Modem Signaling & Cabling Number of different standards defines the signaling over a serial cable. It includes: EIA/TIA-232 X.21 V.35 EIA/TIA-449 EIA-530 EIA-613 HSSI Each slandered defines the signals on the cable and specified the connector at the end of the cable With DB-25 pin connector of EIA/TIA-232 standard, only eight pins are actually used for connecting DTE to DCE The other 17 signals are ignored and are not interesting The eight interesting signals (pins) can be grouped into three categories by their functionality: 1. Data transfer group. 2. Hardware flow control group. 3. Modem control group. DB-25 DTE DCE (Computer or router) RS-232 or (modem) EIA / TIA TXD 2 2 Data RXD 3 3 Transfer Group GRD 7 7 RTS 4 4 Data Transfer CTS 5 5 Group DTR 20 20 Modem CD 8 8 Control Group DSR 6 6 DATA TRANSFER GROUP SIGNALS DESCRIPTION TXD Stands for transmit data. The DTE transmit data to the DCE. RXD Stands for receive data. The DTE receives data from the DCE. GRD Stands for ground. Provides ground reference for voltage measurements FLOW CONTROL GROUP SIGNALS DESCRIPTION RTS Stands for request to send. The DTE has buffers available to receive form the DCE. This is the signal form the computer or router telling the modem when to send data. CTS Stands for clear to send. The DCE has buffers available to take data from the DTE The signal is used by the modem to tell computer or router when to send data. MODEM CONTROL GROUP Signals Description DTR Stands for Data terminal ready. This signal is controlled by DTE. The DTE indicates to the DCE that the equipment (computer or router) is connected and available to receive data CD Stands for carrier detect. This signal is controlled by DCE. It indicates that it has established an acceptance carrier signal with the remote DCE. (it is a DCE-to-DCE connection) DSR Stands for Data Set Ready It gets activate as soon as a modem is turned on. MODEM COMMUNICATION TERMINATION Either the DTE device pr the DCE device may signal for the connection to be terminated. The signals that are used for this function are DTR from DTE or the modem recognizing the loss of CD signal The modem connection can be terminated in two ways: DTE Initiated The access server or computer can chop the DTR signal The modem must be programmed to terminate the connation on loss of DTR and restore to save settings in its NVRM. DCE Initiated The access server detects Carrier Detect (CD) low and terminates the connection. Modem Operation DCE DCE TxD RxD CTS RTS Buffer Buffer Flow Control Compressor Compression Compressor Decompression Packetizer Checksum Packetizer Retransmit Modulator / Modulator / Demodulator Demodulator UART Stands for Universal Asynchrous Receiver Transmitter. A computer component (IC) built-in on the motherboard to handle serial commutation. Every computer contains an UART to manage serial ports. External modems have their own UART IC on the circuit. It is (UART) controlled by a clock usually running at 1.84 MHz and has maximum throughput of 115kbps. UART have a buffer to temporarily hold incoming data. Some of the models of UART’s are: 16 C450 16 C550 uses 16-byte buffer. 16550 af 16750 uses 64-byte transmit & 56-byte receiver buffer. DTE Acronym for data terminal equipments. Device at the user end of a user network interface that serves as a data source, destination or both. DTE includes such devices as computers, protocol translators, multiplexers etc. DCE Acronym for Data Circuit-Terminal Equipments Devices and connections of a communications network that comprises the network end of the user-to-network interface. Modems and interface cards are the examples of DCE. DB Connector Stands for Data Bus Connector. Type of connector used to connect serial and parallel cables to a data bus. DB connector names are in the format DB-X, where ‘X’ represents the number of wires within the connector. Although each line is connected to a pin on the connector, not all pins are always assigned a function. DB Connectors are defined by carious EIA / TIA standards like, EIA / TIA -232 EIA / TIA -449 etc. EIA / TIA Electronic industry Association / Telecommunication Industry Association TRANSMISSION CONTROL PROTOCOL HEADER FORMAT. TCP is a connection protocol. It is a three way handshake protocol. Works at transport layer of an OSI model. Source Port (16) Destination Port (16) Sequence Number (32) Acknowledgement Number (32) Header Reserved Code Bits Window 20 Length (4) (6) (6) (16) bytes Checksum (16) Urgent (16) Options (0-32) if any Data (varies) 0 31 (bits) The TCP header length = 20 bytes. TCP header size = 32 bytes. Source port Shows the port number of the source or calling network devices 16-bits Destination Port Shows the port number of destination ort called network device. 16-bits Sequence Number A number that shows that correct sequence of arriving data at the destination. 32-bits Acknowledgement Number It shows the next expect TCP segment from the source. 32-bits Header length The complete length of the header id TCP. 4-bits Reserved They are for future use and set to zero in the TCP header. 6-bits Code Bits It controls the functions of setup and termination of a specific session. 6-bits Window The window is being calculated which the device is willing to accept at destination 16-bits Checksum Calculates the security measures within the header and data fields. 16-bits Urgent It indicates the end of the urgent data. The urgent data depends upon the priority set at any of the ends where the data is transmitted to and form. 16-bits Options The extra options, features and / or service(s) installed with the TCP header. 0-32 bits (if any) Data The data from upper layers. Varies. Q. Segment to be transmitted from source to destination with the window size of 4. Show the procedure using TCP for complete three way handshake transmission. Source Destination WINDOW 1 1 2 3 4 Send Receive Window1 5 Segments 1,2,3,4 ACK 1 2 3 4 Send Receive Window2 Segments 1,2,3,4 9 ACK 1 2 3 4 Receive Window 3 Segments 1,2,3,4 UDP Header Format UDP stands for User Datagram Protocol. It is a Connectionless protocol. It is a two way handshake protocol. 0 bits 31 Source Port (16) Destination Port (16) Length (16) Checksum (16) 8 bytes Data (if any) Source Port Shows the port of the source or calling network device. 16-bits Destination Port Dhows the port of the destination or called Network Device 16-bits Length The complete UDP header length. 16-bits Checksum Calculates the security features for reliable communication with in the UDP header format. 16-bits Data The upper layer data. Varies. IP Header Format. IP stands for Internet Protocol There are two versions of IP address, o IP v4 o IP v6 IP works at network layer of an OSI model. Data transmits form source to destination using source and destination IP address. IP provides connectionless, best fit delivery routing of datagram. IP is not concern with the contents of the datagram. It looks for a way to move the datagram to their destinations. 0 bits 31 Version Header Length Priority & Total Length (4) (4) Type of (16) Services (8) Identification Flags Fragment (16) (3) Offset (13) Time to live Protocol Header 20 (8) (8) Checksum bytes (16) Source IP Address (32) Destination IP Address (32) IP Option (0 or 32 ) if any Data (varies) if any Version Version number of IP. 4-bits Header Length The length of the IP Header. 4-bits Priority & type of service It shows that number of services that are handled for any IP packet. The first 3 bits are the priority bits. 8-bits Total Length The total length of the header + the data attached wit the header. 16-bits Identification The Identification of unique IP packet. 16-bits Flags It shows weather the fragmentation should occur. 3-bits Fragment Offset Provides fragmentation of packets to allow different MTUS in the WAN. 13-bits TTL Time to Live. The time for which the header will live. 8-bits Protocol The upper layer protocol (transport layer) used and senduip segments to the network. 8-bits Checksum The internal security and integrated check of the header. 16-bits Source IP address The 32-bit IP address of the source network device. 32-bits Destination IP address The 32-bit IP address of the destination network device. 32-bits IP Option The options like testing, debugging, security etc. (IP SEC) 0-32 bits (if any) Data it varies from upper layer protocol. Varies. SWITCHING NETWORKS. For transmission of data beyond a local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes. The switching network is also implemented on LANS. The end stations that wish to communicate with each other using switched network are referred to as stations. These end stations may be computers, terminals, telephones or other communicating devices. The switched networked are a type of networks in which data entering the network from a station are routed to the destination by being switched from node to node. Types of Switched Networks. There are two main types of switched networks which are: o Circuit Switching o Packet Switching Circuit Switching Networks. Communication via circuit switching implies that there is a dedicated communication path between two stations. This path is a connected sequence of links between networks. Communication via circuit switching involves three phases, which are as follows: Circuit Establishments. Data Transfer Circuit Disconnect. 1. Circuit Establishment Before any signals can be transmitted, an end-to-end circuit must be established. The circuit between end-to-end network devices first establishes then data between those devices will flow in both directions. 2. Data Transfer Information can flow now, after the circuit establishment. This data can be half-duplex (one way at a time) or full-duplex (in both directions at the same time). 3. Circuit Disconnect. After some period of data transfer, the connection is terminated, by the action of one of the two stations. Examples of CIRCUIT SWITCHING The well-known example of circuit switching networks is the PSTN (Public Switched Telephone Network). Another example of the PBX (Public Branch Exchange). It is used to interconnect telephones within the building or office. Routing In circuit Switched Network In a large circuit switched network link PTCL (Pakistan Telecommunication Limited), many circuit connections will require a path through more than one switch. When the call is placed, the network determines a route through the network form calling subscripts (e.g. QTA PTCL) to called subscriber (e.g. KHI PTCL) that passes through some number of switches and trunks. Two main requirements for the networks architecture that bear on the routing strategies are: o Efficiency o Resilience There are two main classes of routing algorithms used in Circuit Switching networks, which are: o Alternative Routing o Adaptive Routing Alternative Routing. In this type of routing the possible routs to be used between two end offices are predefined. Each switch is given a set of preplanned routes for each destination, in order of preferences. The preferred choice is a direct trunk connection between two switches. If the trunk is unavailable, then the second choice is to be tried and so on. If there is only one routing sequence defined for each source destination pair, the scheme id known as a fixed alternate routing scheme. S Route 1 S Destination S Route 2 Source Route 3 S Adaptive Routing An adaptive routing scheme is designed to enable switches to react to changing traffic patterns ion the network. This routing scheme requires great management overhead as the switches must exchange information to learn network condition.