- Internetworking Basics 1-3 Characteristics of the OSI Layers. - Characteristics of the OSI Layers. - The lower layers of the OSI model handle data transport issues. - At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B.The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on until it reaches the application layer (Layer 7) of System B. - The header is then removed, and the remainder of the information unit is passed to the network layer. - Figure 1-8 illustrates the IEEE sublayers of the data link layer.. - The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. - Such application programs fall outside the scope of the OSI model. - A frame is composed of the data-link layer header (and possibly a trailer) and upper-layer data. - A packet is composed of the network-layer header (and possibly a trailer) and upper-layer data. - A cell is composed of the header and payload. - A network-layer address identifies an entity at the network layer of the OSI layers. - Error-checking is implemented at a number of the OSI layers.. - Multiplexing can be implemented at any of the OSI layers. - Of the three most widely used LAN implementations,. - WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. - Figure 3-1 WAN technologies operate at the lowest levels of the OSI model.. - They then choose the best path based on that particular system’s definition of “optimal.” The act of determining all routes, however, often requires substantial discovery traffic and a significant amount of time.. - Ethernet provides services corresponding to Layers 1 and 2 of the OSI reference model, and IEEE 802.3 specifies the physical layer (Layer 1) and the channel-access portion of the link layer (Layer 2). - Preamble—The alternating pattern of ones and zeros tells receiving stations that a frame is coming (Ethernet or IEEE 802.3). - Start-of-Frame (SOF)—The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame-reception portions of all stations on the LAN. - Destination and Source Addresses—The first 3 bytes of the addresses are specified by the IEEE on a vendor-dependent basis. - Type (Ethernet)—The type specifies the upper-layer protocol to receive the data after Ethernet processing is completed.. - Length (IEEE 802.3)—The length indicates the number of bytes of data that follows this field.. - Medium-Dependent Interface (MDI)—The MDI is a mechanical and electrical interface between the transmission medium and the physical-layer device.. - Physical-Layer Device (PHY)—The PHY provides either 10-or 100-Mbps operation and can be a set of integrated circuits (or a daughter board) on an Ethernet port, or an external device supplied with an MII cable that plugs into an MII port on a 100BaseT device (similar to a 10-Mbps Ethernet transceiver).. - Media-Independent Interface (MII)—The MII is used with a 100-Mbps external transceiver to connect a 100-Mbps Ethernet device to any of the three media types. - Gigabit Ethernet is an extension of the IEEE 802.3 Ethernet standard. - In Gigabit Ethernet, the FC1 layer will take decoded data from the FC2 layer, 8 bits at a time from the reconciliation sublayer (RS), which “bridges” the Fibre Channel physical interface to the IEEE 802.3 Ethernet upper layers. - Figure 7-20 This figure presents an overview of the operation of the IEEE 802.3 flow control process.. - The protocol type in 802.3 frames are left to the data portion of the packet. - Figure 7-21 This figure shows the fields of the IEEE 802.3/Ethernet frame format.. - FDDI specifies the physical and media-access portions of the OSI reference model. - Possession of the token grants the right to transmit. - Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. - This chapter examines a commonly utilized protocol specification used in WAN networking—the Frame Relay protocol.. - Call Setup—The virtual circuit between two Frame Relay DTE devices is established.. - Idle—The connection between DTE devices is still active, but no data is transferred. - Call Termination—The virtual circuit between DTE devices is terminated.. - Idle—The connection between DTE devices is active, but no data is transferred. - The FECN bit is part of the Address field in the Frame Relay frame header. - The BECN bit is part of the Address field in the Frame Relay frame header. - The DE bit is part of the Address field in the Frame Relay frame header.. - Flags indicate the beginning and end of the frame. - Standard Frame Relay frames consist of the fields illustrated in Figure 10-4.. - Flags—Delimits the beginning and end of the frame. - DLCI: The 10-bit DLCI is the essence of the Frame Relay header. - Congestion Control: This consists of the three bits that control the Frame Relay. - Frame Relay frames that conform to the LMI specifications consist of the fields illustrated in Figure 10-5.. - Flag—Delimits the beginning and end of the frame.. - IEs consist of the following fields:. - IE Length: Indicates the length of the IE.. - It therefore corresponds to the physical layer of the OSI reference model. - The flag field consists of the binary sequence 01111110.. - Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame.. - SIP Level 3 operates at the Media Access Control (MAC) sublayer of the data link layer of the OSI reference model. - SIP Level 2 operates at the MAC sublayer of the data link layer. - SIP Level 1 operates at the physical layer of the OSI reference model. - Figure 14-3 SIP provides services associated with the physical and data link layers of the OSI model.. - SNI—The SNI acts as the interface between the CPE and the carrier equipment.. - Figure 14-5 illustrates the format of the SMDS Interface Protocol (SIP) Level 3 protocol data unit (PDU).. - Address: Occupies the individual SMDS address of the source. - Length—Indicates the length of the PDU.. - Segmentation Unit—Contains the data portion of the cell. - One of the enabling technologies for FTTN is VDSL. - T1E1.4—The U.S. - ETSI—The ETSI has a VDSL standards project, under the title High-Speed Metallic Access Systems, and has compiled a list of objective, problems, and requirements. - The ATM Forum—The ATM Forum has defined a 51.84 Mbps interface for private network UNIs and a corresponding transmission technology. - The ADSL Forum—The ADSL Forum has just begun consideration of VDSL. - The send- sequence number refers to the number of the frame to be sent next.. - QLLC uses the packet-level layer (Layer 3) of the X.25 protocol stack. - X.25 is designed to operate effectively regardless of the type of systems connected to the network. - Subscribers are charged based on their use of the network. - The development of the X.25 standard was initiated by the common carriers in the 1970s. - The X.25 protocol suite maps to the lowest three layers of the OSI reference model. - Figure 17-4 maps the key X.25 protocols to the layers of the OSI reference model.. - Figure 17-4 Key X.25 protocols map to the three lower layers of the OSI reference model.. - The second part of the address is the initial domain identifier (IDI). - When a packet voice connection is made between national administrations to support the activities of a single company—to connect two or more company locations in multiple countries—the application is uniformly tolerated in a regulatory sense.. - Remote user—The client who is dialing ISDN/Public Switched Telephone Network (PSTN) from either home or a remote location.. - NAS—The telecommuting device that terminates the dialup calls either over analog (basic telephone service) or digital (ISDN) circuits.. - Internet service provider (ISP)—The ISP, which supplies the dialup services, can provide for services itself through the NAS or can deliver the dialup remote user to a designated corporate gateway.. - Corporate gateway—The destination router that provides access to the services the remote user is requesting. - the authentication will be manageable by the user independently of the ISP.. - ATM functionality corresponds to the physical layer and part of the data link layer of the OSI reference model.. - The ATM reference model is composed of the following ATM layers:. - Physical layer—Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent transmission.. - ATM layer—Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the OSI reference model. - ATM adaptation layer (AAL)—Combined with the ATM layer, the AAL is roughly analogous to the data data-link layer of the OSI model. - Figure 20-7 The ATM reference model relates to the lowest two layers of the OSI reference model.. - Interleaved packets hence can be identified by the unique VCI value of the source. - LAN emulation client (LEC)—The LEC is an entity in an end system that performs data forwarding, address resolution, and registration of MAC addresses with the LAN emulation server (LES). - LES—The LES provides a central control point for LECs to forward registration and control information. - Broadcast and unknown server (BUS)—The BUS is a multicast server that is used to flood unknown destination address traffic and to forward multicast and broadcast traffic to clients within a particular ELAN. - LAN emulation configuration server (LECS)—The LECS maintains a database of LECs and the ELANs to which they belong. - The LEC must then determine the location of the LECS. - Upon receipt of the. - for the remote end of the circuit. - Origin LSAP—Serves as the SAP of the source device. - Target LSAP—Serves as the SAP of the destination device.
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