge groups. Because LMI transforms the Frame Relay to a LAN-like architecture, this bridging model is identical to the model for remote bridging as described in IEEE P802.1g, which supports the concept of virtual ports. The Frame Relay virtual circuits...
Dial
Part II is devoted to one of the most traditional remote access technologies dial networking. The initial technology information is designated to underline some of the fundamentals and specifics of dial, in addition to the information provided in Part I. The detailed description of modems and the overview of possible provider issues, as well as the detailed description of Point-to-Point Protocol (PPP), is among the main topics covered. The design chapter includes information about text dialin...
T1s and DS and TDM Hierarchy
T1 frames consist of 24 8-bit words plus a framing bit. Each time slot of the frame contains 8 bits of binary information and is called DS0, which is sampled 8000 times per second. Because each DS0 contains 64 kbps (8 k samples sec x 8 bits sample) of user information and 24 DS0s are in a T1 frame, this 1544-kbps signal is commonly referred to as DS1. (DS-1 refers to digital signal level 1, which is a framing specification.) T1 refers to the digital transmission system that happens to operate...
Setting the ISDN Switch Type
The switch types supported in the latest versions of IOS (12.1.x. and later) are outlined in Example 10-1. The most commonly used types in the U.S. and Canada are basic-5ess, basic-dms100, and basic-ni1. Example 10-1 Determining the ISDN Switch Type Example 10-1 Determining the ISDN Switch Type Northern Telecom DMS-100 switch type for the US NET3 switch type for the UK, Europe, Asia and Australia National ISDN switch type for the US TS013 switch type for Australia (obsolete) Besides the listed...
Frequently Asked Questions and Answers
The questions and answers in this section apply to the majority of users. These solutions will generally fix the problems you or your clients are experiencing. If you still have a problem after taking into account everything listed here, search Cisco's web site for more recent troubleshooting information and, if required, open a TAC case for further investigation. Question How can I achieve faster connection speeds Answer To start, some situations simply cannot be helped or improved. First,...
Creating Dial Peers
Dial peers determine the routing of incoming calls to the telephone ports. You can create a total of six dial peers for the two telephone ports. Example 11-9 lists some of the relevant Cisco IOS Software configuration commands. Example 11-9 A Common Procedure for Dial Peers Configuration This command sets up a tag number (1 to 6) 804-isdn(config) dial-peer voice tag pots This command assigns a local directory number ldn The number of the port is defined using the following command...
Address Resolution ARP Reverse ARP and Inverse ARP
The higher-layer protocol encapsulation in Frame Relay formats raises the issue of resolving the upper-layer addresses to Frame Relay addressing schemes. Especially in point-to-multipoint connections, where the hub side has many Layer-3 addresses (typically IPs) assigned, you need to resolve the Layer-2 address (regardless if it is a DLCI or Q.922 address) to the Layer-3 address (IP). The problem can be resolved by polling all subinterfaces for hardware and protocol address resolution, but this...
The Baseline
104 Chapter 4 Troubleshooting Approaches, Models, and Tools The output from these commands returns specific details including device type, IP address, active interface (Ethernet, serial), port type, and port number. The first command provides short output, and the second command provides detailed information about adjacent topology specifics, as shown in Example 4-1. Example 4-1 The show cdp neighbors detail Command Output Example 4-1 The show cdp neighbors detail Command Output Platform cisco...
Information Data Field of the LAPF Control Protocol Frame
In the LAPF control protocol, the information field carries both non-data (signaling) and user data information. A Frame Relay network uses four categories of signaling messages In general, the information field uses two formats as shown in Figure 14-10. In the figure, IE indicates the information elements. Figure 14-10 Information Field Formats The IE fields carry higher-layer protocol information such as user data and higher-layer overhead and routing updates. It is transparent to the Frame...
Using show isdn status to View Service Layers
If the remote user's ISDN is not functioning correctly, you cannot telnet to the router. Your best chance is to work with the remote user. Take time to help the user (if necessary) by teaching him or her how to use the telnet session or the console connection to the router. One of the most useful IOS commands for ISDN troubleshooting is show isdn status, which provides a snapshot of all layers of the service. Example 13-1 shows an ISDN line in the activation stage. The first layer of this...
Van Jacobson Header Compression
Cisco IOS header compression uses the Van Jacobson Algorithm as defined in RFC 1144. Cisco's header compression product supports X.25, Frame Relay, and dial-on-demand WAN link protocols. Because of the processing overhead, header compression is generally used at 64 kbps. For example, a 50 percent throughput improvement can be achieved with telnet, rlogin, and X Windows traffic on a 64-kbps leased line for TCP IP, which conforms to RFC 1144. Example 16-12 shows an interface configured for TCP IP...
Two Binary One Quaternary Coding the Frame Format and the UInterface
ANSI standard T1.601 is used in the U.S. to provide the necessary specifications for the U-reference point because the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) does not define the local loop specifications between NT and the LE across U-reference points, or the otherwise called U-interface. The interface and 58 Chapter 2 Telecommunication Basics line coding is specific for U.S. ISDN, IDSL, HDSL and SDSL local loop specifications. The physical...
Cisco ISDN Cost Effective Solutions
ISDN is offered by most local exchange carriers (LECs). It's used for data, voice, and video exchange. Recently, the use of ISDN for offnet video-conferencing has increased. However, ISDN became less attractive as a cost-effective solution when compared to other emerging technologies, such as digital subscriber line (DSL), cable, wireless, and satellite. The cost of ISDN service is usually composed of an initial installation charge plus usage-based charges. In some cases, a LEC might...
PPP Termination of the Connection
Consider this example After a connection is established and there is no further demand for a network connection, the end user's router sends a request for disconnect -BR0 1 LCP O TERMREQ Open , which is referred to as a termination request. After the expected acknowledgment - I TERMACK TERMsent , you can see LCP Closed, PPP down. Example 12-42 shows the PPP termination sequence if the debug ppp negotiation command is turned on. Example 12-42 PPP Termination Sequence ISDN-6-CONNECT Interface...
Partial Mesh and Full Mesh Frame Relay Designs
Frame Relay networks provide several virtual circuits that form the basis for connections between stations (routers) that are attached to it. The resulting set of interconnected devices form a private Frame Relay group. These groups can be either fully interconnected with a complete mesh of virtual circuits, or only partially interconnected. In either case, each virtual circuit is uniquely identified at each Frame Relay interface by a DLCI. From an architectural point of view, a Frame Relay...
Large Scale Dial Out Configuration
Two types of large-scale dial-out configurations exist. The first uses static dialer maps put into a dialer interface, where they exist within the router configuration. The second type uses the AAA system for storing the dialer maps. In the first example that follows, static dialer maps dial out to each individual client. The number of configurable clients is limited by the size of the configuration. After the configuration is full, no more clients can be added. Although this limitation exists,...
Maximum Number of DLCIs Per Interface
As you already know from Chapter 14, Frame Relay Technology Background, the default DLCI address space is 10 bits. Theoretically, 210 1024 DLCIs can be configured on a single physical link. In reality, certain DLCIs are reserved for management and signaling messages. The available DLCIs for permanent virtual circuits (PVCs) range from 16 to 1007 for Consortium and from 16 to 992 for American National Standards Institute International Telecommunication Union Telecommunication Standardization...
Department of Defense Model
The efforts to create an interface model for internetworking started with the four-layer Department of Defense (DoD) model, which was developed by the Department of Defense in the 1970s for the Defense Advanced Research Projects Agency (DARPA) Internetwork Project (see Figure 4-1). The core Internet protocols adhere to this model, although the OSI model is preferred for new designs. Figure 4-1 The Four Layers of the Department of Defense (DoD) Model The four layers of the DoD model, from bottom...
Enterprise and ISP Designs
From an application point of view, you can break down ISDN into the following main categories. Based on the structure of the core side, you can recognize a separate enterprise design and an ISP design in these categories ISPs, more often than enterprises, use the MMP, which is not mandatory for an enterprise solution. ISPs often use a pool of IP addresses, whereas enterprises often assign an IP subnet from their routable or private address space for remote users based on their needs. ISPs use...
T1 ESF Signal Formats
The SF of ESF is extended from 12 to 24 frames with 24 framing bits. Of the 24 framing bits in an ESF, six bits are used for synchronization, six bits for error checking, and the remaining 12 bits are used for a 4-kbps facility data link (FDL), which is a communication link between CSUs and the telephone company's monitoring devices. The framing bits are used for different purposes than in SF. The ESF takes advantage of new more-reliable conduits (just an analogy with X.25 and Frame Relay...
Annex D Ansi Lmi Type
The T1.617 Annex D defines a polling protocol for use between the router and the network to exchange information about the status of the interfaces and the defined PVCs. The functions of the protocol are as follows Notification of the addition of a PVC Detection of the deletion of a PVC 452 Chapter 15 Frame Relay Design Solutions Notification of the availability or unavailability of a configured PVC Verification of the integrity of the link (UNI) The router periodically polls the network,...
Address Field
The address field has three different formats, shown in Figure 14-6. DLCI--Data link connection identifier is a unique number that provides local identification of the connection. The default address field format is 2 bytes or 10 bits. The extended address frame format can be 3 bytes (16 or 17 bits) or 4 bytes (23 bits). C R Command Response Field provides interconnectivity with higher-layer protocols. Frame Relay does not check it and the router can use this bit for signaling and control. FECN...
LAPF Control Protocol
For frame switching services, the LAPF control protocol is used along side the LAPF core protocol. This protocol is the full Q.922, and it is implemented both in the user's system and in the network (called the frame handler). Figure 14-4 shows one of the common formats, where the information field carries a higher-layer PDU and represents Frame Relay with other end-to-end protocols above LAPF, thus providing error and flow control. Here again, the Flag and Address fields and FCS are the same...
Control Field
In Figure 14-7 and Figure 14-8, two formats of Q.922 are shown. The only difference between the core format and the control protocol format is the control field. The control field has the same format and identical functionality as the Link Access Procedure on the D channel (LAPD) field. The control protocol provides the functions of error and flow control that are missing from the LAPF core protocol. Figure 14-7 LAPF Frame Formats Frame Relay with End-to-End LAPF Control Figure 14-7 LAPF Frame...
Ultra Wideband and WLANs
The origin of ultra wideband (UWB) technology stems from work in time-domain electromagnetics that started in 1962 as a simple concept. Instead of characterizing a linear time-invariant (LTI) system by the more conventional means of a swept frequency response (which is amplitude and phase measurements versus frequency), an LTI system is alternatively characterized by its impulse response. The FCC approved the technology in February 2002, and this could have a significant impact on the WLAN...
Authentication Problems
Before discussing authentication problems, it's important to follow the next rule of the thumb in authentication Establish the basic IP connectivity prior to implementing any authentication. Verify that the link works, then secure it. The authentication problems are easier to resolve when used in an enterprise environment. Two basic solutions are available Local authentication is used where the username (host name) and the password are defined in the particular box to which the remote user is...
Consortium Cisco LMI Type
The Consortium designed the LMI protocol to perform the following functions Allow the network to notify the FRAD about active and present DLCIs Allow the network to notify the FRAD if DLCI is removed or fails Monitor the status of the router-to-network link in real-time through keepalive messages The Consortium LMI defines two messages status enquiry and status. The general format of this LMI type is shown in Figure 15-9. Figure 15-9 Format of Consortium LMI Messages The Message Type Can Be...
Service Assurance Agent
A sophisticated Cisco IOS-based tool is available in IOS versions 12.2 or higher. The Service Assurance Agent (SAA) feature replaces the Response Time Reporter (RTR) in earlier IOS versions and provides extra capabilities such as VoIP metrics, response time, availability of HTTP (port 80) applications, and QoS based on the precedence field of the IP packet. The feature allows you to monitor network performance between a Cisco router and a remote device, which can be another Cisco router, an IP...
Shannons Capacity Theorem
Error-correcting codes can alleviate some errors, but cannot alleviate all the errors introduced by the channel in a digital communications system. Shannon's Capacity Theorem states that error-free transmission is possible as long as the transmitter does not exceed the channel's capacity. N bits constitute a block of information bits that have K bits of error correction tacked on. In terms of the bit error probability (p), the probability of an error can reach 0 as N becomes larger if the...
Scenario 5 Bad Modem
A bad modem is not something generally reported by your clients. A modem that is experiencing problems usually does not mark itself as defective unless there is an obvious hardware problem. Instead, you must analyze the modem with the show modem command. An output from show modem is provided in Example 8-7, where a group of six modems are obviously bad. In the AS5300 access server, modems generally become faulty in groups of six or 12, depending on the types of modems in the device. Example 8-7...
Text Dial Out Network
Dial-out networks are commonly used for support purposes. They are used in conjunction with routers that are configured for text dial-in as remote consoles, and for access to these routers when their primary links are down. They can also be used as a way to script dial-out for polling pieces of equipment that are attached to modems in any geographic location. This includes using a dial-out system to test the functionality of dial-in servers. Cisco's Technical Assistance Center (TAC) uses a...
Home Access Versus Mobility
The final criterion to be considered is the need to access the corporate network from home, and from on the road while in a hotel, convention center, airport, or a customer business partner premises. Depending on the organization's remote access requirement, a mobile solution might be a necessary part of the overall remote access service offering. The Cisco Mobile Office initiative addresses the solutions and considerations that differ based on the need to work on the road, at home, or at work...