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AAL — ATM adaptation layer. A series of protocols enabling various types of traffic, including voice, data, image, and video, to run over an ATM network.

AAL5 mode — ATM adaption layer 5. One of four AALs recommended by the ITU-T. AAL5 is used predominantly for the transfer of classical IP over ATM. AAL5 is the least complex of the current AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability. It is a Layer 2 circuit transport mode that allows you to send ATM cells between ATM2 IQ interfaces across a Layer 2 circuit-enabled network. You use Layer 2 circuit AAL5 transport mode to tunnel a stream of AAL5-encoded ATM segmentation and reassembly protocol data units (SAR-PDUs) over an MPLS or IP backbone. See also cell-relay mode, Layer 2 circuits, standard AAL5 mode, trunk mode.

ABR — Area border router. Router that belongs to more than one area. Used in OSPF. See also OSPF.

access concentrator — Router that acts as a server in a Point-to-Point Protocol over Ethernet (PPPoE) session, for example, an E-series router. access point name See APN.

accounting services — Method of collecting network data related to resource usage.

ACFC — Address and Control Field Compression. Enables routers to transmit packets without the two 1-byte address and control fields (0xff and 0x03) normal for PPP-encapsulated packets, thus transmitting less data and conserving bandwidth. ACFC is defined in RFC 1661, The Point-to-Point Protocol (PPP). See also PFC.

active route — Route chosen from all routes in the routing table to reach a destination. Active routes are installed into the forwarding table.

adaptive services — Set of services or applications that you can configure on an Adaptive Services PIC (AS PIC). The services and applications include stateful firewall, Network Address Translation (NAT), intrusion detection services (IDS), Internet Protocol Security (IPSec), Layer 2 Tunneling Protocol (L2TP), and voice services. See also tunneling protocol.

add/drop multiplexer — See ADM.

Address and Control Field Compression — See ACFC.

address match conditions — Use of an IP address as a match criterion in a routing policy or a firewall filter.

Address Resolution Protocol — See ARP.

adjacency — Portion of the local routing information that pertains to the reachability of a single neighbor over a single circuit or interface.

Adjacency-RIB-In — Logical software table that contains BGP routes received from a specific neighbor.

Adjacency-RIB-Out — Logical software table that contains BGP routes to be sent to a specific neighbor.

ADM — Add/drop multiplexer. SONET functionality that allows lower-level signals to be dropped from a high-speed optical connection.

ADSL — Asymmetrical digital subscriber line. A technology that allows more data to be sent over existing copper telephone lines, using the public switched telephone network (PSTN). ADSL supports data rates from 1.5 to 9 Mbps when receiving data (downstream rate) and from 16 to 640 Kbps when sending data (upstream rate).

ADSL Annex A PIM — See ITU-T Rec. G.992.1.

ADSL Annex B PIM — See ITU-T Rec. G.992.1.

ADSL interface — Asymmetrical digital subscriber line interface. Physical WAN interface that connects a router to a digital subscriber line access multiplexer (DSLAM). An ADSL interface allocates line bandwidth asymmetrically. Downstream (provider-to-customer) data rates can be up to 8 Mbps for ADSL, 12 Mbps for ADSL2, and 25 Mbps for ADSL2+. Upstream (customer-to-provider) rates can be up to 800 Kbps for ADSL and 1 Mbps for ADSL2 and ADSL2+, depending on the implementation.

ADSL2 interface — ADSL interface that supports ITU-T Standard G.992.3 and ITU-T Standard G.992.4. ADSL2 allocates downstream (provider-to-customer) data rates of up to 12 Mbps and upstream (customer-to-provider) rates of up to 1 Mbps.

ADSL2+ interface — ADSL interface that supports ITU-T Standard G.992.5. ADSL2+ allocates downstream (provider-to-customer) data rates of up to 25 Mbps and upstream (customer-to-provider) rates of up to 1 Mbps.

Advanced Encryption Standard — See AES.

AES — Advanced Encryption Standard. Defined in FIPS PUB 197. The AES algorithm uses keys of 128, 192, or 256 bits to encrypt and decrypt data in blocks of 128 bits.

aggregate route — Combination of groups of routes that have common addresses into a single entry in a routing table.

aggregated interface — Logical bundle of physical interfaces. The aggregated interface is managed as a single interface with one IP address. Network traffic is dynamically distributed across ports, so administration of data flowing across a given port is done automatically within the aggregated link. Using multiple ports in parallel provides redundancy and increases the link speed beyond the limits of any single port.

AH — Authentication header. A component of the IPSec protocol used to verify that the contents of a packet have not changed, and to validate the identity of the sender. See also ESP.

ALI — ATM line interface. Interface between ATM and 3G systems. See also ATM.

alternate priority queuing — See APQ.

ANSI — American National Standards Institute. The United States’ representative to the ISO.

Any Source Multicast — See ASM.

APN — Access point name. When mobile stations connect to IP networks over a wireless network, the GGSN uses the APN to distinguish among the connected IP networks (known as APN networks). In addition to identifying these connected networks, an APN is also a configured entity that hosts the wireless sessions, which are called Packet Data Protocol (PDP) contexts.

application-specific integrated circuit — See ASIC.

APQ — Alternate priority queuing. Dequeuing method that has a special queue, similar to strict–priority queuing (SPQ), which is visited only 50 percent of the time. The packets in the special queue still have a predictable latency, although the upper limit of the delay is higher than that with SPQ. Since the other configured queues share the remaining 50 percent of the service time, queue starvation is usually avoided. See also SPQ.

APS — Automatic Protection Switching. Technology used by SONET ADMs to protect against circuit faults between the ADM and a router and to protect against failing routers.

area — Routing subdomain that maintains detailed routing information about its own internal composition as well as routing information that allows it to reach other routing subdomains. In IS-IS, an area corresponds to a Level 1 subdomain.

In IS-IS and OSPF, a set of contiguous networks and hosts within an autonomous system that have been administratively grouped together.

area border router — See ABR.

ARP — Address Resolution Protocol. Protocol used for mapping IPv4 addresses to media access control (MAC) addresses. See also NDP.

AS — Autonomous system. Set of routers under a single technical administration. Each AS normally uses a single interior gateway protocol (IGP) and metrics to propagate routing information within the set of routers. Also called a routing domain.

ASBR — Autonomous system boundary router. In OSPF, a router that exchanges routing information with routers in other ASs.

ASBR Summary LSA — OSPF link-state advertisement (LSA) sent by an area border router (ABR) to advertise the router ID of an autonomous system boundary router (ASBR) across an area boundary. See also ASBR.

AS external link advertisement — OSPF link-state advertisement sent by AS boundary routers to describe external routes that they have detected. These link-state advertisements are flooded throughout the AS (except for stub areas).

ASIC — Application-specific integrated circuit. Specialized processors that perform specific functions on the router.

ASM — The acronym ASM can be either of the following:
  1. Adaptive Services Module. On a Juniper Networks M7i router, provides the same functionality as the AS PIC.
  2. Any Source Multicast. Method of allowing a multicast receiver to listen to all traffic sent to a multicast group, regardless of its source.
AS path — In BGP, the route to a destination. The path consists of the AS numbers of all routers that a packet must go through to reach a destination.

AS PIC — Adaptive Services PIC. See adaptive services.

asymmetrical digital subscriber line — See ADSL.

Asynchronous Transfer Mode — See ATM.

ATM — Asynchronous Transfer Mode. A high-speed multiplexing and switching method utilizing fixed-length cells of 53 octets to support multiple types of traffic.

ATM adaption layer — See AAL.

ATM-over-ADSL interface — Asynchronous Transfer Mode (ATM) interface used to send network traffic through a point-to-point connection to a DSL access multiplexer (DSLAM). ATM-over-ADSL interfaces are intended for asymmetrical digital subscriber line (ADSL) connections only, not for direct ATM connections.

ATM line interface — See ALI.

atomic — Smallest possible operation. An atomic operation is performed either entirely or not at all. For example, if machine failure prevents a transaction from completing, the system is rolled back to the start of the transaction, with no changes taking place.

AUC — Authentication center. Part of the Home Location Register (HLR) in third-generation (3G) systems; performs computations to verify and authenticate a mobile phone user.

authentication center — See AUC.

authentication header — See AH.

automatic policing — Policer that allows you to provide strict service guarantees for network traffic. Such guarantees are especially useful in the context of differentiated services for traffic engineered LSPs, providing better emulation for ATM wires over an MPLS network.

Automatic Protection Switching — See APS.

autonegotiation — Used by Ethernet devices to configure interfaces automatically. If interfaces support different speeds or different link modes (half duplex or full duplex), the devices attempt to settle on the lowest common denominator.

autonomous system — See AS.

autonomous system boundary router — See ASBR.

autonomous system external link advertisement — OSPF link-state advertisement sent by autonomous system boundary routers to describe external routes that they have detected. These link-state advertisements are flooded throughout the autonomous system (except for stub areas).

autonomous system path — In BGP, the route to a destination. The path consists of the autonomous system numbers of all the routers a packet must pass through to reach a destination.

auto-RP — Method of electing and announcing the rendezvous point-to-group address mapping in a multicast network. JUNOS software supports this vendor-proprietary specification. See also RP.

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